Harbor Freight TIG 130A welding aluminum A/C

[junkweld]

This is a great site with lots of great information. This is my first post. I searched but couldn't find any info. of the latest welding project.

I picked up a Harbor Freight ( HF ) TIG inverter 130A box. I was looking at the electrical schematic at Harbor Freight. The schematic shows that the 230V 60Hz AC input is chopped using two IGBTs to make an AC square wave at some other frequency (not 60Hz). Then the AC sq. wave is fed to a voltage reducing transformer. On the output of the transformer is diodes, capacitors, and a choke to make the AC sq. wave into a DC constant voltage for welding. The DC output is used as DCEN for mild steel and stainless steel.

I want to bypass the diodes, capacitors, and choke on the transformer output and try the AC squarewave on aluminum. Has anyone tried this or measured the Frequency of the AC squarewave? I am guessing that knob (potentiometer) on the panel controls the amount of time the IGBTs are on (pulsewidth), but not the frequency they are switched at.

I have a Lincoln 175 TIG (transformer) machine and have welded mild steel, stainless, and aluminum. So I do have some experience but it is just a hobby for me.

[greywynd]

Even if you successfully modify it, you won't have much in the way of capacity or duty cycle. At 130 amps, 1/8" will really make that thing work, to be honest, I suspect that it would be frustrating at best.

[junkweld]

I am replacing the potentiometer on the panel with a foot pedal. That will allow the welding amps to be reduced.

I am mainly concerned with the AC square switching frequency with the DC rectifier diodes bypassed.

EDIT: I just realized that you were talking about only being able to go up to 130 AMPs. The thickest aluminum I weld is 1/8" plate for intercooler end tanks, and small engine bay stuff. I think I could add another fan to increase the duty cycle. I wonder if the AC sq. wave freq. is low and that is why it is rated at 130 AMPs. Using AC I also gain a few amps (maybe up to 10) because the diodes & choke are bypassed.

[M Squared]

Used tig machines don't cost all that much. An intercooler or two could pay for a machine that will do everything you want and even have pulsing and balance controls.

If you really want to weld aluminum with that machine, get some helium and weld it DC.

[Hobart Expert Keith]

Depending on the design of the inverter, the switching frequency could be from 5KHz to 20KHz. This would not be a usable frequency for AC weld output. Also with the AC square wave being 50/50, arc stability would be very poor. I am with the previous poster, look into a good used AC/DC TIG unit. You will be happier that you did.

Keith

[Broccoli1]

What is wrong with the Lincoln?

[junkweld]

Depending on the design of the inverter, the switching frequency could be from 5KHz to 20KHz. This would not be a usable frequency for AC weld output. Also with the AC square wave being 50/50, arc stability would be very poor. I am with the previous poster, look into a good used AC/DC TIG unit. You will be happier that you did.

Keith

I would have expected the switching freq. to be greater than 20KHz. Wouldn't it be audible if it was in the 5KHz to 20KHz range at that power level? After looking at the PCB, it looks like it would be easier to add on an AC chopper to the output of the DC ckt.

I use a Lincoln TIG 175 squarewave and I am happy with it. It is new with less than 40 hours of weld time on it. I like learning new stuff so I figured making the inverter TIG into an AC machine would be an interesting project of learning high-power SMPS.

[junkweld]

What is wrong with the Lincoln?

The Lincoln works great. I am playing with the Harbor Freight unit to learn about SMPS possibly end up with another AC TIG machine.

[B_C]

Unless you are really into all the "Electrics" just buying the right machine for the job seems SO MUCH EASIER in my mind.....When my machines start acting up I just buy a new one. Especially if they are more than ten years old...Technology has gone into the OZONE with tig machines, get a new inverter and you will be HAPPY..

BC;)

[SundownIII]

Junkweld,

Did you ever hear the expression, "You can't make a silk purse out of a sow's ear"?

Fire up that Lincoln and learn to tig weld. You'll be ahead of the game.

[B_C]

http://tbn0.google.com/images?q=tbn:uT5h-lzuZ4oJ::www.eslpod.com/eslpod_blog/wp-content/uploads/2008/05/pig-thumb.jpg

[junkweld]

The Lincoln SquareWave works great. I have welded aluminum, stainless, and mild steel with it. It has more feature than I could ask for.......pulser, adjustable AC duty cycle, etc.

I was looking for a new hands-on toy to play with in terms of making the harbor freight machine capable of doing AC. I figured maybe some other people have wanted to do the same so I posted here.

If I was just looking to weld stuff I would fire up the Lincoln machine and have at it. The Harbor Freight thing is more of a learning project.

[SundownIII]

Crossword puzzles are challenging also.:D

[junkweld]

Crossword puzzles are challenging also.:D

Maybe so, but I can't remember the last time I tried one. Building stuff is so much more fun.

[junkweld]

An interesting thing happened with the Harbor Freight TIG 130 today. I blew out the input diode rectifier (GPBC3510) and the IGBT with a 7018 1/8" rod that stuck using DCEP. The IGBT tests as shorted across the transistors and the diode rectifier has holes in the side of it.
Either these machines don't have short circuit protection or the sense circuit is not working properly. You would think they turn off the IGBT if the rod is shorted. Instead it went into over current and blew parts. Maybe the IGBT went into latch up mode and blew.

[Roger]

I think you identified your problem in the 1st sentence.

[junkweld]

I am not sure what you mean. Are you saying that the machine is not designed with short circuit protection? A transformer machine will melt the rod. The IGBT machine "blew up". How does the Miller, Hobart, Licoln IGBT inverter machines handle a stuck rod?

Thanks

EDIT: I did trace out some of the PCB, IGBT, and transformer. I firmly believe that the IGBT went into latchup mode and the PCB could not turn it off. The output transformer had the direct short (rod stuck) and the IGBT flowed max current creating a latchup condition and this blew the input diode rectifier. The blow rectifier shorted the IGBT and then blew that also. The PCB could not stop the current flow even if it sensed it because the IGBT was latched up. The only fix I can see is to use a contactor or some type of fuse on the 230V input. Also, the TIG manual states the TIG input is 220V at 18 amps. The diode rectifier is GBPC3510 (35 Amp rectifier) and my panel breaker is a 50 amp unit. Maybe just putting a "special" 20 amp breaker in would do the trick. Seems like a hack..........but then again the Harbor Freight TIG circuitry is a hack too.

[Zrexxer]

I am not sure what you mean. Then we'll spell it out for you... It's a poorly designed piece of crap and you're probably lucky it lasted as long as it did. Or unlucky. Whichever.

[junkweld]

Then we'll spell it out for you... It's a poorly designed piece of crap and you're probably lucky it lasted as long as it did. Or unlucky. Whichever.

Oh, OK......thanks. I am curious, have you ever looked inside of one of these units? How do you know it is a poorly designed unit?

Actually, the IGBT is a decent unit made in Germany. The other PCB components are made all over the world. Inverters are pretty straight forward. You DC rectify the incoming AC, then chop the DC feeding a transformer (charge/discharge). This AC voltage is then transformer to a lower AC voltage that is DC rectified. Then sent through a choke to the torch & clamp.

[Zrexxer]

How do you know it is a poorly designed unit?Ummm, maybe because it just burned up under normal use?:rolleyes:

Maybe I'm just lucky since none of the name brand machines I've been using for the last 32 years have done that...

Or maybe you'd prefer not to be reminded of your OWN statements:

but then again the Harbor Freight TIG circuitry is a hack too.

[junkweld]

I think the ckt is a hack because there is no short ckt protection.

My old transformer machines handle a stuck rod OK. This is my first inverter machine so I am trying to figure out how they work in terms of ckt implementation. How do the 32 years of your inverter machines deal with a stuck rod? Could you do a test for me and short out one of your big three inverter machines and let me know what happens? That is, does the output turn off, the machine shut down, or does it blow up. When doing the test please use a 1/8" rod.

Thanks

[SundownIII]

Junkweld,

Sounds like you're on the wrong forum entirely.

Doesn't seem that you're the least bit interested in improving your welding skills through the use of that Lincoln tig machine you have.

Maybe you'd be better served by finding an amateur electronics board to post your questions. Personally, I could care less about all the ways you're finding to "let the smoke out" of that HF POS that you keep posting about.

You just made my "ignore" list.:)

[Archaic]

Junkweld:

Well, you have certainly caused me to think about this one. I can't give you any good answers, as my tig machine, mig machine, and my stick machines are all transformer machines, or DC generator. I have never owned or used an inverter stick machine.

Where a stick machine is concerned I had never considered short circuit protection, as stick welding is, well, short circuit. My smallest stick machine is 225 amp. The largest 300 amp. With a stuck rod, if left stuck, all of them will just cause the rod to turn cherry red, melt in two, and no harm to the machine. And that would be with 1/8" or 5/32" rods.

Short circuit protection for a short circuit machine. Interesting. Who would have thunk it? How little I know about inverters.

Adios-----

[Tux_Rules]

I think you identified your problem in the 1st sentence.


I am not sure what you mean. Are you saying that the machine is not designed with short circuit protection? A transformer machine will melt the rod. The IGBT machine "blew up". How does the Miller, Hobart, Licoln IGBT inverter machines handle a stuck rod?

Thanks

This may be what Roger is refering to:


I want to bypass the diodes, capacitors, and choke on the transformer output and try the AC squarewave on aluminum.

[junkweld]

Junkweld,

Sounds like you're on the wrong forum entirely.


I guess I am. I came across this board because people were building DIY machines so I figured some would like to talk about the low cost machine design. I thought it was a general board about "equipment talk", not just how to better welding skills.



Doesn't seem that you're the least bit interested in improving your welding skills through the use of that Lincoln tig machine you have.

I have been welding with the Licoln TIG, Miller MIG, AIRCO stick. HF stick. All are pretty straight forward and there is only so much to learn about welding. Once you get the hang of it there isn't much to learn. I got into the machine design to learn something new.



Maybe you'd be better served by finding an amateur electronics board to post your questions. Personally, I could care less about all the ways you're finding to "let the smoke out" of that HF POS that you keep posting about.

I have read other threads here about the machine design so I figured I would join up and talk about the Harbor Freight unit if others were interested. Why do you hate the Harbor Freight "POS"? Have you ever tried one? If you could care less about what I post then why do you post in this thread? Why not just ignore it if it is over your head?



You just made my "ignore" list.:)

Same here.

[junkweld]

Junkweld:

Well, you have certainly caused me to think about this one. I can't give you any good answers, as my tig machine, mig machine, and my stick machines are all transformer machines, or DC generator. I have never owned or used an inverter stick machine.

Where a stick machine is concerned I had never considered short circuit protection, as stick welding is, well, short circuit. My smallest stick machine is 225 amp. The largest 300 amp. With a stuck rod, if left stuck, all of them will just cause the rod to turn cherry red, melt in two, and no harm to the machine. And that would be with 1/8" or 5/32" rods.

