Strange SCR power supply (welder) issue

[Tom M]

Do you mean something in control circuit not referenced to the same
ground?

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?

 

[Ignoramus27242]

Your current sensor runs on +/- 15 volts, right? That implies an op amp
right

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.

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.

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?

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

i

 

[Wayne Cook]

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.

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

 

[Ignoramus19949]

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

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

i

 

[Cydrome Leader]

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.

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

 

[Paul E. Schoen]

In rec.crafts.metalworking Ignoramus27242


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

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