Here is the schematic for SCR control of my arc welder

[canoecruiser]

Take a look at ExpressPCB if you want to make a printed circuit card for your design. www.expresspcb.com.

Download the CAD software from their site and make a schematic, then port it over to the layout program. It's intuitively easy; either of you will "get it" right away. The result is top quality and suitable for any use short of a moonshot. They don't mention international ordering (I obviously haven't done so) but I can't imagine it's a problem. We use them all the time at work when we need a fairly simple design, i.e., not an eight-layer board, right away, and we have in-house staff but they can't compete with the low cost and quick-turn delivery.

There are others and probably similar vendors in OZ.

[billbong]

Hi Newt, sounds good.

I like the idea of using the microwave relays from dumped MW's.....so many good parts for nix when sometimes all they fail on is a door relay switch.

As this complete circuit solution per se does not exist as an off the shelf buy it now add on device, I would think someone could be entrepreneurial and start a small business (with very minimal putlay) making complete items....very saleable.

I have noted in previous threads and posts the need for a foot control for many welders, not only the hobby type either.....the circuit won't work with inverter models, but I think they already have a foot control as part of the package, so it's only the trannie type users that will have a need.

I toyed with the idea some time back about using a radio control device as used in model aircraft because the control actuaters are proportional in their movements and would allow you to have a complete wireless connection to control the welder with infinite power variability output.....just and idea.

I can read electronic circuits, assemble and make electronic devices, but the theory and design of even the simplest circuit is beyond my knowledge, although I did make a beat frequency oscillator metal detector purely from suggested ideas I found in a book on simple electronic devices back in 1973, when the BC109 transistor was the main building device for many circuit designs.
Ian.

[Newton Brawn]

Hi Canoecruizer,


Thanks for the site where you can draw the schematic and lay out the pc board. I will use in a next project. So far I am using the autocad to do the schematic and lay out ther pc board. For the TRIAC / FOOT pedal project I made the connections point to point using normal wiring, since it is not is a production line project and adjustments are frequents up to final and complete tested assemble.
Thanks

Newton

[Newton Brawn]

Hi Bill:

Yes, the bc108, bc107, bc109 were born in the 68... and the siliciun bc family were to the market replacing the germanium family...

Just in the time that the mam put his feet in the moon !

Regards

Newton

[Tony Matt]

Hi Newton!

You wrote:

" What is important is to find a transformer rated to 240V, and with very low magnetizing current (2 to 10mA), Small 240V transformer, by construction have low magnetizing current. Our 40x33x15mm 240V transformer has a magnetizing current of 5mA at 240V. The ratio 1:10 up to or 1:20 not is critical. "

How can I measure the magneting current of a tranformer?

Since 2005 I am using the Mike W circuit as published here (Sept 2003) controling my buzzbox. Now I'd like insulate the foot pedal.

Thanks

[Newton Brawn]

Hi Tony

It is easy, to measure the magnetizing current of a transformer.

1 – Make a series circuit wits a resistor of 10000 ohm and the transformer primary winding (240V leads ). Leave the secondary winding open (no load).

2- Energize the above circuit with 240V, 60Hz (or 50Hz).

3- Measure the voltage across the resistor and voltage across the transformer primary winding .

4- Calculate the magnetizing current as voltage reading across the resistor divided by 10000.

In order to have a better figure about the transformer, you may perform a second measurement, identical to above only with a 1000 ohms series resistor. In this case the magnetizing current will be voltage across the resistor divided by 1000.

Let me know your your measurements.

Regards.

[Newton Brawn]

Alo !

Foot pedal insulated from ac line controls the buzzbox.

Here the schematic of the SCR CONTROL modified in order to provide full insulation of foot pedal from the AC power supply.
Please read the previous posts by Newtom Brawn in order to get more info re the coupling transformer T1.


BUZZBOX REV3 jpeg.JPG


THE DRAWING "BUZZBOX REV3 jpeg.JPG " VOIDS AND REPLACE THE PREVIOUS DRAWING .


I shall say thanks to Mike W that has published on 12-09-2003 the circuit in the Hobard Forum, under the Weld Talk Topic Archive Forum


Regards

Newton

[billbong]

Hi Newt, can we assume that a small transformer bought off the shelf with 240 volt primary and 12 volt secondary rated at 250 ma would do the trick without any testing or computing?