Short circuit protection for a short circuit machine. Interesting. Who would have thunk it? How little I know about inverters.

Adios-----

My transformer stick machines just melt the rod also when stuck. Short circuit welding is not exactly short circuit. There is a voltage drop across the arc. Varying the arc length adjust the voltage and current. When a rod is stuck then the max. current flows and melts the rod. With the inverter the max. current goes through the IGBT and input diode rectifier. I need to install a current limiter. Oh yeah, I replaced the IGBT 100A unit with a IGBT 300A unit. Using a 3/32" rod, it will cut 1/8" steel easily. Also, it will weld 1/2" plate now.

This is my first inverter machine so I am trying to learn how the work if others would like to talk about how an inverter machine works.

[Roger]

picked up a Harbor Freight ( HF ) TIG inverter 130A box.
This was your first mistake was.

I once helped make a film about military data base of electronic components used by government contractors. Contractors had to conduct tests proving each component would reliably meet specifications in extreme conditions for civilian electronics but normal for military planes, tanks, spacecraft and be reliable. The data base reduced duplicate testing. Seemingly same IGBT 's with different part numbers made by the same manufacture having same electrical specification can handle over load different. This is why it's a big deal when counterfeit parts are used in aircraft electronics. A little cheaper part here a little out of spec diode and many other details can make a copy of a great welder junk. A few years ago many computer mother boards were failing early because made in china capacitor wasn't as good as what they were copying.

Many places can make great electronics but what separates fair from great is quality control. When low cost is highest priority for a distributer of imported product and the manufacture is trying to maximize profits with little to no liability you might get a product that works sort of ok under normal use. But it could be like baby formula with no nutrition.

[junkweld]

Roger,
I agree with everything you wrote about product design and quality. I have reviewed the PCB and components on the Harbor Freight TIG unit. I have replaced the diode rectifier and IGBT with better products. I used a IGBT quality made unit and stuffed into the machine.

IGBTs have a mode where they latch up. Some are designed so this does not easily happen. Older cheaper designs would latch up more easily. As you stated, the biggest problem with all China products is quality control. For the price I paid for the Harbor Freight TIG unit ($50) I think it is worth messing with installing better parts and modifying it. I have abut $100 into it right now as it sits. I bought the machine to learn about inverter TIG machines and so far it has been a great learning experience and actually welds DC quite well (ha ha, when I don't blow it up). Also, there are two pots on the PCB that I have been tracing back to components. I think they are there to adjust the output current and current sense ckt. I am going to mess with that to see if I can reduce the output of the unit with the 300A IGBT. Right now it will weld 1/8" rod and 3/8" to 1/2" plate with good penetration. It is too much current.

Why do many seem to dislike the Harbor Freight TIG 130A around here? I think it would make a decent unit for the hobbyist that does mild steel and stainless projects.

[bkahler]

For the price I paid for the Harbor Freight TIG unit ($50)

Where did you find it for $50? I'm looking for a small inexpensive welder to throw in the race car trailer for use at the track. I don't want to carry something large or bulky.

Brad

[whateg0]

...I think it would make a decent unit for the hobbyist that does mild steel and stainless projects.

That's what I use mine for. I have access to a PT225, but this is in my shop and was in my price range. It doesn't have to have all the bells and whistles, and if I can get good without them, then after I have them, it should be gravy.

Anyway, I'd like to get a copy of the schematic. I've thought about opening it up and tracing everything out, but I always end up needing it back together again. I started to build a pedal for mine a while back, but it's taken a back burner, as have many other things. I'd like to get back on it. I work in the T&M industry, and like to tinker with stuff like this. I've thought about just replacing the pot with a pedal, but I think I'd rather use the pot to set the max current and then the pedal can work within that range. As long as it's just setting a threshold (which is what I think it does), that should be pretty easy, but if the current through that part of the circuit is important at all, it could get a little tricky.

Dave

[Zrexxer]

Where did you find it for $50? I'm looking for a small inexpensive welder to throw in the race car trailer for use at the track. I don't want to carry something large or bulky. Brad:rolleyes: Every single post this guy has made is about how this POS burned up in a dramatic puff of smoke, and you guys can't wait to buy one. LOL. Ahh well, enjoy your adventure.

[bkahler]

:rolleyes: Every single post this guy has made is about how this POS burned up in a dramatic puff of smoke, and you guys can't wait to buy one. LOL. Ahh well, enjoy your adventure.

Kinda of a smart @ss remark frome someone who has apparently not read all of the posts in this thread. Not everyone has had problems with this welder. For the serious work I've got my share of quality welders. I just don't care to spend a huge amount of money for something thats going to sit in a trailer 99.9% of the time.

To each their own........

[Zrexxer]

Kinda of a smart @ss remark frome someone who has apparently not read all of the posts in this thread. Better than a dumb @ss remark, but someone has that covered already.

Not sure what's apparent to YOU... or not... but yeah, I've read all the posts in this thread. AND the other thread the OP started on the same topic. Like I said, have fun with your POS.

[uhmgawa]

Anyway, I'd like to get a copy of the schematic. I've thought about opening it up and tracing everything out, but I always end up needing it back together again. I started to build a pedal for mine a while back, but it's taken a back burner, as have many other things. I'd like to get back on it. I work in the T&M industry, and like to tinker with stuff like this. I've thought about just replacing the pot with a pedal, but I think I'd rather use the pot to set the max current and then the pedal can work within that range. As long as it's just setting a threshold (which is what I think it does), that should be pretty easy, but if the current through that part of the circuit is important at all, it could get a little tricky.

Dave

A while back I reverse engineered one of the 91811s I own and
created a schematic. The control circuitry is quite simple,
based on an old and proven design which more importantly has
little/no obscured IP.

If one of the big name welding machine manufacturers created such
an open design I'd be far more inclined to buy from them vs. joining
the captive repair part market which they cultivate.

Note this is a DC output only design as the output transformer
is driven at ~20KHz which is out of the range useful for welding
purposes.

Have fun hacking.

[whateg0]

Thanks uhmgawa! Looks like you put a little time into that.

So since we're just feeding a comparator, I don't need to worry about the current going through the pot. That simplifies what I want to do, which is to use the f/p pot as a max current setting and use a pedal for on-the-fly current adjustment. All I need is a voltage.

Now to find time to work on this again.

Dave

[Jake_SS]

My transformer stick machines just melt the rod also when stuck. Short circuit welding is not exactly short circuit. There is a voltage drop across the arc. Varying the arc length adjust the voltage and current. When a rod is stuck then the max. current flows and melts the rod. With the inverter the max. current goes through the IGBT and input diode rectifier. I need to install a current limiter. Oh yeah, I replaced the IGBT 100A unit with a IGBT 300A unit. Using a 3/32" rod, it will cut 1/8" steel easily. Also, it will weld 1/2" plate now.

This is my first inverter machine so I am trying to learn how the work if others would like to talk about how an inverter machine works.

The reason why they increase the frequency of the transformer is so that they can reduce the physical size of the transformer. If you replaced the IGBTs with 300 amp types that would only be there handling capacity, they wont increase your output at all, that is done by the PCB which controls them. All inverters that I know of have protection against short circuiting.
Cheers

[uhmgawa]

So since we're just feeding a comparator, I don't need to worry about the current going through the pot. That simplifies what I want to do, which is to use the f/p pot as a max current setting and use a pedal for on-the-fly current adjustment. All I need is a voltage.

True but note there is an analog switch (U2) which overrides the
user heat setting during arc start. But yea you can drive that with
a voltage source of lower impedance than the 10K pot. Haven't
experimented personally but I'd be careful to avoid induced noise
in the case of an external 10K pot. Shielded cable here is a start.

What I'd been planning to do is remove the control module and
substitute a uC to control the works. Mechanically this would be
fairly simple as all of the control is situated on a separate card
connected to the base PCB with an 18 pin header. Electrically
the 3525 provides some useful features not easily emulated in
a uCs approach particularly in the ability to shutdown IGBT per
cycle drive in under 500ns.


Now to find time to work on this again.

Dave

Good luck with that. It may well be the greater problem to be solved. ;)

[uhmgawa]

The reason why they increase the frequency of the transformer is so that they can reduce the physical size of the transformer. If you replaced the IGBTs with 300 amp types that would only be there handling capacity, they wont increase your output at all, that is done by the PCB which controls them. All inverters that I know of have protection against short circuiting.
Cheers

That is true in this case as well. Moreover there is
unlikely much design margin available to extract
significant additional heat out of this unit (or any cost
constrained design for that matter).

The first limitation likely to be hit would be saturation of
the output transformer, followed by current capacity of
the output rectifier bank, stored energy of the filter
capacitors, current carrying capacity of PCB traces,
internal wiring, current capacity of the input bridge rectifier,
etc..

If the end goal is a substantially higher current welder I'll
have to admit buying one is by far the easiest route. But
I really can't fault anyone experimenting with squeezing
additional performance/features out of a design.

Also it is arguably a viable commercial design worthy of
study, arrived at by someone other than yourself who
funded the series of smoked prototypes.

[junkweld]

True but note there is an analog switch (U2) which overrides the
user heat setting during arc start. But yea you can drive that with
a voltage source of lower impedance than the 10K pot. Haven't
experimented personally but I'd be careful to avoid induced noise
in the case of an external 10K pot. Shielded cable here is a start.

Thank you for posting the schematic. The schematic you created will save many of us wanting to change the design a lot of time. I do not see any form of current limiting in the design. I destroyed the 35A AC to DC rectifier and IGBT on my machine by sticking a 1/8" 7018 rod. I think the ARC start mode was applied and when the rod stuck the mode stayed on until the GPBC3510 rectifier fried. After it blew, the diodes shorted and AC voltage was applied to the IGBT and destroyed that too. I am replacing the IGBT with a slower 300 Amp IGBT that I got used for $35. I realize the turn off and turn on times of the 300 Amp are much slower than the original 100 Amp IGBT. I think at the 20KHz freq. that it will be OK.
After all that, the question is.........Why no current limiting in the design? It appears to me that the design requires a 20 amp breaker max. at the circuit panel. I run a 50 amp so the 35A input rectifier acted as the breaker. I don't see any protection in the design for current limiting when the output (stick or tig) torch is shorted.



What I'd been planning to do is remove the control module and
substitute a uC to control the works. Mechanically this would be
fairly simple as all of the control is situated on a separate card
connected to the base PCB with an 18 pin header. Electrically
the 3525 provides some useful features not easily emulated in
a uCs approach particularly in the ability to shutdown IGBT per
cycle drive in under 500ns.

I was thinking of doing the same thing eventually. The Atmel AVR devices at 16MHz could do a reasonable job in tems of shutting down the IGBT within 500ns when using Assembly instructions for that function.

