Older Thunderbolt AC/DC shorting when turned on, diodes at 31kohms?
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  1. #1
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    Apr 2013
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    Older Thunderbolt AC/DC shorting when turned on, diodes at 31kohms?

    Hi folks,
    One of the welders here at my non-profit vocational welding school in West Africa has developed a short somewhere. When you turn it on, it buzzes loudly (not like a normal transformer machine idling) and the shop lights dim.

    It's an older black-face, from before there was a weld output selector - you select by plugging the leads into different terminals for AC and DC (read: DC is always connected to the transformer).

    I got into it and disconnected the transformer tap that runs to the rectifier circuit. Doing so removes the problem. No excess current draw, no buzzing, no lights dimming. So, it seems like the problem has to be somewhere from the rectifier circuit on. I have also seen machines where the terminal lug shorts to the chassis, so I disconnected that and reconnected the transformer tap to rectifier circuit connection, and the problem persists, so that's not the problem.

    I removed the four diodes. Three of them are open one way and around 31kohm the other with a DMM. The fourth is open both ways, so that's obviously bad.

    My question is, I was expecting the good ones to be open one way and a dead short the other (1 ohm or less). Are the other three bad as well? I'm using a decent Fluke meter, so I don't think it's an instrument problem.

    Thanks!

  2. #2
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    Re: Older Thunderbolt AC/DC shorting when turned on, diodes at 31kohms?

    This might help you with your question. https://sciencing.com/test-miller-we...s-8046199.html

    Welding diodes flow one way, block the other way - as I understand it anyway.
    Last edited by MinnesotaDave; 05-08-2018 at 02:09 PM.
    Dave J.

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  3. #3
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    Re: Older Thunderbolt AC/DC shorting when turned on, diodes at 31kohms?

    Quote Originally Posted by MinnesotaDave View Post
    This might help you with your question. https://sciencing.com/test-miller-we...s-8046199.html

    Welding diodes flow one way, block the other way - as I understand it anyway.
    Yep, that's how all diodes work - my question is, normally a diode tested in the direction that allows current to flow will show virtually no resistance, like 1 ohm or less. My limited experience would say that a diode reading 31k in that direction means the diode has failed - but if that's the case it would mean that all four diodes in the machine failed at the same time, which also seems a little unlikely.

    Anyway, if anyone has more first-hand experience with these diodes' typical passing resistance, that'd be helpful. The little buggers are $26, which isn't as bad for one as all four! Worth it either way though, this is a nice little machine.

    Thanks!
    -Jason

  4. #4
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    Re: Older Thunderbolt AC/DC shorting when turned on, diodes at 31kohms?

    Quote Originally Posted by JasonPAtkins View Post
    Yep, that's how all diodes work - my question is, normally a diode tested in the direction that allows current to flow will show virtually no resistance, like 1 ohm or less. My limited experience would say that a diode reading 31k in that direction means the diode has failed - but if that's the case it would mean that all four diodes in the machine failed at the same time, which also seems a little unlikely.

    Anyway, if anyone has more first-hand experience with these diodes' typical passing resistance, that'd be helpful. The little buggers are $26, which isn't as bad for one as all four! Worth it either way though, this is a nice little machine.

    Thanks!
    -Jason
    You should disconnect one side of the diode from circuit before testing. Test all of them for correlation. They should be close.

    The OHM reading isnt a good way to test but will work. Different OHM scales will give different readings because diodes are non-linear. I think you need highest scale but it could be lowest OHM scale to get best reading in OHM's. I use a diode test meter. In diode mode; the meter supplies 9 volt to diode and the meter will read actual voltage drop. For silicon diodes, voltage drop should be 0.7 volts. I have seen 1.414 volt drops but I think it was a dual diode. 31 K OHM's sounds hi.

    If one diode is out, I would replace all four. Ebay_ sells 3 phase diodes that have 4 diodes or a full wave bridge. I dont know how the power is in W Africa but if it isnt clean 50-60 Hz, it could be a problem. If you get brown outs, shut the machine down before the voltage completely drops or it will damage your transformer.

  5. #5
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    Apr 2013
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    Re: Older Thunderbolt AC/DC shorting when turned on, diodes at 31kohms?

    Quote Originally Posted by Insaneride View Post
    You should disconnect one side of the diode from circuit before testing. Test all of them for correlation. They should be close.

    The OHM reading isnt a good way to test but will work. Different OHM scales will give different readings because diodes are non-linear. I think you need highest scale but it could be lowest OHM scale to get best reading in OHM's. I use a diode test meter. In diode mode; the meter supplies 9 volt to diode and the meter will read actual voltage drop. For silicon diodes, voltage drop should be 0.7 volts. I have seen 1.414 volt drops but I think it was a dual diode. 31 K OHM's sounds hi.

    If one diode is out, I would replace all four. Ebay_ sells 3 phase diodes that have 4 diodes or a full wave bridge. I dont know how the power is in W Africa but if it isnt clean 50-60 Hz, it could be a problem. If you get brown outs, shut the machine down before the voltage completely drops or it will damage your transformer.
    Thanks a lot!

    Miller's support system was down yesterday (email undeliverable) but they got back to me today and are confirming that the 31k ohm reading isn't a problem as long as they're showing high resistance or open the other way, which they are, so they agree with the hypothesis that just the one failed.

    Our entire vocational school (including welding and machining) run from a big off-grid solar power system, so while we have to be careful about power use, the power itself is quite clean.

    Replacing them with a 4 diode unit would require some reworking, wouldn't it? Two of them connect to one backing plate, two to the other (one each from the transformer tap and one each from one of the output terminals.) Do you worry that the one failing means the others likely aren't far behind, or that this one failing may have weakened the other three?

    Thanks for your input.

  6. #6
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    Re: Older Thunderbolt AC/DC shorting when turned on, diodes at 31kohms?

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