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Diodes in parallel

Discussion in 'Electronic Basics' started by Jack// ani, Feb 5, 2005.

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  1. Jack// ani

    Jack// ani Guest

    Hi there,

    Can I increase the current rating of my psu which is having 1N4007
    diode bridge rectifier, to 2amps by placing similar diodes in parallel
    to each diode in the bridge rectifier? Actually my pcb is not having
    enough room for 1N540X series diode, but I can easily put 1N4007 diodes
    in parallel!

    Thanks for any help
  2. sparky

    sparky Guest

    Just increasing the diodes will not increase the capacity of the psu.
    You must also increase the rating of the other components.
  3. Whether the power supply can provide more amperes is not just
    dependent on the diodes. But to address the diode question,
    paralleling with a similar type will allow a bit higher current
    without destroying the diodes, but not twice as much. The diodes will
    not share the current exactly and each will heat the ambient air near
    he other, so each will get hotter for its share of the current than if
    it were operating at that current without its partner near by.

    Even alone, a 1 amp diode is not normally expected to deliver 1 amp
    average, except under the most ideal of conditions (leads heat sunk to
    25 C a short distance from the body, for example).
  4. Don Bruder

    Don Bruder Guest

    This fellow's question brings to mind a similar concept I had over on an
    automotive group - Alternators are fairly tough little beasties - Most
    of the guts of them are all but indestructible short of physical damage.
    But a very common failure mode for them is for the diode trios to barf
    if/when they're asked to try to charge a mostly-dead battery.

    I had the thought not too long ago that paralleling multiple diodes per
    "trio position" should give at least some added "anti-fry" protection in
    terms of how dead a battery the alternator can charge without burning
    itself (or more specifically, its diode trios) up.

    Since it's such a common failure mode, and cheap to repair (compared to
    "buy a new/reman alternator and replace as a unit") the benefit would
    potentially be huge to the person doing such a mod to their alternator.

    Now, it sounds like you're saying that rather than improving the
    situation, such a setup would likely be an actively BAD thing?

    Is the "extra heating" issue still a significant problem in a forced-air
    environment such as what I'd have in my particularly alternator? (diode
    trios in it are heat-sinked, and are the first thing that the air, being
    actively pulled in through the back of the alternator by its built-in
    fan, encounters)

    It would seem to me, at least at first glance, that "stacking" multiple
    diode trios (let's say to three diodes per phase and polarity, for a
    total of 18 discrete diodes (versus the standard 6 -- two, one for each
    polarity, on each of three phases)) in this beast would be beneficial,
    if only in terms of "Well, that one over there cooked, but I'm still
    doing fine, and so is the one next to me" style backup - should one
    fail, the remaining two for that phase pick up the slack. (Of course,
    with such an arrangement, if there's an overload big enough to blow one
    of the diodes, I think it unlikely that the other two would survive as
    they were forced to pick up even more of the load that managed to burn
    out one of the three already...)

    Which doesn't even start to address the whole "Since three of them are
    sharing the load (however unevenly they might be portioning it out
    amongst themselves) there should be little or no reason for one to fail
    in the first place" concept...

    Gimme a sanity-check on this idea, would ya, John?
  5. I didn't say anything about a bad thing. I said that two 1 amp diodes
    in parallel and also in close proximity should not be expected to
    deliver 2 amps. They will deliver somewhat more than one diode would
    be capable of, just not twice as much.
    Forced air definitely redefines close proximity.
    Blown diodes are normally short circuits, so they will not take turns
    as you describe. A better way to protect the alternator might be to
    add a resistor between the regulator and the armature winding, so that
    the excitation current was limited to a value slightly less than the
    design maximum. Since the alternator normally uses way less than the
    maximum possible excitation, this would affect its operation only at
    lowest speed and during overload situations.

    I am working on a new regulator for my motorcycle that includes an
    adjustable current limit for the excitation, but for a completely
    different reason.
    There would be less heat in any one of the parallel diodes than if
    only one were carrying the whole current. The problem is that the
    hottest diode tends to have the lowest forward drop and hog most of
    the current, making sharing worse. So all sets need similar heat
  6. Don Bruder

    Don Bruder Guest

    OK, clarification noted. So does it make sense to expect that
    "unloading" any single diode by splitting the load across (as per my
    previous example) two more diodes wired in parallel with it would have a
    positive effect as far as "diode-death prevention" is concerned?