Thanks again for the info. you posted on this machine. It is great to see some wanting to learn about and make it better.

[junkweld]

The reason why they increase the frequency of the transformer is so that they can reduce the physical size of the transformer. If you replaced the IGBTs with 300 amp types that would only be there handling capacity, they wont increase your output at all, that is done by the PCB which controls them. All inverters that I know of have protection against short circuiting.
Cheers

I agree about the higher freq. in order to use a smaller transformer. I replaced the IGBT module with a 300 amp unit only because it was used for $35 rather than about $100 for a new original unit. Yes, the freq. would need to be lowered to charge up the inductor more to created more welding current. Like uhmgawa said, the inductor is probably already at the limit of being saturated (charged) with the current design freq.

Something is up with this machine right now. I can stick weld 3/8" plate with it using a 1/8" rod. It seems to be putting out way too much current on the high side.
I still don't see any form of current limiting in the design. The ARC start looks to increase the current starting an arc. This would want to destroy the input AC-DC rectifier every time a stuck rod happens.

[uhmgawa]

I do not see any form of current limiting in the design.

Current limiting does exist in the design. However it
isn't a per-cycle function. Note the current transformer
between the IGBT and the power transformer. The
secondary of the current transformer is rectified by
D3-D6 and a voltage proportional to the IGBT current
is developed across 1R1+1R2. This is averaged and
fed to the inverting input of the error amp in U1.

However this scheme simply folds back the duty cycle
as current increases -- it also possesses a time lag.
The SG3525 does support per-cycle current limiting but
an instantaneous over-current sense must generate a
positive going pulse on the shutdown input (U1 pin #10)
in order to terminate a cycle.

This can be done with an additional comparator sensing
the existing current ramp from the current transformer
compared to a reference voltage. However the leading
edge of the current ramp is likely quite noisy and unless
suppressed will lead to false over-current triggering.
Leading Edge Blanking designs suppress the noise found
in the initial part of such a current ramp. But they weren't
part of the scene back in the mid 70's when the 3525 was
thunk up.

So I speculate this is why the less noise sensitive (but less
effective) duty cycle foldback was used besides it being
provided in the 3525. Note some manufacturer's IGBTs
used in the 91811 have internal current limit schemes.
So it is possible the IGBT which failed in your machine
either didn't have this feature or the current limit pot W1
was substantially out of calibration.


I destroyed the 35A AC to DC rectifier and IGBT on my machine by sticking a 1/8" 7018 rod. I think the ARC start mode was applied and when the rod stuck the mode stayed on until the GPBC3510 rectifier fried.

The arc start mode only is active for ~1sec after the electrode
is lifted from the work. That said it isn't maximum available
heat but about 66% of full duty cycle.


After it blew, the diodes shorted and AC voltage was applied to the IGBT and destroyed that too.

I'd use a 50A bridge given the fact there is no soft-start
relay between it and the filter capacitors. A series resistance
between the bridge and caps would drastically decrease
the turn-on stress seen by the bridge. Here the series
resistance would be shunted by a relay after a time delay
when the caps had reached 80-90% of full voltage. The
series resistance needs to be sized and dissipation rated
to withstand the momentary surge itself will see.


I am replacing the IGBT with a slower 300 Amp IGBT that I got used for $35. I realize the turn off and turn on times of the 300 Amp are much slower than the original 100 Amp IGBT. I think at the 20KHz freq. that it will be OK.

The IGBTs used in the 91811 have switch times around
500ns. So coming reasonably close should be sufficient.


I was thinking of doing the same thing eventually. The Atmel AVR devices at 16MHz could do a reasonable job in tems of shutting down the IGBT within 500ns when using Assembly instructions for that function.

That had crossed my mind as well back when I had been
looking to toss the 3525. However it is going to be a tad
intricate as while the AVR is running the PWM drive you'll
need to look for the per-cycle overcurrent input and
terminate drive. Also during PWM generation the code will
need to sample the user heat setting and adjust the PWM
cycle appropriately. Both phases of the drive need to be
symmetric to avoid imbalanced drive although the capacitively
coupled design will compensate. The 3525 also has a
stronger output drive than you'll typically find on a uC and
the device is operating at 12V needed to drive the push-pull
stage driving the IGBT base transformers.

A stronger case for a uC displacing the 3525 would be that
of variable frequency drive. But given the design frequency
is fixed at 20Khz that isn't a benefit here.

I'd like to see the 3525 displaced but 30+ years later it
does the simple job which it was designed for quite well
(sans needing some additional circuitry to effect per-cycle
current limit).

[uhmgawa]

...the inductor is probably already at the limit of being saturated (charged) with the current design freq.

The output inductor is another limiting factor. But my concern
was rather of saturating the output transformer driven by the
IGBT. As the core begins to saturate it acts less as a
transformer and more as a low impedance short.

[Jake_SS]

That is true in this case as well. Moreover there is
unlikely much design margin available to extract
significant additional heat out of this unit (or any cost
constrained design for that matter).

The first limitation likely to be hit would be saturation of
the output transformer, followed by current capacity of
the output rectifier bank, stored energy of the filter
capacitors, current carrying capacity of PCB traces,
internal wiring, current capacity of the input bridge rectifier,
etc..

If the end goal is a substantially higher current welder I'll
have to admit buying one is by far the easiest route. But
I really can't fault anyone experimenting with squeezing
additional performance/features out of a design.

Also it is arguably a viable commercial design worthy of
study, arrived at by someone other than yourself who
funded the series of smoked prototypes.

There are inverters that are rated at say 140 Amps but you can fit another PCB to increase the output to say 170 Amps without any damage to other components but they have to be the same brand, not all manufacturers machines are suitable for this procedure though.

[junkweld]

Current limiting does exist in the design. However it
isn't a per-cycle function. Note the current transformer
between the IGBT and the power transformer. The
secondary of the current transformer is rectified by
D3-D6 and a voltage proportional to the IGBT current
is developed across 1R1+1R2. This is averaged and
fed to the inverting input of the error amp in U1.

Thank for posting this stuff. It is exactly the kind of things I was looking to talk about for the 91811 box. I looked at the current sense transformer and see what you wrote about sensing the IGBT to transformer pulsed AC current. It is DC rectified, filtered, voltage divide to the error amplifier. I measured the pot setting on my PCB and it is set at 20K ohms just as the schematic shows. Since it seems like my machine stills does not current limit properly, I am going to check the other caps and resistors.



However this scheme simply folds back the duty cycle
as current increases -- it also possesses a time lag.
The SG3525 does support per-cycle current limiting but
an instantaneous over-current sense must generate a
positive going pulse on the shutdown input (U1 pin #10)
in order to terminate a cycle.

I agree. It looks to average the current limiting. The current limiting should have acted before blowing up parts in my case where I blew the rectifier and IGBT. The 1/8" rod was stuck for about 3 seconds before the parts went. The shutdown pin appears to be the only way to quick current limit as you stated.



This can be done with an additional comparator sensing
the existing current ramp from the current transformer
compared to a reference voltage. However the leading
edge of the current ramp is likely quite noisy and unless
suppressed will lead to false over-current triggering.
Leading Edge Blanking designs suppress the noise found
in the initial part of such a current ramp. But they weren't
part of the scene back in the mid 70's when the 3525 was
thunk up.

Using a Ucontroller, it would be easy to do a synchronous current sample and edge blanking to do the shutdown control.



So I speculate this is why the less noise sensitive (but less
effective) duty cycle foldback was used besides it being
provided in the 3525. Note some manufacturer's IGBTs
used in the 91811 have internal current limit schemes.
So it is possible the IGBT which failed in your machine
either didn't have this feature or the current limit pot W1
was substantially out of calibration.

My machine had a EUPEC BSM100GB60DLC IGBT in it. It does not have built in current limiting. The IGBT that I am replacing it with does not have current limiting protection either. My belief is the W1 pot is out of cal, but it is set at 20K ohms so probably is something else wrong. I also wonder about IGBT "latchup". That could cause the IGBT to lose the gate control and over-current due to shorting out both IGBTs at the same time. What are your thoughts on the latchup theory blowing the input 3510 rectifier?



I'd use a 50A bridge given the fact there is no soft-start
relay between it and the filter capacitors. A series resistance
between the bridge and caps would drastically decrease
the turn-on stress seen by the bridge. Here the series
resistance would be shunted by a relay after a time delay
when the caps had reached 80-90% of full voltage. The
series resistance needs to be sized and dissipation rated
to withstand the momentary surge itself will see.

I might try just putting a 20 Amp circuit breaker on the 91811 input 220v when I do the adjustment of the W1 pot to turn down the output. I could do the switched resistance later on if I go the Ucontroller route.



The IGBTs used in the 91811 have switch times around
500ns. So coming reasonably close should be sufficient.

The IGBT in my box has specs much faster than the IGBT part # in the schematic. I am using an IGBT that is more like the schematic part # switch times.



That had crossed my mind as well back when I had been
looking to toss the 3525. However it is going to be a tad
intricate as while the AVR is running the PWM drive you'll
need to look for the per-cycle overcurrent input and
terminate drive. Also during PWM generation the code will
need to sample the user heat setting and adjust the PWM
cycle appropriately. Both phases of the drive need to be
symmetric to avoid imbalanced drive although the capacitively
coupled design will compensate. The 3525 also has a
stronger output drive than you'll typically find on a uC and
the device is operating at 12V needed to drive the push-pull
stage driving the IGBT base transformers.

A stronger case for a uC displacing the 3525 would be that
of variable frequency drive. But given the design frequency
is fixed at 20Khz that isn't a benefit here.

I'd like to see the 3525 displaced but 30+ years later it
does the simple job which it was designed for quite well
(sans needing some additional circuitry to effect per-cycle
current limit).

Yes, the AVR would need some output drive support and input ckt condition, but I think it would all easily fit on the daughter card off of the PCB. Like you said, the transformer is designed for 20KHz so changing the freq. would not be useful. The control could be fine tuned but that is all.

[junkweld]

The output inductor is another limiting factor. But my concern
was rather of saturating the output transformer driven by the
IGBT. As the core begins to saturate it acts less as a
transformer and more as a low impedance short.

I was referring to the primary and secondary inductors of the transformer. If the freq. is decreased then they need to get bigger to keep them out of saturation. Like you said, once they are fully charged they will act like a short and provide no more energy (weld amps).

[junkweld]

There are inverters that are rated at say 140 Amps but you can fit another PCB to increase the output to say 170 Amps without any damage to other components but they have to be the same brand, not all manufacturers machines are suitable for this procedure though.

You would need to look at the transformer current charge slope to see if you could get more amps out of the machine. The new PCB would simply lower the freq. and increase the duty cycle or keep it constant. That will charge/discharge the transformer longer and create more output current if the transformer does not go into saturation.

