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Thread: Help trouble shooting a HF 125

  1. #1
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    Help trouble shooting a HF 125

    Hi
    I was using my HF 125 today and then it stopped working. I have done the bridge rectifier/capacitor modification. The welder powers on. When I push the switch I hear a click. The wire does not feed. It sounds like a motor is trying to run. If I touch the wire to what I am trying to weld there is a tiny spark.

    Where do I start to trouble shoot this problem?

  2. #2
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    Re: Help trouble shooting a HF 125

    If your wire feed motor wont turn, I'd start there. Take the wire all the way out and see if the motor turns then. Then see how much voltage you have going to the motor. I have no idea how much. Maybe someone else can say. Since you performed the rectifier mod and the unit AC from the factory, you should know how to bypass that stuff to eliminate the possibility of the problem being the rectifier or capacitor.

  3. #3
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    Re: Help trouble shooting a HF 125

    Looks like the bridge rectifier has failed. It was a 150 amp 1600v rectifier. I took the rectifier, capacitor and resistor out of the system and the welder works.

  4. #4
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    Re: Help trouble shooting a HF 125

    Good to hear you found the problem. Can you get one with a little higher amp rating? You would think 150 amp would be enough but I'd probably go a little higher if you can get one.

  5. #5
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    Re: Help trouble shooting a HF 125

    Was the rectifier a 3 phase or a single phase ?

  6. #6
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    Re: Help trouble shooting a HF 125

    The rectifier was a 3 phase. The info I found online said that a 3 phase would work. I bought the same rectifier referenced in that article. I am concerned about buying the same rectifier again. One thing that I like about it was the built in heat sink. I am just guessing that the problem lies in the rectifier because I was not getting any voltage coming out of the rectifier. My knowledge of electronics is limited at best and closer to none. I don't know if a bad capacitor would cause no voltage at the rectifier.

    In looking at rectifiers some are described as "SQL" and others are "MDQ" The SQL rectifiers have a heat sink and are 3 phase. The MDQ rectifiers do not have a heat sink and come in either single phase or 3 phase. What is the difference between SQL and MDQ rectifiers.
    Last edited by mickri; 01-27-2020 at 01:01 PM.

  7. #7
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    Re: Help trouble shooting a HF 125

    A 3 phase rectifier uses 6 internal diodes to get 150 amps.
    If you use a 3 phase diode as a single phase rectifier, you are using 4 of the diodes , so 2/3 of 150 amps is 100amps.
    You would need to use about a 250 amp 3 phase diode .

  8. #8
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    Re: Help trouble shooting a HF 125

    Thanks Bluewelders for the explanation on 3 phase rectifiers. Makes perfect sense to me. When I was first looking into modifying my welder I wondered about that very issue. Not knowing diddly squat about electronics I went with what people online said about a 3 phase working. I will get a single phase rectifier as a replacement.

  9. #9
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    Re: Help trouble shooting a HF 125

    Using two of the three phases in that rectifier, the two active legs won't be 180 degrees out of phase with each other. My first impression is the output would be DC of varying voltage because the phases don't work opposite each other like a 2-phase rectifier.

    Is this a theoretical or practical consideration?
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  10. #10
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    Re: Help trouble shooting a HF 125

    It is theoretical.

  11. #11
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    Re: Help trouble shooting a HF 125

    phasing is dictated by the inputs phase angle, not the diodes. A 3 phase rectifier just means it has 3 diodes in one body, not 2. the diodes never know what the other diodes in a rectifier are doing phase-wise. I have a degree in electrical engineering so it's not just my "random welders opinion".
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  12. #12
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    Re: Help trouble shooting a HF 125

    Quote Originally Posted by Chad86tsi View Post
    phasing is dictated by the inputs phase angle, not the diodes. A 3 phase rectifier just means it has 3 diodes in one body, not 2. the diodes never know what the other diodes in a rectifier are doing phase-wise. I have a degree in electrical engineering so it's not just my "random welders opinion".
    Ok put that way, it makes sense. The output will have the phasing of the input, 180 degrees apart, and that third diode is irrelevant. So in summary buying a 3 phase rectifier to replace a 2 phase is harmless, the only negatives might be cost or mounting space.

    I was mistakenly visualizing this in different terms: My car had weak headlights for a couple of years after I saw 'Check Engine' once then never again. I seldom use that car at night so I hadn't thought about it until finally it cranked slow. An automotive electrical specialist put some kind of diagnostic tester on it and identified that one of three diodes had failed.

    In this automotive alternator case, the power into the diodes was likely 3-phase and the output was at best 2/3 of spec, possibly a lot worse if the bad diode was passing AC. But this example isn't relevant to using a 3 phase rectifier to replace a 2 phase. Sorry for stirring confusion!
    HF MIG-180 with all the mods.
    Grizzly H8153 Stick/Tig Pulse 130/160.
    Wards PowrKraft AC-230. Stick & carbon arc.

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