Reading voltage

Date 04-02-2014 22:43

Out of sheer curiosity, I decided to check the voltage at the mig gun today. We have CC/CV power sources with voltmeters and ammeters but no pulse. The power source was set at 19 volts. When the trigger was depressed, the reading on the power source alternated between 19 and 85 volts. The handheld voltmeter showed the higher voltage continuously. What's up with that?

By OBEWAN

Date 04-02-2014 23:11

Were you welding or just taking a reading without any current flowing? And, what kind of meters do you have? Are they analog needle or digital designs? And, where did you attach your meter leads?

At first blush, I might speculate that the open circuit voltage is higher than the welding voltage - assuming you were not welding.

By newinsp (**)

Date 04-03-2014 00:34

Ah ha!

There was no welding. The drive rolls were opened so as to not feed the wire.

Digital readouts. Using an average multimeter.

Thanks OBEWAN, that may be the problem. I'll report back after class tomorrow.

By OBEWAN

Date 04-03-2014 00:44

I think we might have solved it.

If you go to the power supplier's spec sheet you might find a listing of the open circuit voltage.

I had an instructor that used to say "It's all Ohm's law". That did not make his class any easier, but it stuck with me.

Once the current starts flowing, there will be a voltage drop due to resistance in the welding circuit.

You will find the most accurate reading by measuring directly across the welding arc, but this might be hard to do with a hand meter without being blinded.

By newinsp (**)

Date 04-05-2014 08:43

Yes, that's exactly what it was. The spec sheet has a chart that shows it.

By PhilThomas (**)

Date 04-05-2014 02:35

Sounds like the display is alternating between the setpoint (19) and the open circuit voltage (usually between 70-80 depending on the power supply). Since there is no arc, the circuit is "open" and the OCV is what is being measured.

By newinsp (**)

Date 04-05-2014 08:49

Yes, that's what it was. I tested between the lugs, while welding was in progress, and got the same reading as the digital readout on the power source.