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Strange SCR power supply (welder) issue

Your current sensor runs on +/- 15 volts, right? That implies an op amp somewhere in the loop. It may have a small offset that is causing a small (12 amp) current even when the input is 0 volts. You may be able to trim this offset to zero with a small pot to the output if there is enough source resistance. Or some additional resistors may be needed.

Can you get the current control to go slightly below 0 volts? Maybe take the low arm of the control one diode drop (or less) to the - 15 volt supply?

Reply to
Tom M
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right

I do not think that it is the case. It would be great if it was, and your suggestion makes a lot of sense, but the output from the current sensor is very proportional to current, no offset. I measured i with my harbor freight multimeter.

I think that maybe I could add a tiny bit of voltage to the current sensing input to the SCR firing controller?

i
Reply to
Ignoramus27242

Keep in mind that the SCR's are controlling the reactor and not the main transformer.

Reply to
Wayne Cook

Yes, they are controlling the main rectifier, not the transformer.

i
Reply to
Ignoramus19949

That's fine instrument there. Don't question it.

Reply to
Cydrome Leader

It's not bad for the $3 I paid for it, but a 9 VDC battery reads 18.9 VAC one way and zero the other. Well, hey, the average is pretty close! No precision rectifier, no capacitor, just a single diode in series. But maybe you have the deluxe version?

Paul

Reply to
Paul E. Schoen

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