Thoughts on this used welder?

[Mark @ Everlast]

I'd be interested to know whaat the ocv is coming out of the machine on dc and ac...bet that will tell us a lot more about the welder than anything.

[Shield Arc]

Hey Mark can you explain OCV to me? Why do our SA-200s put out 40-volts, and weld fantastic, but these inverters put out 60 to 70, and really weld nice too?

[joshuabardwell]

I'd be interested to know whaat the ocv is coming out of the machine on dc and ac...bet that will tell us a lot more about the welder than anything.

Mark,

I have measured the voltage using a multimeter on the DC output. I measured 50 volts DC. On a hunch, I switched the multimeter over to AC and measured about 20 volts AC as well. That's on the DC outputs, mind you. I don't know how normal that is. It's my understanding that rectifier arrays are not 100% perfect at filtering out the AC waveform, but I don't know enough to know whether that much AC leaking through is normal or not, or whether it would affect the welder's performance.

I haven't measured the AC outputs' voltage because I haven't had any problems with AC welding, but if you think it would be useful, I can.

[California]

Maybe rig an ordinary 110v light bulb across the output to make a dummy load, then measure the ac volts?

[joshuabardwell]

Maybe rig an ordinary 110v light bulb across the output to make a dummy load, then measure the ac volts?

You mean, on the DC side, to see if the AC voltage goes away when it's actually loaded?

[California]

Yes.
.

[joshuabardwell]

I just did what you suggested with the light bulb. With the welder set on the lowest amperage setting (20 amps DC), I measure 57 volts DC and 22 volts AC with the light bulb not in circuit--in other words, just measured between the leads. With the light bulb in circuit, I measure about 27 volts DC and about 39 volts AC. The numbers are basically the same with the welder set on its highest setting (140 amps DC).

It's not completely out of the ordinary that the DC voltage drops when a load is added, although I wouldn't expect that from something as beefy as the welder pushing through a load as relatively small as a 120V light bulb. It is really surprising that the AC voltage increases, though.

Another interesting fact that may be relevant is that a couple times, when I turned the welder on in preparation for running a test, it would show just 1 volt AC, until I touched the leads to the light bulb, after which it would jump to the above readings and not return to 1 volt AC until I turned the welder off and back on again.

But what does it all mean?

For those who may have come late to the thread, I have tested all of the diodes in the rectifier bridge individually, by running a 12 volt battery and light bulb through them and simultaneously using my multimeter's diode test function. All of them showed the expected voltage drop of about 0.7 volts. None of them showed any voltage in the opposite direction. So at this point, the going hypothesis is that the diodes are good, unless there is evidence to the contrary.

I performed the same test on the AC side of the welder, in the interest of science. It showed about 63 volts, dead steady whether the bulb was in or out. It showed zero DC volts, as you would expect.

EDIT: And here is a photo of my test setup, because you know you want to see it.

[CNC Dan]

Another interesting fact that may be relevant is that a couple times, when I turned the welder on in preparation for running a test, it would show just 1 volt AC, until I touched the leads to the light bulb, after which it would jump to the above readings and not return to 1 volt AC until I turned the welder off and back on again.

If it is a newer welder, it could have a voltage reduction device that reduces the OCV. But I highly doubt it.

[joshuabardwell]

If it is a newer welder, it could have a voltage reduction device that reduces the OCV. But I highly doubt it.

It doesn't seem to be a very complicated device internally. Zero circuit boards, for example. Just a dang huge multi-output transformer that drives the AC side, and then for the DC side, feeds into a choke coil, and a rectifier array. Also, bear in mind that it was the AC voltage that was at 1 volt until the circuit was completed. The DC voltage was at 57 volts from the moment the welder was turned on. I'm honestly not even sure why there is so much AC voltage on the DC output at all. Shouldn't the diodes be preventing this?

[CNC Dan]

I'm honestly not even sure why there is so much AC voltage on the DC output at all. Shouldn't the diodes be preventing this?

Its un-filtered DC. If you were to use an osiloscope(sp?) to view the waveform it would look like cursive 'm's all strung together.
Think of the usual AC sine wave, but flip the lower parts up to the upper half.

The AC that you are measuring is the ripple of the preaks and valeys.