Buying one means you have to first buy one and then test it without knowing if that range stocked by the seller is OK for your use, and even more so if bought on Ebay.

What would determine the magnetizing current in any transformer design....the amount of iron in the circuit?

The turns ratio of the wire would have to be as required.

Does this mean that there are transformers that have less iron in the circuit than would be desired for the current?

In other words are the cheaper ones less desirable and take more primary current in heating the iron (due to the lack of enough of it) while outputting to the load?

The amended complete circuit layout with values you posted is just what is needed for the average constructor (like myself).

All it needs now is a supply of circuit boards to fit the circuit and everyone can fly....LOL.....so all those with those mini routers looking for a project, now's the time to step in and fill the gap, otherwise we need a printed circuit layout so that one can be etched as needed, and even I can etch a circuit board when needed.

If the trannie is a simple aquisition then the problem of the insulated foot control is solved.

I hope this project doesn't fizzle out from lack of interest as it's so near to success I can taste it....LOL.
Ian.

[Newton Brawn]

Hi Bilbong !

Thanks for the questions. I will answer between the lines, ion such way you can follow up ther issues.

1 -- Hi Newt, can we assume that a small transformer bought off the shelf with 240 volt primary and 12 volt secondary rated at 250 ma would do the trick without any testing or computing?
>Yes, the comercial transformers are economically designed to use the hi induction of the iron WITHOUT saturate the core. Normally the core is laminated silicon stell. Typical design induction in small (and medium) size tranformer is 1.0 tesla (weber /sq meter). The manufacturer, in order to produce a transfomer, follows a standart procedure of calculation and design, and in the end it is easy to predict the characteristics of the transformer.

2 - Buying one means you have to first buy one and then test it without knowing if that range stocked by the seller is OK for your use, and even more so if bought on Ebay.
>Yes, you shall buy one, and test in order to have the exact parameters of mag/ exi current. As the circuit is not critical and allows a wide range or tolerance, a 240V primary / 12V 250mA transformer will work fine. Normally the seller does know the magnetizing or exitation current of the transformer.

3 - What would determine the magnetizing current in any transformer design....the amount of iron in the circuit?
>The B/H curve of the stell used in the core, the cross section of the core, the mean lenght of magnetic path in the core, the line or primary voltage, the line frequency, the number of primary turns are the datarequired for calculate the mag current.

4 The turns ratio of the wire would have to be as required.
>The turns ratio is not critical. You may use 240/6, 240/12, or even 240/15V tansformer as shown in the ckt, MATCHING the foot pedal potenciometer. If you have a 240/24 or even a 240/55V transformer just use a 2-5k ohms foot pedal potenciometer.

5 Does this mean that there are transformers that have less iron in the circuit than would be desired for the current?
>Yes, small core cross section results im more turns to avoid saturation, and more turns results in higher primary inductance, resulting in smaller primary mag current.


6 In other words are the cheaper ones less desirable and take more primary current in heating the iron (due to the lack of enough of it) while outputting to the load?
> not nescessarelly..

7 The amended complete circuit layout with values you posted is just what is needed for the average constructor (like myself).
> I believe that the major care are to be done with the SCR connections, heathsinks assembly, soldering skils of the constructor. Some experience in SCR ckts are required.

8 All it needs now is a supply of circuit boards to fit the circuit and everyone can fly....LOL.....so all those with those mini routers looking for a project, now's the time to step in and fill the gap, otherwise we need a printed circuit layout so that one can be etched as needed, and even I can etch a circuit board when needed.
> It is not nescessary to a printed circuit board, so few components, it will be easy and quickly assemble the resistors, diodes, diacs and caps direct in the wire terminal board.


If the trannie is a simple aquisition then the problem of the insulated foot control is solved.

I hope this project doesn't fizzle out from lack of interest as it's so near to success I can taste it....LOL.

Ian.

I hope I have answered all issues, if you have further questions, please let me know

Regards

Newton

[billbong]

Many thanks Newt, all the questions answered very clearly.
Ian.