Welder High No-load Current Draw

Whatever the reason for the current draw is probably why the diode blew.

Whoever was able to test the capacitors should also be able to check the coils and other components.

Though, for energy conversion efficiency, it might be just better to purchase a new welder.

Thank you for the detailed answer spades. In reference to the diode: the diodes on my machine are not separate diodes but a group of smaller diodes arranged in parallel on a heat-sink board with both positive diode groups on one board and the negative diode groups on another board. Before I bought the machine there was a problem with one of the diodes and the effected diode group was cut off and replaced with a single diode(7209, 300UR25A). This was the diode that went bad.

The fan, which runs on 115 vac, works fine. I have cleaned the machine(Vacuumed & Blown out) and sprayed contact cleaner on all exposed terminals.

I checked the caps for shorts between the two terminals and terminal to case with an ohm meter and didn't find a problem. The caps were removed from the machine to be tested as I don't have a reliable tester. They were tested using a multi-meter at the electricians shop and declared good. The caps don't look as if they have expanded but as old as they are it's hard to tell. They are attached to the machine with a steel band around their middle that would prevent them from expanding much in that direction. I guess I'm going to have to assume the caps are bad and replace them.

As I can't see the arrangement of the diodes, I can't offer any rational comment.

Have you actually tested the caps for capacitance values? Many multimeters have this functionality and will give a pretty accurate reading - takes a few seconds to do so while the capacitor charges.

Try the unit without the caps in circuit, they are only for pf correction so the unit will work OK without them (assuming no other problems).

As I suggested earlier, put the polarity switch to AC and you can test the rest of the unit without even replacing the diode.

If you are still having troubles you will have to check voltages on the secondary side, check also the stabiliser cap as they can also fail.

If it all works at that point, then you can consider spending some cash.

A multimeter is only very low voltage. May I suggest a megger! Though I personally have not tested HV caps for many years with a megger myself......if ever......is it a valid test or not????

Looking here suggests that Meggers ARE DC, so they could be used to test the caps (with eye protection at least!!). You need DC to see that the cap (even though it an AC cap), is holding a charge in both directions, though I think you should just replace them with no more ado....

Are Meggers DC?

Do remember that the caps, even if not at their best, will/could store a VERY NASTY shock for anyone touching them afterwards, up to the voltage being used for test, so check with a DC meter before touching that the voltage is gone.....do not use a good metal tool to short this out, you might weld the tool....use a ceramic resistance of between 500 and 1000 Ohms and eye protection, that can handle say 5-10 watts, from an old TV maybe....it may still get hot so be careful....

CHECK CHECK CHECK.....

Remember old capacitors do NOT like being left "lying around", the chemicals dry out/leach out or whatever. They need to be used regularly.

I think that you proved that they are defective by taking them out of the circuit and stopping that heavy current.......just simply replace them......

You might blow some warm air through the welder as well, in case any damp is still in it, or out it out in the sun on a hot day, covers open....on the central heating or whatever.....

Good luck.

A Megger may well use a test voltage in the kV range, (MIT1025-US uses 10kV) which the 460V rated caps will get very upset about.

Totally true....

That was one of the reasons I wrote "check" in capitals 3 times.

But checking with am Ohmmeter was almost a waste of time.....

I would just replace them if it was me.

I was within 20 feet of some big DC caps that "took off" many years ago. It like a hand grenade and the chemicals that get spread around are also nasty.....luckily the cover contained that, bur was severely bent by the forces involved..

Thats what happens when a 48VDC power supply (80s design) uses 50 Volt Electrolytic caps and a series/shunt transistor goes short circuit, giving them 75VDC!!!

I remember I shocked the local engineer, I simply removed the offending transistor, replaced the blown caps and others that had "bulged" and put if back together without the transistor.....he went purple in the face.

I simply switched it on an everything worked as designed. I told him that the device must run and we had no spare transistors handy and that he could replace it if he wished the following week when a spare could be obtained.

A year later it was still running with one transistor missing!!! I asked!!

If you re-read my post you will see that I was not referring to the ohmmeter function, but to the capacitance tester which is a feature on many multimeters. I have a number of them with this facility and use them regularly for testing motor start caps etc., they are very accurate.
Megger (being a brand name) along with many other manufacturers make capacitance testers, many of which are incorporated with normal multimeter functions. These devices utilise low voltage to charge the cap and take readings (my Fluke and Digitech units both put less than 1 volt across the cap during testing).
Whilst often used, a traditional DC high voltage insulation tester could very well punch a hole in the thin foil plates if its output voltage exceeds that of the capacitor's safe working voltage, and it still doesn't give a read of capacitance, which is what you really need in order to assess the integrity of the cap.

Such a low voltage meter will not test a High voltage cap properly. Testing such caps is a fraught and worrying job!!

The OP has removed the caps and the problem is gone.

That has to be a good indication of where the problem possibly is!!

Old caps of certain styles are known to give problems after not being used for a long time. I cannot see them from here to know the type exactly.....

If he wants to be more certain before investing much, then he could take all the suspect caps out, measure current.

Add one new one, measure current.....if the high current does not return with the new cap. Buy two more....fit!

Problem solved.....

Most likely a diode blew, which increased the ripple on the DC side, which over heated the caps and they swelled, now they are shorted, maybe to the case and thereby ground. How did someone test the caps?

Could have gone the other way too, if the welder sat unused for over a year or so. If they are electrolytic caps, they lose their oxide film over time with no power on them. When you just re-energize them a year later, the film layers burn through, you get less capacitance and they become prone to failure very quickly after that. Then with the bad caps, the dI/dt through the diodes was too much for them and one of them shorted.

As they are pf correction caps - and therefore on the AC side - they are more likely to be Polymerics rather than Electrolytics.

Polymerics generally fail open as they have a pressure interrupter (normally a notched wire which breaks as the case expands) which open circuits the cap if excess pressure occurs, electrolytics don't have that interrupter. Both can get very close to a dead short as they become unable to withstand the applied voltage, and before total failure occurs.

Both types generally swell when excess pressure occurs, the cross that you see etched in the top of many caps is a weak spot designed to allow gentle rupture rather than violent explosion, some have a rubber plug that pops out and others just rely on the weakness of the case.

I wonder how these caps were tested - probably just connected a battery across them to see if they sparked!!

Don't know if this will help but we had a welder, and I am pretty sure it was a Lincoln, a long time ago that had been sitting around in retirement. When we went to use it we got a small arc and a blown mains fuse. Eventually we had to dismantle parts and clear the rust off the transformer plates and choke. It worked well after that. I only have a hazy memory of the details but we did remove lots of rust of a powdery nature. Hope it helps you