First, a little background. I picked up a Canadian-made 1971 Lincoln Idealarc 250 on the cheap with the intention of turning it into a constant voltage capable machine for wire processes. The fan ran rough and AC selection was not working when I got it. I fixed those things easily and moved on to getting this rig to MIG. The end goal is to be able to vary the Voltage output on the welder from 20 to 45V.
I managed everything alright, time delays, SSRs and thermal overload for the transformer. Decided on regulating voltage with a semi-controlled full wave rectifier. The SCRs I bought have both a gate and what looks to me is a second cathode, which threw me.
I've built a few rectifiers to convert AC only arc welders over to DC but this phase control thing has me a bit stumped. I understand you need to trigger the gate at certain times to change the firing angle. I'm unsure as how to accomplish that though.
If the SCR is controlled by trigger and quench pulses - should I be using a generic AC control circuit? I was wondering if someone wiser than I could clarify this and/or point me to a circuit that will work. These SCRs were a bit of money. I really don't want to pop them.
72 OCV - 250A DC - 300A AC
Here's an simplified diagram of what I have so far.
EDIT - Updated Diagram. I was wondering if I could get away with just using a cheap PWM controller? Also tried to integrate one of the circuits Antonio suggested but I don't think I did it correctly, and didn't want to try it just yet in case I risk some sort of catastrophic failure. I also discovered something called a "chopper board" I'm wondering If this is something I could build or pick up a generic one online somewhere? I was pretty sure a semi controlled full wave rectifier was the way to go since I read somewhere they're good for 60 - 80% efficiency, which is alright since the welders voltage is so high. Found out about these boards that chop the waveform so I'm not sure now. Looking for the easiest solution to implement. With the exception of regulating voltage on the Primary side. Without Power factor correction, the welder draws something like 98a @ 240VAC when maxed out on the constant current side of things. So, I guessed if i want to run it at 36v DC@250a - 0.035 solid-core or 38v with 0.40, I'm stuck regulating the secondary unless I want to use some massive gauge cable for the power cord to carry the main supply.
Edit: updated diagram. Tried PWM controller - didn't work. Also tried a few other circuits I found online unsuccessfully. The particular chopper board I need for my machine is from a Lincoln Power-Mig 300. They are a discontinued transformer MIG welder so the choppers for them are no longer available. I found one used board for $3600 CAD which is wayyy too much. So back to fashioning a method of voltage regulation for myself.
Discovered this circuit from learnabout-electronics.
Its for a transformer that's for 240v to 12v that drives a 100ma lamp as opposed 240v to 72v @ 250a DC so the capacitor and resistance values won't do. I'll have to fiddle with those a bit.
It can't be that easy, can it?
I was also curious if it were possible to regulate the voltage off the secondary, before the rectifier, with 2 big SCRs back to back?
I was also wondering if I should tap the secondary closer to the centre of the winding. The secondary has 15 windings. If I were to tap number 10 that should net me 48v. Would tapping that winding a little earlier help make this project a little easier? The windings are aluminum, with a decent air gap so I can get in there easy with chill bars and a torch if needed.
Trying to figure out what would be the easiest manner to get me some variable voltage capability. Sorry for the all newbie questions and the wiring diagrams done in paint. Can't get Circuit lab figured out.
Thanks for reading.