Strange SCR power supply (welder) issue

Ignoramus17359 · 2006-07-08T19:13:46+00:00

I have a welder that I made from a old Hobart CyberTIG, by using aamodern SCR firing controller by PCTI, Inc and a Cubloc microcontrollerusing BASIC.It all seems to work, but there is a bewildering issue.Current and voltage are regulated using potentiometers. (notmicrocontroller outputs, for now)When I have current pot set to 0, and connect work and electrode leadsto make a short, the current is not 0, but is instead about 12 amps. I verified that output voltage from the potentiometer is zero. It isnot some sort of floating ground issue for the input of the SCR firingboard -- they all share the same ground.So.. What could it be? Why is it outputting current when it is set to0?Just as a side note, it sets voltage just fine and goes to zero voltswhen the voltage pot is set to 0.i

Jul 08, 2006 25 Replies

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Tom M 20 years ago

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?

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Ignoramus27242 20 years ago

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?

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Wayne Cook 20 years ago

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

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Ignoramus19949 20 years ago

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

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Cydrome Leader 20 years ago

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

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Paul E. Schoen 20 years ago

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

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