Strange SCR power supply (welder) issue

I have a welder that I made from a old Hobart CyberTIG, by using aa modern SCR firing controller by PCTI, Inc and a Cubloc microcontroller
using BASIC.
It all seems to work, but there is a bewildering issue.
Current and voltage are regulated using potentiometers. (not microcontroller outputs, for now)
When I have current pot set to 0, and connect work and electrode leads to 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 is not some sort of floating ground issue for the input of the SCR firing board -- they all share the same ground.
So.. What could it be? Why is it outputting current when it is set to 0?
Just as a side note, it sets voltage just fine and goes to zero volts when the voltage pot is set to 0.
i
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Ignoramus17359 wrote:

What is the maximum current?
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about 200A.
i
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On Sat, 08 Jul 2006 19:13:46 GMT, Ignoramus17359

Leakage.
Cold Fusion
A blessing from Our Lady of Perpetual Motion
Gunner
"If thy pride is sorely vexed when others disparage your offering, be as lamb's wool is to cold rain and the Gore-tex of Odin's raiment is to gullshit in the gale, for thy angst shall vex them not at all. Yea, they shall scorn thee all the more. Rejoice in sharing what you have to share without expectation of adoration, knowing that sharing your treasure does not diminish your treasure but enriches it."
- Onni 1:33
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That's close to my list of hypotheses.
Forgot to say, I use a LEM current transducer for current feedback. It has + and -15V inputs. I thought for a moment that perhaps if the + and - inputs are not precise opposites, it would exhibit the observed behavior (since there is not ground input, the zero is the middle point between + and - input), but I know that inputs are very close, like 15.03v and 15.01v. (according to my multimeter)
i
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Ignoramus17359 wrote:

My first thought is you have an offset on your ammeter, that offset being - tada! - about 12 amps.
GWE
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That's a good one, but, that's not the case. There is real current. The needle is at zero if I have leads disconnected. When I touch them, there are tiny sparks, and then current starts flowing.
i
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wrote:

What is the problem here, aside no troublshooting skills?
you have 12 amps coming from somehwere, find out where. What's between the power source and where you're reading 12 amps?
electricity flows in a circuit. You can can figure out where it's flowing.
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On Sat, 8 Jul 2006 22:09:26 +0000 (UTC), Cydrome Leader

1 gauge wire, with work lead shorted to electrode holder.
i
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wrote:

It's not going to 0 volts. It doesn't take much voltage to get a current flow into that low impedance. Recheck with a good DVM. I would guess you have an offset voltage problem in your control circuit.
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Do you mean something in control circuit not referenced to the same ground?
i
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wrote:

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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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
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wrote:

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

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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wrote:

Good, now unless that wire is picking up some serious magnetic fields it won't be generating any current.
next step. check your power source. Maybe the current is coming from there.
Circuit = complete loop. check all the parts that complete it. Electricity doesn't come from nowhere.
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On Sun, 9 Jul 2006 01:15:05 +0000 (UTC), Cydrome Leader

Yea, that's where it is coming from, the power source!

I am sure that it is coming from the SCR rectifier, there is nowhere else it could come from, the question is why.
So far I see the following possibilities:
1) SCR firing board is getting wrong control signal inputs (ie, wrong desired voltage signal). Maybe due to incorrect grounding.
2) SCR firing board gets bad feedback signals from the current sensor
3) SCRs are bad and "leak electricity"
Case 1 can be tested by forcibly connecting the "desired current" input of the SCR firing board to the ground (as well as the negative end of where the potentiometer is to be wired to the SCR firing board). If current disappears, I know that I have bad input.
Case 2 can be tested by supplying 4 volts to the current sensor input of the SCR firing board (that means that the board is led to believe that the current is too high, as my max current setting is much lower). So, if it is any good, it should go as low as possible.
Case 3 is easy to test for -- there should be no current when 3 phase AC is applied to the input of SCR rectifier and SCR firing board is turned off (ie it is not supplied with 110V power to run at all). If current persists, we know that I have one or more bad SCRs.
Any more thoughts?
i
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On Sun, 09 Jul 2006 01:43:06 GMT, Ignoramus17359

Not the case. Grounding desired current input did not change anything.

did not have time to test

also not the case
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wrote:

Most SCRs have some sort of snubber connected across them to limit dV/dT turn-on. Usually, they draw only 100 mA or less. If you have an output transformer with a high ratio, it would cause higher output current, but your 12 A would mean a ratio of 100:1 or so. It would be helpful to look at the output waveform to see if there may be any high frequency spikes, which could be caused by transients in the supply line. They would be conducted by the snubbers and cause possibly high currents.
Paul
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Ignoramus17359 wrote:

The board or the scr firing ckts may need to be balanced. IF you have current it is coming through the scrs. The other case may be you have a bad scr. You could put a scope on the primary side of the firing ckt but make sure you don't get on the scr side of the firing transformer unless you have one of the battery operated scopes designed for 600 volt insulation or better. Even that sometimes doesn't help though, my 221 Tek scope blew up the test lead when an internal metal shield in the scope broke free andwent across the scope lead input line inside the scope. It was scary. It took out the 30 kw drive too.
John
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