Help understanding welder current control circuit

[avitsur]

Hi
I would like to ask for your help and opinion regarding a scr based current control circuit that I've found on the web. first, let me explain that I'm aware of the fact that there are compact IGBT welders at a reasonable price. the purpose of my attempt is sort of educational and hobby. so, to the point. I have a old "buzz box" welder (230vac single phase input) it has up to 150amp output welding current, selected by taps on the transformer. what I would like to do, is to add this scr based current control circuit that I've found. it is of Russian origin and I've managed to achieve the high current diodes and scr's. I would like to ask for your opinion about it. there is at least one question that I would like to make clear. as you can see in the circuit attached, the cathodes of VS1 and VS2 that produce the "+" output of the welding current are connected to the "earth clamp" of the welder while the "-" the anodes of VD1 and VD2, are connected to the welding rod (elektrod). that is not uncommon, there are situations that reverse polarity welding is needed. now comes my question, as you can see, the negative output of the control circuit rectifying bridge is connected to the positive output of the welding current. is it wired so in order to enable to turn off the scrs? (the control circuit voltage is 24v).thanks for your help.

[Ian.M]

No.  There is no provision in the circuit for forced commutation so it cant turn the SCRs off.  They have to turn off naturally, when the current through them drops to near zero.

Its wired to delay the turn-on of the SCRs like a simple (leading edge) TRIAC light dimmer.  R1 + R2 & C1 provide the variable delay time constant.  VD6 (a 5.6V Zener) and SCR VS3 replace the DIAC in the light dimmer circuit, providing the threshold the voltage on C1 must reach before firing.  R3 limits the firing pulse current.  Diodes VD3 and VD4 feed the firing pulse to whichever SCR VS1, VS2 is forward biased to fire it, and presumably also act to prevent the gate voltage of a SCR already on from the previous half cycle trigging the other one prematurely.