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  • Help finding diode equivalent

    Hey all,

    I'm trying to fix a coworker's stick welder. It's an old Ward's PowerKraft AC/DC machine and it won't run a bead...just a slight spark. I opened it up and tested the diodes. Three were fine, but one is open in both directions, so I'm pretty confident that's the problem. Unfortunately, I can't imagine anybody stocks parts for it, and I don't know who made it originally to try cross-referencing.

    The bad diode is marked 70HF30. I can find that number but all of the places like Mouser or Digikey have it as a non-stock item, 20 week wait time, and minimum order of 100. From my limited knowledge of electrical components, it would seem this is a 70A diode, forward/normal polarity and 300 volt max. If I can find 70A, forward, 400 volt max, would it be a problem? My thought was if I replaced the pair it would work, but figured I may be missing something.

    Thoughts?

  • #2
    Only other concern would be for the polarity of the diode. I assume they are stud mounted? Sometimes they have two with the stud as the cathode and two with the stud as the anode. Since you have three that are working, you're in luck. Look at the polarity of your meter leads on the diodes when they are conducting (or not conducting, for that matter). Let's say you put the negative on the stud and the positive on the top terminal. Do they all give the same reading (either conducting or not conducting) when you put the leads on all three of them the same way? If so, it's easy--they are all the same. If you find a difference, you will need the one with the different polarity of the three remaining good ones, since the dead one would be the other half of that pair.

    What are you basing the 70 amps on? That feels a little light to me, but may be OK. The direct answer to your question, though, is yes, just find one with a higher voltage rating and go for it. Just be sure it's one with the mounting stud of the correct polarity. You could even go for a higher current rating, it will just cost more. If it's forward resistance is lower than the old one, it might cause the other half of it's "pair" to fail, but that's not a sure thing...try it.
    Last edited by Aeronca41; 01-02-2021, 07:49 PM.

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    • #3
      Originally posted by Aeronca41 View Post
      Only other concern would be for the polarity of the diode. I assume they are stud mounted? Sometimes they have two with the stud as the cathode and two with the stud as the anode. Since you have three that are working, you're in luck. Look at the polarity of your meter leads on the diodes when they are conducting (or not conducting, for that matter). Let's say you put the negative on the stud and the positive on the top terminal. Do they all give the same reading (either conducting or not conducting) when you put the leads on all three of them the same way? If so, it's easy--they are all the same. If you find a difference, you will need the one with the different polarity of the three remaining good ones, since the dead one would be the other half of that pair.

      What are you basing the 70 amps on? That feels a little light to me, but may be OK. The direct answer to your question, though, is yes, just find one with a higher voltage rating and go for it. Just be sure it's one with the mounting stud of the correct polarity. You could even go for a higher current rating, it will just cost more. If it's forward resistance is lower than the old one, it might cause the other half of it's "pair" to fail, but that's not a sure thing...try it.
      Thanks!

      They are stud mounted and you're right...two have the stud as the cathode and two as the anode.

      The machine is 140A max DC output, so that would work for two 70A diodes, or at least it would seem so.

      They appear to be only about $5-6 each, so I'm thinking of replacing all four while I have it apart. I seem to recall reading something along those lines somewhere else....keeps things balanced. Besides, it's not my money for a change

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      • #4
        Great! That's a good price--where did you find them? Agree completely--do 'em all while you're doing it. Save the old ones for spares.
        Now the 70 amps makes sense, too, but that's right on the edge of the spec. Might want to check on what it would cost to go to some rated a bit higher. Good design practice is to always leave some headroom. However the 70 in the part number does lead one to your conclusion.

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        • #5
          Originally posted by Aeronca41 View Post
          Great! That's a good price--where did you find them? Agree completely--do 'em all while you're doing it. Save the old ones for spares.
          Now the 70 amps makes sense, too, but that's right on the edge of the spec. Might want to check on what it would cost to go to some rated a bit higher. Good design practice is to always leave some headroom. However the 70 in the part number does lead one to your conclusion.
          I changed the part number to 70HF40 and immediately got hits from all the big suppliers...Mouser, Newark, Digikey, etc so it must be a more commonly used part. All were roughly the same price, although when I looked up the reverse polarity part some of the suppliers were almost twice the price. I think Newark had the best price with both standard and reverse polarity for just under $6 each...have to go back and check.

          I did a search just going up to an 80A version and the price triples! I guess I see why they didn't go oversized when they made it

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          • #6
            That's really good to know--valuable information. I expected the higher current varieties to be a little more expensive, but not that much! I guess when they have them and you need them, we have no choice but to pay up. I'd heartily agree with the 70-amp units in that case That price is crazy.

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            • #7
              Try Profax they have great pricing on diodes.

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              • #8
                Originally posted by Aeronca41 View Post
                Great! That's a good price--where did you find them? Agree completely--do 'em all while you're doing it. Save the old ones for spares.
                Now the 70 amps makes sense, too, but that's right on the edge of the spec. Might want to check on what it would cost to go to some rated a bit higher. Good design practice is to always leave some headroom. However the 70 in the part number does lead one to your conclusion.
                I went back and checked again and saw the diodes actually all have the same polarity, they're just mounted differently. So, I guess it's just four of the same part...makes it a bit easier. I did a couple of Google searches on the welder model name and actually found a YouTube video of a guy with a slightly newer version of the same machine changing the diodes....the part he listed was the 400V version so it seems like they may have changed over time. We'll see!

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                • #9
                  You can always go with higher voltage or current; just don't go lower.

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