[uhmgawa]

There are inverters that are rated at say 140 Amps but you can fit another PCB to increase the output to say 170 Amps without any damage to other components but they have to be the same brand, not all manufacturers machines are suitable for this procedure though.

If I understand your thought here, you are suggesting to parallel
output stages to increase available amperage. That is theoretically
possible in general but practical complications will arise. One
being eddy currents flowing between the multiple units due to
output voltage differences. Another concern is verifying the arc start
control isn't affected buy the ganged outputs.

Trying to balance energy output of the replicated PWMs is going to
be a headache unless all output stages are either driven from one
controller or the controllers are synchronized -- the 3525 controller
does provide for the latter.

Not impossible but it isn't clear this would be the easiest route to
achieving increased output amperage.

[uhmgawa]

You would need to look at the transformer current charge slope to see if you could get more amps out of the machine. The new PCB would simply lower the freq. and increase the duty cycle or keep it constant. That will charge/discharge the transformer longer and create more output current if the transformer does not go into saturation.

Lowering the PWM frequency isn't going to increase maximum
energy throughput of the transformer. What it will do is increase
the ramp time of the transformer and drive it closer into saturation.
Subject to other circuitry constraints a higher switching
frequency will allow a given transformer to pass a greater
amount of energy due to reduced flux swing. But for the
transformer core material alone this is a trade-off as higher
switching frequencies correspond to increased core loss.
Add to this losses in the drive and rectification circuitry
which increase with frequency and create their own upper
limit in the design.

Realistically a $199 TIG welder which is designed to be
profitably manufactured/sold isn't going to have much (if
any) design margin to eek out significant additional heat.
Aside from paralleling units which has its own challenges,
attempting to scale the output of a single unit will require
upgrade of: output transformer, output rectifiers, IGBT,
half bridge AC coupling capacitors, bulk DC storage
capacitors, input rectifier bridge, output inductor, etc..
The current capacity of the internal wiring and PCB traces
as well will become a limitation.

Irrespective of whatever prospective scheme might be used
to increase available heat you'll also need to reckon with the
limitation of the TIG torch which appears to be a Weldcraft
WP-9V (or knock-off) with a design capacity of 125A.
So we're already at a slight deficit there on a 130A machine.

In short the 91811 is an interesting, minimalist design worthy
of study as arguably it has converged by the usual trial and
error and smoked prototypes funded by someone else. But
I don't feel pursuit of increased output amperage from the
design is likely to be productive.

[junkweld]

I agree. After looking at the schematic and components I realize it isn't worth trying to make this machine weld aluminum. It would be cheaper to start from scratch or just get a low end machine. I will stick with using my Lincoln 175 TIG for doing aluminum.

The schematic is interesting in that it uses the basic switching supply control devices. I still don't like the method it uses for current sampling and limiting. I also measured the freq. my machine switches the IGBT at. It was at 16KHz.

[walker]

Interesting how a brand new poster appeared just for you to talk to. His writing style is even similar to yours. Hmmmmmmmmmmmm.:eek:

[junkweld]

We are two different people that both own the Harbor Freight TIG unit be the sounds of it.

[junkweld]

Current limiting does exist in the design. However it
isn't a per-cycle function. Note the current transformer
between the IGBT and the power transformer. The
secondary of the current transformer is rectified by
D3-D6 and a voltage proportional to the IGBT current
is developed across 1R1+1R2. This is averaged and
fed to the inverting input of the error amp in U1.

However this scheme simply folds back the duty cycle
as current increases -- it also possesses a time lag.
The SG3525 does support per-cycle current limiting but
an instantaneous over-current sense must generate a
positive going pulse on the shutdown input (U1 pin #10)
in order to terminate a cycle.


I looked over the schematic for a while. I see that the 50:1 current transformer creates an AC voltage that is then full wave diode rectified with the two 5.1 ohm resistors. Then sent to a diode and filter. Then the W1 pot voltage divider. Then to the Error Amplifier which is inverting with a gain of 1 using 100K resistors. The error amplifier is used to reduce the IGBT Pulsewith when the voltage is too high as compared to what the front of the machine pot/knob is set to.

The problem I see with this is that it is average current like you said to control the IGBT gate. When a short at the torch is created, there is no pulse by pulse current limiting. Most IGBTs need the current limited within 10usec. The averaging ckt doesn't do that. Also, there is no IGBT gate clamping when the output is shorted and the Miller capacitor holds the IGBT on until it destroys itself (latch-up)



This can be done with an additional comparator sensing
the existing current ramp from the current transformer
compared to a reference voltage. However the leading
edge of the current ramp is likely quite noisy and unless
suppressed will lead to false over-current triggering.
Leading Edge Blanking designs suppress the noise found
in the initial part of such a current ramp. But they weren't
part of the scene back in the mid 70's when the 3525 was
thunk up.

So I speculate this is why the less noise sensitive (but less
effective) duty cycle foldback was used besides it being
provided in the 3525. Note some manufacturer's IGBTs
used in the 91811 have internal current limit schemes.
So it is possible the IGBT which failed in your machine
either didn't have this feature or the current limit pot W1
was substantially out of calibration.

Yes, controlling the shutdown will do pulse by pulse current limiting, but it doesn't help if the IGBT is in latch up due to the Miller Capacitor. It needs a limiter on the IGBT gate that senses the voltage across the Source and Drain.



The arc start mode only is active for ~1sec after the electrode
is lifted from the work. That said it isn't maximum available
heat but about 66% of full duty cycle.


The arc start is one for 1 sec. I thought the heat was set to 50% (15k voltage divider)

[uhmgawa]

I looked over the schematic for a while. I see that the 50:1 current transformer creates an AC voltage that is then full wave diode rectified with the two 5.1 ohm resistors. Then sent to a diode and filter. Then the W1 pot voltage divider. Then to the Error Amplifier which is inverting with a gain of 1 using 100K resistors. The error amplifier is used to reduce the IGBT Pulsewith when the voltage is too high as compared to what the front of the machine pot/knob is set to.


The current transformer will sense the current ramp on its primary
and provide a voltage proportional to the primary current to U1.1.
So we limit the drive pulse width by how large we allow the current
ramp to rise.



The problem I see with this is that it is average current like you said to control the IGBT gate. When a short at the torch is created, there is no pulse by pulse current limiting. Most IGBTs need the current limited within 10usec. The averaging ckt doesn't do that. Also, there is no IGBT gate clamping when the output is shorted and the Miller capacitor holds the IGBT on until it destroys itself (latch-up)

Both scenarios can be addressed via terminating each per-cycle pulse
via the shutdown pin. Doing so will terminate active drive and essentially
short the primary of the drive transformer which will dissipate the
accumulated charge contained in the drive circuitry and IGBT base
parasitic capacitance.

It almost seems like per-cycle current control may have existed
in some design iteration as portions of the mechanism are in
place. Sampling of the current ramp off of 1R4 is conveniently
isolated from 1C7 averaging by D7. I'd hazard you'll still need a
few uS leading edge blanking to prevent false limit triggering by the
spike during ramp start. But otherwise it is a simple matter of
terminating the cycle via shutdown on U1.10.

[uhmgawa]

Interesting how a brand new poster appeared just for you to talk to. His writing style is even similar to yours. Hmmmmmmmmmmmm.:eek:

Odd sometimes how even common knowledge can be slow
to circulate. So just for the record, everyone in this forum
besides yourself is actually a single individual with excessive
time on their hands. ;)

[SundownIII]

I really like uhmgawa's assessment of the 130A machine in question.

Bottom Line:

You can take a turd, dip it in milk chocolate, put sprinkles on top and when all is said and done, YOU STILL GOT A TURD.

Personally, I'd rather see a discussion about welding/fabrication than I would a discussion about building a Heathkit welder. Of course, the OP already knows all he needs to know about welding. He told us so in a previous posting.

[junkweld]

The current transformer will sense the current ramp on its primary
and provide a voltage proportional to the primary current to U1.1.
So we limit the drive pulse width by how large we allow the current
ramp to rise.


Yes that is what it does. On average it will track the pot/knob setting and maintain the proper welding current. The problem is when a direct short occurs at the input. I don't see how it can react fast enough to shutdown the IGBT without destroying them.



Both scenarios can be addressed via terminating each per-cycle pulse
via the shutdown pin. Doing so will terminate active drive and essentially
short the primary of the drive transformer which will dissipate the
accumulated charge contained in the drive circuitry and IGBT base
parasitic capacitance.

It almost seems like per-cycle current control may have existed
in some design iteration as portions of the mechanism are in
place. Sampling of the current ramp off of 1R4 is conveniently
isolated from 1C7 averaging by D7. I'd hazard you'll still need a
few uS leading edge blanking to prevent false limit triggering by the
spike during ramp start. But otherwise it is a simple matter of
terminating the cycle via shutdown on U1.10.


I have been looking at adding another inverter and D flip flop with a small RC time constant. If the sense voltage is zero (use the output of the LM393 comparator for electrode at 0V) for an RC time constant then shutdown by activating a diode that is wired ORed into the shutdown pin (with the other 2 ire ORed diodes). It requires another hex inverter package and DFF package.

I have also been thinking of chucking the AC current sampling with the 50:1 toroid and just installing a $10 current shunt between the output diodes and transformer. Put a comparitor on that to trigger shutdown. I think that is a much better option. I am not sure which ckt I will end up with at this time. It is a fun project to play with though.

[junkweld]

I really like uhmgawa's assessment of the 130A machine in question.

Bottom Line:

You can take a turd, dip it in milk chocolate, put sprinkles on top and when all is said and done, YOU STILL GOT A TURD.

Personally, I'd rather see a discussion about welding/fabrication than I would a discussion about building a Heathkit welder. Of course, the OP already knows all he needs to know about welding. He told us so in a previous posting.

There are plenty of discussions like the one you would like to see on this forum. Why not go and read them instead of wasting your time posting to this one with no relevant information.

[Roger]

So this welder is Alpha version sold to suckers wanting assembled poorly tested product to evaluate trouble shoot and repair from first time they try to use it as advertised. This is a new concept to me growing up in USA with every product having at least implied warranty of working as advertised for a few months to years not couple minutes or days.

This is not software.

[junkweld]

So this welder is Alpha version sold to suckers wanting assembled poorly tested product to evaluate trouble shoot and repair from first time they try to use it as advertised. This is a new concept to me growing up in USA with every product having at least implied warranty of working as advertised for a few months to years not couple minutes or days.

This is not software.

I prefer to keep this topic about technical stuff, but you are claiming that this concept is new to you. If so, then you must be typing your posts on a Linux machine and not a microsoft or apple machine. They have always released alpha code and have users (you call them suckers) like you testing it for them....but you are OK with that because it is USA made.

