Electricians/Welder Guys

Horse Plains Drifter

What would be an event that would cause all six rectifier diodes to go open all at once? Sticking the rod for a period of time? Or? My neighbor has a Miller Big D4 welder. A couple weeks ago his neighbor kid wanted to use it. They had trouble with it not wanting to strike an arc at that time. The kid messed with it and got it welding, and used it. Fast forward two weeks, and the kid wanted to use it again. Trouble again with no arc. The kid doesn't remember what he did to get it to weld. The guy that owns it checked it out, and couldn't find anything obvious. I checked it over today, and it appears that all of the rectifier diodes are open. We removed three, and checked them, and they are open both directions. Haven't pulled the other three, but at this point we are assuming they are gunnybag too. Any ideas? TIA for any input.

Alpine

The welder generates AC. The kid was welding in DC and shorted it out.

bambihunter

As you likely know, rectifier diodes are to limit current to one direction. Is it possible to run that welder where polarity could be reversed? Is there any possibility current could be fed from another source (i.e. another welder nearby)?
Just a few ideas that popped in. My only experience with rectifier diodes were in early college classes, and when I used to install audio sometimes people would wire their stuff backwards and pop them in their amps.

babun

The Big D4 is an DC welder only IIRC. You can not "break it" by reversing poles. You go from ground positive to ground negative change over because of the rods you use or welding position.
Just for shiitts and giggles check the voltage and capacity of the 120 volt Ac outlet it should have on the control board. Plug a heavy load into it , like a grinder/saw. If ok, and it doesn't seem to run slow, your probably okay in the generator end.
The machine is old {right?} it may have just been losing the Ac to Dc diodes slowly, now all are gone.
It may have been welding a long time on AC current or reduce capability without you knowing it, now all are shot.

Horse Plains Drifter

babun wrote:

The machine is old {right?}

Yes sir! Inside one of the side panels somebody checked the antifreeze, and in marker wrote "-30 9-79". I will check the outlet, but I don't think it worked beforehand. I did some work on this welder a couple years ago, and IIRC the outlet did not work then. And yes, it is a DC only machine.

dav1965

This could cause the machine not to weld correctly but has nothing to do with the diodes.

The ground has to be fairly clean. I have welded a lot and for a living and there were times i knew my ground was clean enough to weld and it was not.

Also make sure the ground clamp is not rusty or coated with oxidation.

mag00

I'm not familiar with the D4, I had Pipeliners.

The pipeliner had to be polarized properly or it would not work. I do not know if that is the case with the D4.

Check the generator output. Is it a/c or d/c? How many volts?
You may need to look into the generator itself, being old may need brushes or commutators or polarizing.


You mentioned the diodes being open. What did you use to check them? Some meters will not work on larger diodes. Car battery and a light bulb would be definitive. You know how a diode works, so take it from there.

If diode are bad? I'm at a loss on what would cause it, they go bad so rarely. Seriously, one way switches, don't really matter what you hook up and from what end or polarity. Unless lightening hit it. It would take alot, and there should be signs of scorching. Unless Miller had some fundamental flaw which I highly doubt.

Then the other thing that comes to mind is if the motor idles then powers up upon striking an arc. That is another circuit to look into.

You mention the 110 did not work, is that a dc or ac outlet?