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Discussion Starter #1
Like the title says, what does a mig machine act like when a diode is out? This is on a Linde 100 welder I recently acquired if it matters...

Thanks in advance!
Erik
 

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I don't know how they'd act, but they are easy enough to check. Put an ohmmeter across the diode and then reverse the leads. Diodes should only conduct in one direction, otherwise, they are not a diode, but a conductor.
 

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Discussion Starter #3
Yep, I know how to check them but haven't had time to yet. I was just sitting here at work so I thought I would throw it out there. I talked to a couple of my friends who have been welding for years and neither of them have ever experienced a diode failure so they couldn't tell me either. It is a question that I'm curious to find out about now.
 

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My old Century MIG just wouldn't weld when it blew a diode. Fan ran, wire feed ran, just some sparks from the wire, though...
 

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Discussion Starter #7
PAToyota, I would guess that your diode failed open (insulated) then, correct?

To expand on the original question a bit, I'm wondering if I possibly have some AC voltage present while I'm welding with this welder. This welder welds ok, but the welds definitely do not look as nice/smooth as the ones that I can do with the Lincoln Mig that I'm used to using. It might just be a setup issue that I haven't figured out yet, but I'm not sure.
 

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I would guess so... Also, when I opened it up it was obvious which one was bad - charred and toasted...
 

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Hi, I had a diode go south in my 240volt mig welder several years ago. The fan would run when it was turned on and the wire would feed when you pulled the trigger however the second you tried to strike an arc it would blow the circuit breaker or fuse in the fuse box of our house. It was frustrating I had 2 shops have a look at it and they couldn't work it out. I ended up finding it with a multimeter myself. It all still looked brand new inside. Apparently lots of stitch welding is fairly hard on them. Hope you've found your problem.

Cheers Ben.
 

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Try talking to the instructor of the machine repair class at your local community college. He may thank you for allowing him to use it as an example in class and it will probably be returned in working order.
 

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Discussion Starter #13
Check the polarity of the work leads. Flux core or MIG?
It has always been set up as MIG. My buddy asked the same thing when I ran the question past him. I don't know 100% certain that the polarity is correct, I will check it when I head back over to the shop (hopefully today). I'm also going to check the diodes. I'll post up the results when I get them checked out.

I'm thinking that this is just the way the welder works at this point based on the descriptions of a bad diode. My friend who I bought it from did mention that it welds differently than other MIG welders he has used. It might just take a bit of time to learn the quirks of this machine.

Thanks!
 

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It has always been set up as MIG....I'm thinking that this is just the way the welder works at this point based on the descriptions of a bad diode. My friend who I bought it from did mention that it welds differently than other MIG welders he has used.
Hmmm... Always welded this way? Always welded different from any other MIG used? I'd definitely check the polarity. Could have "always" been set wrong...
 

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I'm not to sure how big the diode is in that welder but be careful checking it with your average ohm meter. Your average ohm meter wont put out enough voltage to properly check high load capable diodes.
I had this issue while troubleshooting a diode stack that saw a few thousand volts on them. It made me question what the working range of a standard ohm meter was.
 
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