It really doesn't matter if it is software or not. Heck, they could have everything controlled by a Ucontroller or FPGA.......and then it would be software, but really what does that matter.

The 91811 design is a well known power ckt design that has been around forever. You see it in all kinds of power inverters. This one just happens to be a welding machine. Every design can be ripped apart and improved on. We just happen to be picking on the 91811 here. This could also be done for the USA machines that you claim are never alpha.

I don't see why people get so offended when talking about welding boxes like this one. I have welded with Miller 210 MIGs, Hobart 210 MIGs, AIRCO 300amp stick, Lincoln 175 TIG, Clarke 130 MIG, Chicago Electric 85 flux core, Lincoln AC225, Mitech AD/DC TIG inverter, HF 130A DC TIG, Miller 130 MIG, Solar 120 MIG, weldernator using GM 100amp alternator and 6HP Tecumseh engine, and probably a few others I forgot about. But really, who cares. They all welded decent for what they were intended for.

I am not claiming the 91811 is the best DC TIG machine in the 130 amp range. It suites a certain class of users. That is, it is for the hobby builder. A professional shop would be more geared to a Hobart, Miller, Lincoln, ESAB machine that would take more day to day abuse.

If you are going to claim the USA machines are so superior then provide some proof by showing the design is clearly better. Just saying that the 91811 is an alpha box is not correct in that maybe I have severely abused my machine in a manner that a USA machine in the same class would have failed also.

[Ronnie]

Get on with tig welding!!!!

You can't make chicken soup from chicken chit

Ronnie

[Roger]

I prefer to keep this topic about technical stuff, but you are claiming that this concept is new to you. If so, then you must be typing your posts on a Linux machine and not a microsoft or apple machine. They have always released alpha code and have users (you call them suckers) like you testing it for them....but you are OK with that because it is USA made.

specify
I am using Linux on this computer. Well aware buggy software is sold by microsoft and others.
Some bugs are fixed by no cost up dates some not. Linux also has bugs.

I am using a computer made and assembled in China. Is hard to find one that doesn't use made in China parts. They can make great quality products. When they are competing to make lowest priced product they do. Quality suffers if it isn't part of specification including testing in China and by distributer. From what I see Harbor Freight doesn't specify high quality but others are even worse. Japan once exported very low quality cheap products. Trade associations and regulations solved the problem. Japan now competes with quality and price, but uses their quality control with low China labor cost and environmental costs to have low price.

junkweld knows they have higher failure rate as shown by this quote and other posts.
Quote Post 02-21-2009, 12:31 PM by junkweld
For those that dislike the far east machines......yeah I know.......I should throw it away. I like messing with these things. The USA units hardly ever break so they are no fun to play with.

Must be a reason Miller and Lincoln welders are sold in China.

I also know every machine breaks and some models in each manufacturer's product line are more reliable than others.

[uhmgawa]

So this welder is Alpha version sold to suckers wanting assembled poorly tested product to evaluate trouble shoot and repair from first time they try to use it as advertised.

It appears one of the primary motivations has been somehow
lost in translation. The reason the welder under discussion
invites reverse engineering is due to the fact it is an open
design with essentially no locked up intellectual property.
It also allows those suitably skilled in the art to effect repair
of the machine without being limited to repair parts from the
machine's vendor at whatever premium they choose to charge.

I doubt Kende Welding Equipment (manufacturer of the
unit) is intentionally doing the world a favor by not locking
up the welder's control logic in an opaque microcontroller,
but as it stands the design is attractive to those so inclined
for this reason.


This is not software.

Well yes, precisely. ;)

[junkweld]

uhmgawa,
I just picked up another 91811 machine that is missing all of the attachments. I am looking for the gas inlet fitting and outlet fittings. I checked the Western Enterprises catalog but nothing seems to match up.
The 91811 looks like the outlet fitting is a 1/2" unknown TPI or metric female fitting. No oxy RH gas fitting matches up to this (A size is 3/8" and B size is 9/16).
The 91811 inlet looks like a 1/2" male unknown TPI which no inert gas fitting will match up (A size is 3/8" and B size is 5/8")

Any idea where I could get these fittings? Harbor Freight will be my last choice if I can't find them anywhere else. I might also find USA thread bulkheads if they are exotic metric fittings that are hard to find. I can always use the gas valve out of it and put the AC H-bridge on the output of my Lincoln buzzbox later on after testing with the 91811 unit.

Also, I picked up an IGBT H-bridge that is 200amp continuous capable. Someday I might get around to making the 91811 unit weld aluminum. Have you ever tried welding alum. using a squarewave inverter without High Freq/voltage start?

Thanks,
Junk

[uhmgawa]

uhmgawa,
I just picked up another 91811 machine that is missing all of the attachments. I am looking for the gas inlet fitting and outlet fittings. I checked the Western Enterprises catalog but nothing seems to match up.
The 91811 looks like the outlet fitting is a 1/2" unknown TPI or metric female fitting. No oxy RH gas fitting matches up to this (A size is 3/8" and B size is 9/16).
The 91811 inlet looks like a 1/2" male unknown TPI which no inert gas fitting will match up (A size is 3/8" and B size is 5/8")

Any idea where I could get these fittings?

No idea as they appear to be an oddball thread, neither
an SAE nor metric standard size. Might have just found
Kende's only attempt at securing IP. ;)

The easiest thing to do is pick up standard size brass
fittings and convert the inlet/outlet of the machine. The
internal polyurethane tubing appears to be ~3/16" ID.


Harbor Freight will be my last choice if I can't
find them anywhere else.

I predict trying to get matching fittings from HF to be more
difficult than attempting to change a flat tire on a vehicle
sailing down the highway.


I can always use the gas valve out of it and put the AC H-bridge on the output of my Lincoln buzzbox later on after testing with the 91811 unit.

No idea what you paid for the unit but trying to scrounge
an equivalent set of replacement parts will easily exceed
half the cost of the machine, likely more.


Also, I picked up an IGBT H-bridge that is 200amp continuous capable. Someday I might get around to making the 91811 unit weld aluminum.

That's the only realistic way to get usable welding AC out of
the machine -- generate it from the DC output. Incidentally
this how commercial variable frequency AC output TIG machines
do so. That lonely 91811 you picked up is a lightweight
inverter DC power source. All you need is the H bridge to
generate AC output and a some thought into what you'd like
for control/timing features.


Have you ever tried welding alum. using a squarewave inverter without High Freq/voltage start?

No, personally I have not. But lift-start should be trivial to
prototype quickly. Pull up the torch output similar to the
ELECTRODE SENSE input in the 91811 and detect the
ground make-to-break transition. The work/clamp output
would be driven to ground by turning on the lower IGBT in
that leg of the bridge during this startup sense operation.

In this mode of course the 91811 inverter is just run wide
open at maximum duty cycle with the H bridge timing providing
control of the actual heat output.

[paradox]

I've just installed a pedal on my Lil' Harbor Freight 130 Amp tig.
It sure is nice to be able to modulate the power now.
Here are the instructions I followed in a gun forum,if anyone is interested,and has not seen it.

http://www.akfiles.com/forums/showthread.php?p=310671

Question for one of the electronic guy's posting in this thread:
I would like to install a Microswitch in the pedal to shut down power to
the torch at the end of each bead.

I saved the print uhmgawa provided. Could one off you guys recommend where in the circuit,would be best to tie in?

I do have a sweet Hobart 187 handler mig,that I use for most of my projects...

This cheapy tig is just for fun,playin' out in the shop,as junkweld has said. :-)

[Roger]

Great project.
Foot pedal is almost as big as the welder.

[uhmgawa]

I've just installed a pedal on my Lil' Harbor Freight 130 Amp tig.
It sure is nice to be able to modulate the power now.
Here are the instructions I followed in a gun forum,if anyone is interested,and has not seen it.

http://www.akfiles.com/forums/showthread.php?p=310671

A concern I have about moving the pot external is the
potential for noise feeding back into the PWM control
during welding. This is possible via induction of welding
output into the cable between the pedal and the machine.
Particularly as shown above, the unshielded cable
being used is quite susceptible to this. I'd at least
recommend using shielded cable, ideally over all three
pot conductors with the shield connected to the control
circuit's ground. A better way would be use a digital
pot in the unit and control it externally however the
interface does become more complex.


Question for one of the electronic guy's posting in this thread:
I would like to install a Microswitch in the pedal to shut down power to
the torch at the end of each bead.

I saved the print uhmgawa provided. Could one off you guys recommend where in the circuit,would be best to tie in?

The simplest way would be to add a N.C. switch
in series with the thermal cutout. One of the
terminals of the TC is at control ground (J1.5)
and the other (J1.6) prevents R19 from resetting
U4 with a positive level. Even better would be
to add a diode along side D3, D4 to create its
own shutdown input which a N.O. external switch
would pulse to V+. Adding a 1K resistor to ground
on the anode of the diode is preferable to lower
the impedance of the external line and reduce
sensitivity to noise.

Attached is an updated schematic which addresses
a few minor errors in previous versions.

[paradox]

uhmgawa
Thank you for the response,and your work developing the schematic.
You,and junkweld have done the best job of explaining the inner workings
of the "little red toaster" that I've found written on the net so far.
I'll bet there are a bunch of tinkers "googling" this unit,and landing here just as I did.

It's nice that the flamers have backed off now.

While I do not completely comprehend all that you fellers are talking about,
I do find it fascinating.

Per your recommendation on the pedal wiring,I will tie in the existing 3-con shielded cable that came installed with my Casio pedal to the control circuit ground.I trust that an unshielded extension about 10" long inside the cabinet at the front panel to reach pin J1-5 at the themal overload,(an easily accessible chassis ground point) will be suitable...


I need to go through my junk boxes to find a suitable switch to shut down the torch power at the extreme bottom of the pedal travel.Then,try the simple method of tying into the thermal cutout as suggested. I'll need to come up with some flexible shielded cable with a minimum 5 conductors before trying that mod though.

Thanks again.

[junkweld]

No idea as they appear to be an oddball thread, neither
an SAE nor metric standard size.
Yes, they are odd ball threads. I gave up on locating them and decided to change to a different bulkhead.



No idea what you paid for the unit but trying to scrounge
an equivalent set of replacement parts will easily exceed
half the cost of the machine, likely more.
Yes, it is easy to get carried away and end up spending more when trying to save by getting a junk unit that seems like a "good deal".
I have the following for parts:
One 197x Lincoln AC-225 buzz box (free)
One 200x Harbor freight 91811 box ($50)
One WP-9 torch with Lanthanated 1/16" ($40)
One IGBT H-bridge with large heatsink ($100)
A couple of 132 amp SCRs and diodes ($30)

The thought is to use the SCRs and diodes for the AC-225 unit to do adjustable
current and DC output. The H-bridge would be installed on the back of the AC-225 later on.



