Parallel Bridge Rectifiers?

Discussion in 'General Technical Questions and Answers' started by W4XKE, May 12, 2010.

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  1. W4XKE

    W4XKE Ham Member QRZ Page

    Is it feasible to parallel bridge rectifiers for the purpose of increasing the current capability?

    I need to feed about 25 amps output from a single phase power supply and have on-hand some 8-amp bridge rectifiers. I’ve never seen this done before. (Maybe a good reason why?) Wondering if I can parallel 4 of these to make a 32-amp circuit.

    I’m aware that attempting to parallel a pair of power supplies will end in disaster as one will try to hog the load and burn itself out. Will the bridge rectifiers do the same? Thought I’d ask somebody who’s tried it before I enter into another school of hard knocks.

    Never mind... found the answer and it's just as I suspected. One fellow likened it to paralleling 2 relays. One set of contacts will always close first, taking all the current and burning up. Same with the diodes. Even if they're perfectly matched, it's doubtful they'll stay that way. (Off to Digi-Key for a heftier component.)
     
    Last edited: May 12, 2010
  2. K7JEM

    K7JEM Ham Member QRZ Page

    When you parallel diodes, one will always conduct more current than the other, so it's not a good idea. You might be able to install balancing resistors, but still not the best idea.

    A 25 amp bridge can be purchased for just a very few dollars. I would go that way.

    Joe
     
  3. W4XKE

    W4XKE Ham Member QRZ Page

    Thanks, Joe. You just confirmed what I was told. What is it they say about hams utilizing good engineering practices? :eek:

    Wouldn't it be nice if we could absorb all the knowledge base of these old timers before they go SK? Drat! Each generation has to learn all over. It isn't fair!
     
  4. K8MHZ

    K8MHZ Ham Member QRZ Page

    Fair or not, it sure is fun!
     
  5. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    And please don't forget the "25A bridge" is only "25A" when bolted to a very good heatsink using a thin layer of thermal compound (heatsink grease). The sink does most of the work with these units, because "25A" modular bridge rectifiers actually have 3A diode cells internally, so they can handle about 6A or so before they burn up without a heat sink.

    The heat sink's the trick.
     
  6. WB3BEL

    WB3BEL Ham Member QRZ Page

    And if you need 25A operating current the rectifier needs to be bigger. I'd shoot for 3 to 4X the current rating myself...Luckily they are pretty inexpensive.
     
  7. KA9MOT

    KA9MOT Ham Member QRZ Page

    Like the Quickening on Highlander....with-out all the head removal stuff. When a Ham goes SK the nearest Ham gets his knowledge.

    I'm afraid I'd be out doing the deathwatch thing.
     
  8. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    The old timers all had to learn by either experimentation or by studying what other "older" timers did before them.

    If I recall correctly that used to be called "learning," kind of like the stuff we went to school for.:)
     
  9. WA7KKP

    WA7KKP Ham Member QRZ Page

    An old Indian trick . . .

    Instead of paralleling bridge rectifiers, lead for lead, why not . . .

    Use four bridges, one for each leg. Tie the AC input (cathode to anode) connections across on each bridge. This now makes a single equivalent diode, at twice the PIV and current. Two diodes in series; two in parallel.

    Now, if your PS has a low DC filtered voltage, you might have issues with the added IR drop of the diodes. If you're starting out with 24v or better, you'll have enough headroom to make it work.

    You'll see that in many high current supplies, that the output is rarely over 25-30A. After this point, the power losses are enough to make SCR regulation at the primary voltage a viable, economical solution. Also you'll see a FW CT (2 diodes) instead of the bridge, because of the IR drop across the second diode in conduction. The ultimate goal is to have barely enough DC headroom ahead of the regulator to minimize power dissipation. Any extra headroom usually just goes up as heat . . .

    Just use a good solid connection, and #12 or larger wire for the high current. Don't use sta-kon connections -- they will corrode and burn up after time. Solder everything, silver solder is best. Even a fraction of an ohm here can spell trouble later.
     
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