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Testing an old welding rectifier

Discussion in 'Electronic Basics' started by [email protected], Mar 30, 2007.

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  1. Guest

    My uncle recently bought a 550a 3ph welding rectifier with the idea of
    converting his welder to dc. When he tried checking out the bridge it
    conducted both ways.

    So drilled out all the rivets that connected the sections and tried
    testing it one section at a time. Using my DMM on diode check, I drop
    ..3 volts one way and 1.2v the other way.

    The sections remind me of a selenium rectifier if it helps. Could
    some one educate me on what I'm seeing and how to properly test this?
    It sure doesn't test like a normal silicon diode.


  2. Guest

    Been a loooong time since I've had to deal with selenium rectifiers,
    but conduction characteristics aren't anything like a silicon or
    germanium diode. They've got a large forward voltage drop and the
    reverse characteristic isn't anything like completely "off". In a lot
    of battery charger circuits, they were used as a primitive voltage
    regulator in addition to DC-producing duties. That's the reason large
    ones look like giant heatsinks, they're very lossy. Really big ones
    had fan cooling. It's unlikely your solid-state meter puts out
    anything like enough voltage to test a stack properly. I always used
    a scope and hooked the test victim up to AC(read Variac), if they put
    out anything like half-wave DC, they passed. Mostly, when I
    encountered an iffy one in an old TV set, I replaced it with a 10 cent
    silicon diode and a 5 cent resistor. That was back in the days when
    it paid to fix TV sets.

    Usually, failure amounted to a small fireball and letting copious
    amounts of stinky smoke out, somewhere between a garlicky belch and a
    sulfur candle.

    Big amperage silicon diodes are/were readily available on the surplus
    market, your relative would have been far better off buying some of
    those. I've seen 300 amp bridges for $15-20 in the past, google
    should be able to locate some.

  3. Thanks for a good informative post.

    Many years ago as a kid I worked on an older battery charger whose
    selenium rectifier had gone bad. I never could get it to work right
    with just a diode....too high of a charging voltage.

  4. <snippety>

    Others have already created some good advice for fixes.

    I just wanted to mention that, for some bizarre reason, my gray
    matter parsed the subject line as "wedding rectifier."

    Go figure. ;-)
  5. And when they go bad, they REALLY smell bad. Like rotten eggs.
    And about 20 volt drop instead of Silicon's .6
  6. John

    John Guest

    Selenium rectifiers test funny..... Thats the only thing I remember
    about them.... If they smell bad they usually are.

  7. Ray

    Ray Guest

    Where would I look to find 300 amp silicon bridges for a reasonable price ?
    I have googled to no avail...
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