That's the only realistic way to get usable welding AC out of
the machine -- generate it from the DC output. Incidentally
this how commercial variable frequency AC output TIG machines
do so. That lonely 91811 you picked up is a lightweight
inverter DC power source. All you need is the H bridge to
generate AC output and a some thought into what you'd like
for control/timing features.
I agree. putting an H-bridge on the DC output and changing the "sense" input is the only way to go with the 91811 unit. I could always put the second TIG 91811 in parallel with the one I have. That is, create a 2*130amp output tying the DC outputs together and tweaking the control board. Feed that into the H-bridge. Too many ideas and not enough time for all these projects.

[junkweld]

The simplest way would be to add a N.C. switch
in series with the thermal cutout. One of the
terminals of the TC is at control ground (J1.5)
and the other (J1.6) prevents R19 from resetting
U4 with a positive level. Even better would be
to add a diode along side D3, D4 to create its
own shutdown input which a N.O. external switch
would pulse to V+. Adding a 1K resistor to ground
on the anode of the diode is preferable to lower
the impedance of the external line and reduce
sensitivity to noise.

Attached is an updated schematic which addresses
a few minor errors in previous versions.
I also think wiring in another diode to the wired ORed shutdown input is the way to go. When adding a foot pedal the best option to me always seemed like removing R15 (4.7K ohm) and installing the foot pedal with a 4.7K ohm in its place. This allows the front panel knob to set the max. current still. Basically, it would operate just like my Lincoln TIG 175 squarewave machine. You set the max. current on the machine and the pedal adjusts between zero output up to the level set on the panel of the machine.

You don't really remove R15, you just disconnect the "low" side wire from the panel 10K pot and wire it to a shielded pair to the 4.7K pot in the pedal. The return from the 4.7K pedal pot is tied to ground on the PCB (pin J1-5).

[paradox]

When adding a foot pedal the best option to me always seemed like removing R15 (4.7K ohm) and installing the foot pedal with a 4.7K ohm in its place. This allows the front panel knob to set the max. current still. Basically, it would operate just like my Lincoln TIG 175 squarewave machine. You set the max. current on the machine and the pedal adjusts between zero output up to the level set on the panel of the machine.

You don't really remove R15, you just disconnect the "low" side wire from the panel 10K pot and wire it to a shielded pair to the 4.7K pot in the pedal. The return from the 4.7K pedal pot is tied to ground on the PCB (pin J1-5).





That sounds like a fine idea.

Could an in stock,5k 1/2W pot in the following link be substituted without too much change in the concept?
There is a Radio Shack down the street,and up the road... :-)

http://www.radioshack.com/product/index.jsp?productId=2102789

[paradox]

Tonight,I decided to try wiring the foot pedal to only vary the power "up to" the limit set on the panel control pot.

First thing I did was to remount the original 10k pot into the panel.(I had left a little "pig-tail" with the ribbon colors still on it,when I removed the pot from the machine the other day.)

Reinstalled J2-5 (wht) to "heat-high" panel pot terminal.
Reinstalled J2-6 (gry) to "heat-select"panel pot terminal.
Left the third pot "low-heat" term open.

J2-7 "heat-low" to foot pedal pot wiper,and the return (resistance increasing as pedal is backed off) to J1-5. (chassis ground)
The shield of the foot pedal cable was also tied into the chassis ground at J1-5.
I just left the 10k linear pot in the pedal for now(5k at mid stroke) just to see what the modulation would be like.

This has resulted in no control over the low end of the welding power.
No mater the positions of either pot,I'm getting "way" too much current
into a piece of ~.080 mild steel tubing scrap I've been testing with.

I picked up a 5k pot at Radio Shack but have not opened the pkg yet,as I'm not sure if that's a suitable part.

Any Ideas?

I can put this thing back to regular "full amp" foot control in just a little while,but just thought it would be neat to have the foot pedal function as just a "fine control". This is for fun anywho... :D

[paradox]

Today,I took a few minutes to put the wiring back to stock just to check the low end current,as it was far too high after the mod in the last post. Now it's back to normal.(It has always run a little hotter than I'd like from the get go.)

Just had to unsolder J2-7 purple lead,(heat-low)that had been attached to the cable going to the pedal,and solder it back onto the 10k panel pot.

Could one of the electronic guys take a look at the PDF print that uhmgawa drew up, listed in post #67,and tell me what I did wrong in the last post?

Basically,I would like the pedal to control "up to" the max setting on the panel pot.
If the "low end" could be made a bit lower at the same time that would be sweet. :-)

[Zrexxer]

Interesting how a brand new poster appeared just for you to talk to. His writing style is even similar to yours. Hmmmmmmmmmmmm.:eek:I think we're up to three personalities now :rolleyes:

[usmcpop]

Correct me if I'm wrong, but when these little inverters first showed up at Harbor Freight, weren't they in a white case, came in a wooden crate and were labeled "Made in Russia"? Maybe it was a different unit I'm thinking of?

[junkweld]

Today,I took a few minutes to put the wiring back to stock just to check the low end current,as it was far too high after the mod in the last post. Now it's back to normal.(It has always run a little hotter than I'd like from the get go.)

Just had to unsolder J2-7 purple lead,(heat-low)that had been attached to the cable going to the pedal,and solder it back onto the 10k panel pot.

Could one of the electronic guys take a look at the PDF print that uhmgawa drew up, listed in post #67,and tell me what I did wrong in the last post?

Basically,I would like the pedal to control "up to" the max setting on the panel pot.
If the "low end" could be made a bit lower at the same time that would be sweet. :-)

Using a 10K pot in the pedal instead of the required 4.7K pot will make the SG3525 control/error_compare voltage higher and make the weld output current higher. You want a 4.7K pot in the pedal.
You want the 4.7K ohm pot connected so that when the pedal is NOT pressed, the ohm across the pedal pot reads close to 0 ohms (min. weld output). When the pedal is pressed, then the pedal pot leads should read around 4.7K ohms (so it looks to the TIG ckt like a normal 4.7K ohm resistor is there and up to the panel pot setting amps with flow at the weld output). It sounds like you have it wired backwards in terms of the pot resistance vs. pedal position.

Do not use a 10K pedal pot when bypassing the 4.7K ohm resistor on the PCB. It will make the weld output too high. You want a 4.7K ohm or slightly less pot. The 5K pot you have should be OK, but the weld output will be slightly higher than normal with the pedal fully pressed (full weld output).

You could also connect the pedal with the PCB 4.7K ohm resistor in parallel with foot pedal 100K ohm pot inside of it. This is probably the best setup, but VERY GOOD shielding is required. With this setup if the pedal 100K pot goes bad (open) or the pedal is not connected then front panel 10K pot will operate as normal. That is, hook the 100K pedal pot to the "low" side of the panel pot. The other lead of the 100K pot is connected to the GND of thermal switch (on J2 ?? connector).
This should allow the pedal to control the output from about zero up to the panel pot setting when the pedal is connected. With the pedal disconnected, it should work as it did from the factory (front panel pot sets the output constant). *** Noise coupling in could be a problem doing it this way with the 100K pedal pot ***

[junkweld]

Correct me if I'm wrong, but when these little inverters first showed up at Harbor Freight, weren't they in a white case, came in a wooden crate and were labeled "Made in Russia"? Maybe it was a different unit I'm thinking of?

Not sure. This unit has "KENDE" on the PCB. It was manufactured by Kende in China.
The little Harbor Freight 85 amp output, 120 vac input stick welder I just got for $40 at my door is white and also made in China.

[paradox]

Thanks for clearing that up junkweld.

I think I'll take the unused 5k back to the 'Shack and swap it out for a 100k pedal pot,then wire it in parallel with the PCB 4.7K ohm resistor as you suggest,in order to get finer control over the low end.


Regarding the noise coupling with the 100K pot for the pedal.
I've tied the nicely braided shielded cable to chassis ground.The pedal is hardwired/non-removable.Is that likely sufficient,or is there something else I might be able to find in the Radio Shack junk draws while I'm over there?

[junkweld]

Thanks for clearing that up junkweld.

I think I'll take the unused 5k back to the 'Shack and swap it out for a 100k pedal pot,then wire it in parallel with the PCB 4.7K ohm resistor as you suggest,in order to get finer control over the low end.


Regarding the noise coupling with the 100K pot for the pedal.
I've tied the nicely braided shielded cable to chassis ground.The pedal is hardwired/non-removable.Is that likely sufficient,or is there something else I might be able to find in the Radio Shack junk draws while I'm over there?

With the 100K ohm in parallel you only get about 1/4 movement of the pedal for control. The 100K does a good job of making the original 4.7K ohm do its job. The 100K ohm only starts to change the resistance the ckt board sees when it gets in the 25K ohm range. That is why is starts to have an affect at 1/4 of the pedal which is 1/4 of 100K. Ideally, you want want to change all the PCB resistors and make it have 100% fine control with the pedal.
Using a 50K ohm will give more usable pedal movement (about 1/2), but will lower the overall output weld current a little. It all depends if you want to sacrifice output amps a little for more foot pedal control. The 100K ohm will give the original output amps, but has less pedal control.

[paradox]

The "little red toaster" is behaving itself on the low end,like never before on the mild steel bicycle tubing I'm playing with.

Following junkwelds instructions,with the panel potentiometer at about 30%,this thing will amp on down till the flame go's out. (1/16" 2% thoriated)

The used Casio VP-2 pedal with shielded cable from the 'Bay ran me about $12.00 shipped. (Ya have to watch)

The 100K ohm,1/2 Watt pot was $2.99 stocked at Radio Shack Part#271-092

This being a Hobart forum,it's understandable that some of the senior membership,are not especially open to the servicing,and modding of this tiny,no frills,el'cheapo,Asian toy.
Far Eastern motorcycles excepted,of course.

Looking at the traffic Junkweld's thread has generated for this site,it does seems some folks,have a least a passing interest for whatever reason.

Mucho thanks to Him,and Uhmgawa.

[junkweld]

Good to hear you have it working. Yes, the 100K ohm pot is stocked at Radio Shack. The 50K ohm pot is not. The nice part with pedal connected this way is you can weld as normal with the foot pedal installed (and not used) when doing odd positions. You could also add a $10 two-prong connector (get male and female) that tmvtaylor on ebay sells for the china plasma/tig units. Could also go with any two-prong bulkhead plug that is laying around.

You did better than me on the foot pedal deal. I spent about $25 on mine.

I wonder if some will say a fourth personality has emerged.

[uhmgawa]

When adding a foot pedal the best option to me always seemed like removing R15 (4.7K ohm) and installing the foot pedal with a 4.7K ohm in its place. This allows the front panel knob to set the max. current still. Basically, it would operate just like my Lincoln TIG 175 squarewave machine. You set the max. current on the machine and the pedal adjusts between zero output up to the level set on the panel of the machine.