    Basically, I'm working under the concept of "A draw that would kill a
    "lone" diode should (generally speaking, and for the sake of discussion,
    ignoring the possibility/effects of uneven load division/thermal runaway
    that you mention later on) be insignificant when three diodes are
    splitting the load."

    Do I need a new concept?
    Kinda figured it would :) Seems to me that heat would become (at least
    for practical purposes) a non-issue in such a situation.
    Hmmm... not a concept that I had thought about - That would tend to
    throw a bit of a wrench in the works, wouldn't it? Again, though, it
    comes back to the whole "Shouldn't the load that would blow a single
    diode be seen as effectively insignificant by three similarly
    rated/heat-sinked/etc diodes wired in parallel?" - If the diode is
    unloaded enough by paralleling multiple "copies" of itself in its
    position in the circuit that even under ridiculaous loading, no single
    diode of the group should fail in the first place, it's pretty much
    irrelevant that a given diode "usually fails shorted" when it dies, no?
    May I be so bold as allow my curiousity to run wild long enough for me
    to enquire about what this completely different reason is?
    Ahhh, yes... Good ol' thermal runaway.

    Makes good sense, now that I consider it. That would seem to indicate
    that one would want to carefully match the diodes being used to
    construct the "trio set" - A task with the potential to be anywhere from
    "trivial - I did it three times while sleeping last week" to "outright
    impossible", I imagine...
  7. Jamie

    Jamie Guest

    since paired diodes need to be match perfectly to insure both share an
    equal load it's not practical.
    one diode may start conducting before the other, and this happens the
    other diode normally does not hardly even or at all conduct. thus one is
    doing all of the work.
  8. A positive effect, yes. A multiple of the current capability equal to
    the multiple of diodes, no.
    No. Paralleling can be some use. But a bigger diode and more
    effective heat sinking may do better.
    If paralleling prevents failure, then, yes, you don't have to deal
    with failure.
    I have a motorcycle that has its idle speed loaded variably by the
    alternator. If I adjust the idle when the battery is under heavy
    charge, then it races after the battery is fully charged. If I adjust
    the idle when the battery is fully charged, it has trouble stalling
    the next time the battery needs charge. I am going to make the
    regulator a switcher for improved efficiency and include a current
    limit so that the low RPM maximum torque load can be reduced. I also
    expect it to keep the battery more accurately charged, compared to
    what the electromechanical regulator accomplishes, now.
  9. JanRwl

    JanRwl Guest

    No. Minute differences between the diodes will cause one to handle all or
    nearly all of the current. The "parallel second diode" will not conduct much
    at all.
    Use 1N5402 diodes, even if you have to "stand 'em up" off the PCB. Some clever
    fiddling should get 'em all-four in place!
  10. Don Bruder

    Don Bruder Guest

    Right. Got that part. Pessimistically assuming that you'd actually LOSE
    some capacity to the paralleling (For the sake of discussion, let's pull
    a number out of the air and say each diode would only handle 75% of its
    rated load) that would still give 125% more "head space" over that which
    is available from a single diode in that position when the group is
    considered as a unit. One would HOPE that an alternator rated for 55
    amps is never going to be asked to supply nearly 125! Of course, should
    that ever happen with the "ganged" diodes, the failure point may well
    end up being the coil cooking open, rather than a diode frying...
    No argument there. "How much better, and for what price - in both
    dollars and difficulty?" probably becomes an issue quickly. As of yet, I
    haven't figured out what the price on one of the diodes from the trio
    would be. (For that matter, I haven't yet figured out what the installed
    diodes actually are.) I would expect that, as is usually the case when
    messing about with rectifiers and regulators, moving up in current
    capacity implies moving up to a bigger price tag.
    Sounds like a fun mess with the idle... NOT!

    Electromechanical? As in the old-style "solenoid and gap" type? Oi,
    vey... those things always were a mess. I gotta give 'em credit - They
    worked amazingly well. But keeping them working correctly... Wotta
  11. Ray

    Ray Guest

    Listen to Nike, just do it. You "can easily put the diodes in parallel" so
    do it and take current reading thru both your 1n4007 diodes at the same
    time to see if one conducts more or not. Post your results, sounds
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