Dear members,

I just got 2 transformer TIG welding machines which got a broken control 
board. I love the transformer type because they run more stable in 
countries with unstable voltages. And I feel pitty to dump those 
transformers since they are still in good shape.

Though I am neither an electrician nor an electronic expert I could 
check diodes, condensators and thyristors of those unit and they are all 
o.k.

Also the transformator works as it should.

High voltage for iginition is also still doing its job.

But the main controlboard seems to be totally broken.
Anyhow it is far to old to be fixed. There are 5 LM324 on and some other 
ICs besides the passive elements. The supplier does no longer deliver 
spare parts for this type. Since all the wiring (200 Ampere) is done 
nicely and in good shape, and I still find proper diodes and thyristors, 
I kind of dislike to use the new technology (MosFET).

I attached my first approach to this project, still need to learn more 
about single chip microcontrollers. But in order to not waste too much 
time being on a complete wrong track I would really appreciate some 
contributing ideas and remarks.

You see the transformer (28V 200 Ampere and 2x 24V, 1A) which I will 
just use for welding up to 160Ampere, and the existing diodes and two 
thyristors, the resistor of 0.001 Ohm and a throttle which avoids that 
high voltage during ignition will damage the diodes and thyristors.

I added some PUT to control the thyristors since I could not find any 
optocopplers with 250mA 400mW output.

My may concern is the measurement of running current using the shunt of 
0.001 Ohm. Maximum voltage expected there should be 0,16V in case of 160 
Ampere.

Is it possible to measure the voltage (difference) directly with an 
Arduino one? If anyone may give a hint how this could be measured best 
it will safe me a lot of time.

Thanks in advance

Ottosan

The lowest referecne voltage that most Arduino Boards support is ~ 1 V. 
YOu want to measure a voltage of max. 0,16 V. So the maximum numeric 
value you will get is ~160.

So you get values between 0 and 160. Is that enough resolution?

Most Arduino devices have no differential ADC input, but the Arduino DUE 
has. The voltage must be below 3,3V.

I assume that this is not the case in your device. If I am right, you 
need additional parts anyway.