The existing voltage divider gives:

Vref + 10K + 10K(pot) + 4.7K + GND

which allows the pot wiper to traverse from 59.5% to 19%
of Vref. If we conditionally vary the 4.7K lower resistor, the
divider will be altered such that we'll be lower than 19% of
Vref at the lower range of the 10K pot. For example if the
pot is set at 10% range 9K + 1K, removing the 4.7K from
the divider will give us 5% of Vref on the pot wiper which
is out of range of the current 19% lower limit. So we can't
use the full range of the pedal and encounter over-sensitivity.
We'll also have a dead zone at the start of pedal travel.

There is enough unused circuitry on the board which can
provide a conditionally selectable external heat input
unencumbered by the existing voltage divider. Attached is
a modification to the circuit which details this, changes
noted in red. As shown the welder will operate normally
until /EXT_SEL is pulled to GND, at which point the voltage
on EXT_HEAT will determine the pulse width.

Below the modification is a circuit (drawn in blue) which
will allow us to set upper and lower heat boundaries, and
establishes a voltage range through which the full action
of the pedal will sweep. We can also trade off pedal
sensitivity by narrowing the range between upper and
lower limits.

In this case the random pot value found in a scrounged
foot pedal is, within reason, not critical. Values between
1K and 100K linear taper should function equally well
except for lower impedances being more resistant to
induced noise. Here again the use of shielded cable for
the remote pedal potentiometer is strongly recommended.

Note in the case the pedal UP (at rest) threshold is set to
a higher heat than the DOWN (fully depressed) threshold,
the pedal action will be reversed and heat will decrease as
the pedal is actuated.

I think that's about as far as I'd personally go with this as
any more aggressive of a modification just screams for a
$3 uC implementation. To that end, additional signals and
modifications needed for separate control are called out in
green. ELECTRODE_CONTACT, OCV_DETECT,
OVERHEAT_DETECT, and CURRENT_SENSE are inputs.
PWM_SHUTDOWN and /GAS_SOLENOID are outputs.
That's it.

Still I wouldn't offload the SG3525's functionality to a uC
unless for some reason the entire control board was to
be replaced. Given the design constraints imposed by
the existence of an output transformer, namely balanced
phase-phase duty cycle and a 20KHz switching frequency,
the 3525 trivially tends to that task while dealing with the
IGBT drive.

All which is needed to interface to the 3525 is either an
inexpensive serial DAC or digital pot. Even here I
wouldn't futz with hard enforcement of the existing 40.5%
[19..59.5]% Vref range. An 8 bit DAC (or 256 tap digital
pot) swinging from Vref to GND will still provide in excess
of 100 discrete heat settings in that range. The remaining
59.5% [19 <, > 59.5]% unusable range can simply be
disallowed by the uC.

The previously discussed PWM shutoff input to U4 is
tossed in there as well. And while it resets the ON state
maintained by the latch and ultimately the shutdown
input of U1, discharging C5 is part of the process. Thus
this isn't able to provide the speed required to effect per
cycle current control. Fortunately U1.10 is able to be
directly driven externally due to R5 being in series with
the output of U4.2. Apparently U1.10 clamps the line too
low for the other driven inputs to function correctly.

[NNY528i]

........
There is enough unused circuitry on the board which can
provide a conditionally selectable external heat input
unencumbered by the existing voltage divider. Attached is
a modification to the circuit which details this, changes
noted in red. As shown the welder will operate normally
until /EXT_SEL is pulled to GND, at which point the voltage
on EXT_HEAT will determine the pulse width.

Below the modification is a circuit (drawn in blue) which
will allow us to set upper and lower heat boundaries, and
establishes a voltage range through which the full action
of the pedal will sweep. We can also trade off pedal
sensitivity by narrowing the range between upper and
lower limits.

In this case the random pot value found in a scrounged
foot pedal is, within reason, not critical. Values between
1K and 100K linear taper should function equally well
except for lower impedances being more resistant to
induced noise. Here again the use of shielded cable for
the remote pedal potentiometer is strongly recommended.




Hi there,

This is a fascinating discussion you guys really know your stuff. I have this welder and have been contemplating adding a pedal for a while. Today i hit the gold mine at work when we scrapped some stuff on a job and I found a Miller RFCS 14(stock #043554) Tig foot control pedal(much better than the old singer sewing machine pedal i was trying to modify)

Will this pedal work in the application you describe above? or is the 1k resistor too small? Also could the switch set in the pedal be connected to that themal shutdown you were discussing to act as an arc shutoff signal?

Thanks for any help you can offer.

Again also great stuff here.

Jason

[uhmgawa]

found a Miller RFCS 14(stock #043554) Tig foot control pedal(much better than the old singer sewing machine pedal i was trying to modify)

Will this pedal work in the application you describe above? or is the 1k resistor too small? Also could the switch set in the pedal be connected to that themal shutdown you were discussing to act as an arc shutoff signal?

I'd hazard a qualified yes to both questions. The 1K impedance
is just about ideal for the application as a remote heat selector
since it should be relatively immune to induced noise. The foot
control switch would need to be wired in series with the welder's
(normally closed) thermal switch and would require N.C. contacts
which would open when actuated. Some thought is needed as well
of how to switch the foot control shutoff in/out of the welder control.
The greater concern is of bringing that relatively high impedance
line out of the welder via the foot control in terms of noise sensitivity.
A quick fix would be adding a small 12V signal relay in the welder
for this purpose, N.C. contacts wired in series with the T.C., which
the foot switch would activate when depressed. That would
dismiss the need for N.C. contacts in the switch as well as potential
noise concern.

On the subject of remote heat input, I'd realized a much simpler
procedure existed which although addresses only that need,
requires virtually no surgery to the welder's PCB. I posted it
elsewhere some time ago, included below verbatim:




Here is an update to the 91811 schematic. This
contains the OEM version of the machine [pages
1,2], my original modification for external heat and
timing control [page 4], and a substantially simplified
circuit providing external heat control alone [page 3].

This simplified external heat mod involves no surgery
to the control board. Tie-in to the PWM controller
is achieved by disconnecting the 3-pin connector from
the existing front panel 10K heat pot and routing it to
the board containing the added circuitry. This board
will supply it's own 3-pin cable/header which will then
connect to the original 10K pot PCB. The worst of
the modification is extracting GND and +12V from the
main board which can be found on the voltage
regulator U5, at pins #2 and #3 respectively. With
some soldering skill one should be able to make the
two connections without needing to unmount the
main PCB from the heat sink.

Actually the larger share of tedium in the above
is the issue of how/where to mount the additional
two limit potentiometers and the internal/external
select switch given the scarcity of front panel real
estate on this machine.

[NNY528i]

I'd hazard a qualified yes to both questions. The 1K impedance
is just about ideal for the application as a remote heat selector
since it should be relatively immune to induced noise. The foot
control switch would need to be wired in series with the welder's
(normally closed) thermal switch and would require N.C. contacts
which would open when actuated. Some thought is needed as well
of how to switch the foot control shutoff in/out of the welder control.
The greater concern is of bringing that relatively high impedance
line out of the welder via the foot control in terms of noise sensitivity.
A quick fix would be adding a small 12V signal relay in the welder
for this purpose, N.C. contacts wired in series with the T.C., which
the foot switch would activate when depressed. That would
dismiss the need for N.C. contacts in the switch as well as potential
noise concern.

On the subject of remote heat input, I'd realized a much simpler
procedure existed which although addresses only that need,
requires virtually no surgery to the welder's PCB. I posted it
elsewhere some time ago, included below verbatim:

I did see that circuit and description

The circuit I was looking at was the one shown on page 3 of your unified schematic which looked pretty clean and easy to deal with with a single 324 chip to deal with. I like the idea of the upper and lower limits as I am l;earning so i can keep from burning holes while I get used to the pedal action, then I suspect I will keep the limits wider open.

I inspected the innards of the pedal and the microswitch in the pedal has both N.C. and N.O. contacts so I figured i would probably have to switch them, do you think the welder will care if the thermal contact STAYS closed any time you are not welding or should that be just fine? (if its fine then that would be good as regularly I have bumped the tip while repositioning myself or the workpiece and triggered the gas to cycle) I also noticed that I had to lift the torch to kill the arc which removed the post flow gas from the weld area before it was cool which is why I would really like to be able to just shut down the arc as needed.

Is any kind of noise management need on the external circuit board? I am thinking a board mounted to the backside of the 2 pots with a line connection to the existing heat pot would be the way to go with this but i didn't know if induced noise would be a problem.

Thanks for the help.

[uhmgawa]

I inspected the innards of the pedal and the microswitch in the pedal has both N.C. and N.O. contacts so I figured i would probably have to switch them..

The N.C. contact will certainly work. The only consideration I had in mind
was of reestablishing the thermal cutoff circuit when the pedal was
disconnected.


do you think the welder will care if the thermal contact STAYS closed any time you are not welding or should that be just fine? (if its fine then that would be good as regularly I have bumped the tip while repositioning myself or the workpiece and triggered the gas to cycle)

Note the thermal cutoff opens on reaching 65*C and disables
the welder PWM. So your external N.C. cutoff switch will be wired
in series with the thermal cutoff.

Flip-flop #1 of U4 is used to enable the the PWM controller U1.
This latch is reset via V+ on pin U4.4 (disabling the welder) by
any of: power-up, OCV being reached at the torch after ~2s, or
the thermal cutoff opening. However you don't want to leave
the U4.4 pin asserted as this will override the arc start logic.
IOW just close the remote switch before you try to start the arc.



Is any kind of noise management need on the external circuit board? I am thinking a board mounted to the backside of the 2 pots with a line connection to the existing heat pot would be the way to go with this but i didn't know if induced noise would be a problem.

That approach seems reasonable. The greater concern is of
inducing noise into the connection between welder and pedal.
But even here with a 1K pot you probably could omit the use
of shielded cable without concern. The T.C. circuit is incidentally
bypassed by the power up reset timing cap C5 causing that
input to have an effective low impedance as well.

[NNY528i]

The N.C. contact will certainly work. The only consideration I had in mind
was of reestablishing the thermal cutoff circuit when the pedal was
disconnected.


Would some kind of TPTT switch for internal/extenal control with one pole switching the heat control and the other switching the T.C. circuit back in line.( i think I have seen them with 2 NC and 1 NO or something along those lines) Otherwise i could just use a seperate switch to switch it in and out.




Note the thermal cutoff opens on reaching 65*C and disables
the welder PWM. So your external N.C. cutoff switch will be wired
in series with the thermal cutoff.

