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? :confused2:

 

[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.

 

[CNC Dan]

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.

 

[joshuabardwell]

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.

Interesting. So the AC component can be ignored then? What about the fact that the DC voltage drops with the bulb in circuit on DC, but not on AC?

 

[joshuabardwell]

Also: I thought the whole point of DC welding was that the voltage didn't pass through zero 60 times a second. But the image above shows the rectified DC hitting zero just as often as AC--just not ever going negative.

 

[CNC Dan]

Also: I thought the whole point of DC welding was that the voltage didn't pass through zero 60 times a second. But the image above shows the rectified DC hitting zero just as often as AC--just not ever going negative.

Thats unloaded output. Put a heavy load on it and the inductance of the choke (coil) will keep the Current going. Kinda works like flywheel.

Plus, the Voltage waveform and the Current waveform won't be in sync.

Isn't AC fun? You should see all the math that goes with it.

 

[CNC Dan]

Interesting. So the AC component can be ignored then? What about the fact that the DC voltage drops with the bulb in circuit on DC, but not on AC?

That's what happens with an un-filtered, un-regulated supply.

The meter is reading the peaks of the un-filtered DC. But those peaks are narrow so they can't provide much current. Under load, they flatten out.

 

[joshuabardwell]

Thats unloaded output. Put a heavy load on it and the inductance of the choke (coil) will keep the Current going. Kinda works like flywheel.

Fascinating.

That's what happens with an un-filtered, un-regulated supply. The meter is reading the peaks of the un-filtered DC. But those peaks are narrow so they can't provide much current. Under load, they flatten out.

I'm confused though. The welder ought to be able to provide up to its rated 140 amps when welding, right? Does that not count as "under load"? Maybe not, since welding basically creates a short between the rod and the work piece.

Sounds like you're saying nothing is very anomalous about my readings. Yes?

 

[CNC Dan]

Fascinating.

I'm confused though. The welder ought to be able to provide up to its rated 140 amps when welding, right? Does that not count as "under load"? Maybe not, since welding basically creates a short between the rod and the work piece.

Sounds like you're saying nothing is very anomalous about my readings. Yes?

You are right, it should be able to provide rated amps, but not at the OCV. Usualy they are rated at 25 volts.

Can't figure whats going on with the super low volts at startup though.

Check all the connections on the DC side of the diodes. I still think something simple is out of whack.

 

[joshuabardwell]

Can't figure whats going on with the super low volts at startup though.

Just to reiterate, the super-low volts are the AC volts, not DC. As far as I can tell, we have concluded that the AC voltage is just the result of the non-regulated rectified DC, and so it is not a problem or worthy of further investigation.

If the welder is rated at 25 volts, then I believe everything is nominal, since it drops to 27 volts with the bulb in circuit. What it seems we are shaping up to is that voltage is correct and the diodes test out good, but something in the circuitry is preventing delivery of full amps when welding. Someone made a suggestion to stick a rod on max output for ten seconds, then un-plug the welder (for safety) and look for a spot inside the welder that is hot. I think that may be the next step.

 

[CNC Dan]

Someone made a suggestion to stick a rod on max output for ten seconds, then un-plug the welder (for safety) and look for a spot inside the welder that is hot. I think that may be the next step.

I've done that on cars that wouldn't start due to a bad connection.
Have the welder all opened up so you can get to all the suspect connections quickly.

 

[joshuabardwell]

Well, I made something useful today!



I had some hooks from some ratchet straps that had to be retired, so I put some screw threads on them so they could screw into the wall and hang tools from and such.

Don't bother critiquing the welds. They are total crap and I know it. But I twisted and pulled on them and they didn't wiggle, so I think they'll do.

BTW: AC 1/8" E6011 at about 120-130 amps.