Flip-flop #1 of U4 is used to enable the the PWM controller U1.
This latch is reset via V+ on pin U4.4 (disabling the welder) by
any of: power-up, OCV being reached at the torch after ~2s, or
the thermal cutoff opening. However you don't want to leave
the U4.4 pin asserted as this will override the arc start logic.
IOW just close the remote switch before you try to start the arc.



By this do you mean that, before attempting to start an arc i would need to open the pedal switch(ie foot off to kill the arc, then slight pedal befor restrikeing the arc which seems the natural way to do things)




That approach seems reasonable. The greater concern is of
inducing noise into the connection between welder and pedal.
But even here with a 1K pot you probably could omit the use
of shielded cable without concern. The T.C. circuit is incidentally
bypassed by the power up reset timing cap C5 causing that
input to have an effective low impedance as well.

Great that really simplifies things, I should be able to track down the Welder side 14 pin female plug from Miller and just mount that in the body of the welder somewhere.

Time to dig out my bread boards and soldering iron and make a trip to the Shack for some parts.

[crawler]

I prefer to keep this topic about technical stuff, but you are claiming that this concept is new to you. If so, then you must be typing your posts on a Linux machine and not a microsoft or apple machine. They have always released alpha code and have users (you call them suckers) like you testing it for them....but you are OK with that because it is USA made.

It really doesn't matter if it is software or not. Heck, they could have everything controlled by a Ucontroller or FPGA.......and then it would be software, but really what does that matter.

The 91811 design is a well known power ckt design that has been around forever. You see it in all kinds of power inverters. This one just happens to be a welding machine. Every design can be ripped apart and improved on. We just happen to be picking on the 91811 here. This could also be done for the USA machines that you claim are never alpha.

I don't see why people get so offended when talking about welding boxes like this one. I have welded with Miller 210 MIGs, Hobart 210 MIGs, AIRCO 300amp stick, Lincoln 175 TIG, Clarke 130 MIG, Chicago Electric 85 flux core, Lincoln AC225, Mitech AD/DC TIG inverter, HF 130A DC TIG, Miller 130 MIG, Solar 120 MIG, weldernator using GM 100amp alternator and 6HP Tecumseh engine, and probably a few others I forgot about. But really, who cares. They all welded decent for what they were intended for.

I am not claiming the 91811 is the best DC TIG machine in the 130 amp range. It suites a certain class of users. That is, it is for the hobby builder. A professional shop would be more geared to a Hobart, Miller, Lincoln, ESAB machine that would take more day to day abuse.

If you are going to claim the USA machines are so superior then provide some proof by showing the design is clearly better. Just saying that the 91811 is an alpha box is not correct in that maybe I have severely abused my machine in a manner that a USA machine in the same class would have failed also.

What does typing on a Linux machine have to do with posts? Are you able to see my posts? My PC at home is running Fedora Linux.

[junkweld]

Alpha software that is paid for. Yes, your post are fine. Fedora is a great choice.

[junkweld]

Just an update:
I came across some steel foot pedals on ebay advertised as TIG 160. The seem to be good steel construction quality with a 10-15 foot lead. The connector is 7-pins:

2 pins tied to each other (jumper)
3 pins connected to a 5K ohm pot.
2 pins connected to a N.O. switch

It works great with my Mitech. I think the Riland and Mitech take the same 7-pin connector. No, I am not selling them and don't know the seller. The pedal would work great with the modifications that uhmgawa posted for the HF TIG machine.

With the Mitech, the 2 pins with the jumper make it so the foot pedal has control instead of the panel knob. the other 2 pins is for the High Freq. arc start and the 3 pins control the weld heat.

The pedal works nice when you can do stuff in normal positions. Using the fixed heat setting and the HF start switch on the torch works great for out of position.

[NNY528i]

I'd hazard a qualified yes to both questions. The 1K impedance
is just about ideal for the application as a remote heat selector
since it should be relatively immune to induced noise. The foot
control switch would need to be wired in series with the welder's
(normally closed) thermal switch and would require N.C. contacts
which would open when actuated. Some thought is needed as well
of how to switch the foot control shutoff in/out of the welder control.
The greater concern is of bringing that relatively high impedance
line out of the welder via the foot control in terms of noise sensitivity.
A quick fix would be adding a small 12V signal relay in the welder
for this purpose, N.C. contacts wired in series with the T.C., which
the foot switch would activate when depressed. That would
dismiss the need for N.C. contacts in the switch as well as potential
noise concern.

On the subject of remote heat input, I'd realized a much simpler
procedure existed which although addresses only that need,
requires virtually no surgery to the welder's PCB. I posted it
elsewhere some time ago, included below verbatim:


There seems to be an error in your schematic for the pinouts on the LM324 pin #14 is marked 2 times, I suspect the input to the opamp should be pin 12 but I dont know for certain.

[uhmgawa]

There seems to be an error in your schematic for the pinouts on the LM324 pin #14 is marked 2 times, I suspect the input to the opamp should be pin 12 but I dont know for certain.

That's correct, the non-inverting input to opamp #4 should
have been pin #12. I've corrected this in the schematic.
Thanks for pointing it out.

[NNY528i]

That's correct, the non-inverting input to opamp #4 should
have been pin #12. I've corrected this in the schematic.
Thanks for pointing it out.

No problem, I have finally collected all the bits needed to try this out, have started laying things out but now need to track down my iron. Will post results when I have some.

[200acdc]

Just an update:
I came across some steel foot pedals on ebay advertised as TIG 160. The seem to be good steel construction quality with a 10-15 foot lead. The connector is 7-pins:

2 pins tied to each other (jumper)
3 pins connected to a 5K ohm pot.
2 pins connected to a N.O. switch

It works great with my Mitech. I think the Riland and Mitech take the same 7-pin connector. No, I am not selling them and don't know the seller. The pedal would work great with the modifications that uhmgawa posted for the HF TIG machine.

With the Mitech, the 2 pins with the jumper make it so the foot pedal has control instead of the panel knob. the other 2 pins is for the High Freq. arc start and the 3 pins control the weld heat.

The pedal works nice when you can do stuff in normal positions. Using the fixed heat setting and the HF start switch on the torch works great for out of position.

Hi all Fresh to this forum and resonably fresh to tig welding, I am wondering if anyone can offer any tips specific to the mitech/riland type foot pedals/ machines and how to be able to make these pedals have a finer control. Also what happens if the 2 pins jumpered together are separated? does this allow machine to set max current of pedal? I believe my cheap tig welder from ebay to be of this style welder.

[SundownIII]

To all these hobbiest guys who don't have a clue about tig welding and are trying to make a silk purse out of a sow's ear, I have one little piece of advice.

JUNK WELDERS PRODUCE JUNK WELDS

[SidecarFlip]

or is it....... JUNK WELDORS PRODUCE JUNK WELDS ?

Far as a sows ear goes, they make great dog chews when processed.

[JRouche]

or is it....... JUNK WELDORS PRODUCE JUNK WELDS ?


And thats the key!!! My welder is much better than the weldor. Funny!! JR

[CASH0615]

I recently took a job at ATI in Dallas, TX. I am eager to find our graduates jobs. I don't know anything about welding; yet. Any recomendations?

[B_C]

or is it....... JUNK WELDORS PRODUCE JUNK WELDS ?

Far as a sows ear goes, they make great dog chews when processed.

I used to buy "PIG EARS" for my dogs to chew on until I read where they can give your dog diseases and parasites......Seems that they look fine but have hidden problems.....Hmmmmmmmm I wonder if that is like the cheap welders...:D

[ching0n]

a bit of necromancy may be in order.

I Picked on of these about 2 years ago for like 100 bux. The welder would stop arcing after a few seconds. I sourced the problem to a detached solder that goes from two wires at the outputs to the pcb (i'm guessing current or electrode sensing). The thing did not work as expected until recently when I dug back into it. I managed to source the problem to the same leads, one of them had no continuity (shorted inside sheath). I'd recommend to check these for continuity (and/or resistance) if you have arc stability problems as the quality of that particular set of wires was suspect at best (in my machine).

I've ordered a set of pedals off an arcade and will try to go by uhmgawa's modifications...

[mcostello]

That's the OTHER reason people stress quality.:)

[Sberry]

I am sure there are people from other professions that laugh at us welders and I am not as pretentious about my skill set as I used to be. I know how to do more **** well than most people but it don't make them any less smart, just different or they can live because they are more specialized, etc.
As for the "newbie" here asking "dumb" questions or discussing subjects that don't interest me I chose not to participate most of the time, can shoot my mouth off on occasion but as one said,, some of it isn't for the thin skinned.
I got to agree that everyone has to start somewhere and for the most part as I can see its likely the guy coming here looking has some general skill sets, I ran in to a guy the other day that started blowing about welding, being proud of the self taught route with 6013, after I got by that a bit and looked at the work it was really good and the "welding" was good, wouldn't have given it a second thought for paying for the job, well designed, safe, etc. This guy don't know **** about it and in conversation he isn't accurate, never been to school, thats obvious but where the rubber hits the road he is pretty proficient even though he isn't some "master" of 40 yrs and expert thru and thru. The guy just got new 175 tig too. I bet he can run it pretty good, maybe not for pipe welders but he is no true hack either.
I suspect there are some fools from time to time that come here, the guy wanting to weld the gas tank in and under the car types cause he has a 300A tig, ok, but lots of them are great and learn fast. I watch people on a couple forums and we got some guys we should be pissing all over ourselves to be honored for their presence. Guys with minds way greater than my own,,,,,, this thread proves it.
I was thinking down these lines a few minutes ago, I got some stuff I do that I do super well, that opinion is from others also, but I lack real intellectual ability and academic discipline. At installing the equipment we use I can perform at masters level but how the **** works is beyond me. Different skill,,,, more than 3 or 4 wires and I am lost, would have to look at the pics to do a 3 way,,, ha but all those wires, circuits, controls, complicated hydraulics,,,, if I had to fix the radio beyond hooking up the battery we would never get off the island. A hole in the boat I may manage.
Even though I solve a lot of problems its a general common sense trouble shooting thing, I can get by,,, sometimes I make phone calls, sometimes a little trial and error or previous experience but some stuff is plain over my head period, I just plain am not smart enough, I can accept that, I get help.
Having babbled that,,, this is a great thread, I even forget what its about but its fine work and I had wondered how cheap it could go, we got a couple guys here sharing some hard earned info, ingenuity at its finest. I will agree that if the goal is to become a welder or need production its all a waste of time, buy a machine, but from a design, engineering, hobby deal, even though its over my head I have some interest and can glean some general info, its great, I think it even helps Hobart, certainly created some traffic here, glad some guys took time to share, never know what becomes of it.

[Ketch]

Sberry: One of the best posts I've read on this site!

Thanks for putting this run in an often lost perspective!

Phil :D

[half cab]

sberry: One of the best posts i've read on this site!

Thanks for putting this run in an often lost perspective!

Phil :d

x2 on what phil said:d