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Why extra Diode?

Discussion in 'Electronic Components' started by Watson A.Name - \Watt Sun, the Dark Remover\, Aug 15, 2004.

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  1. I was examining a defunct ATX PS that had a Really Bad Day; two of the
    electrolytics exploded and unwound the aluminum foil and sprayed
    dielectric and other guts all over the resr of the components.

    Anyway, I found this arrangement:

    Main o------|<-------+
    xformer |
    of an |
    ATX PS +-----|<-----> to inductor
    | and filter caps

    All diodes are FR104, Fast Recovery 1 Amp, 400V or something like that.

    Why would they use such an arrangement? Maybe to get a bit of voltage
    drop? Just curious.

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  2. Mark (UK)

    Mark (UK) Guest


    That reminds me of an incident that happened in my first job. I was a
    trainee building prototypes of power supplies, and had just finished
    building the first of this new huge switch mode thing with 48v at loads
    of amps output.

    Anyhow, so I proudly plugged it in and turned it on - there was small
    pause as the lights flickered in the lab, then BANG - the mains
    rectifier electrolytics deposited their entire contents over me, the
    ceiling and anything within range - turned out I put them in backwards :)

    I never did that again.

    Yours, Mark.
  3. Jamie

    Jamie Guest

    start up voltage.
    some switchers require a min voltage to allow for its feed back
    voltage in the coil to get started. the inrush current on the caps
    will not allow for enough voltage to be present so a Diode is a
    good idea to use since it will give you a approximately .6 volts of a
    putting both together you would end up in the range of 1.2 volts
    on the average of at least 1.2 volts to be present at the coils on
    start up when the caps are in initial inrush.
    anything below that would constitute a short in the diodes which
    maybe the reason why your caps exploded it the start up didn't
    have a time delay shut down from the voltage not reaching at least its
    50% mark.
  4. Rich Grise

    Rich Grise Guest

    Mark (UK) top-posted:

    I was trying to learn how to build a switcher from some chip's data
    sheet. I was working with ferrite vendors as well as the chip vendor,
    and decided on a core, bobbin, turns, and all that. So, I've got this
    cup core or pot core, I'm not sure which it's called - it's the one
    with two cylindrical halves and a hole through the middle.

    So I've got this carefully committee-designed transformer, about to
    fire up this unit at 1MHz, everybody's nervous - I'm cranking up the
    variac (this is when I learned that it's hard to ramp up the input
    of a switcher and have it behave predictably), and realized, too late,
    that I had forgotten you're not supposed to fasten a cup core
    transformer down with a steel screw. FzzzzzPow!

    I never did get a switcher designed, or even built. After about three
    months, and I don't know how many thousands of the client's dollars,
    we gave up. We concluded, in 1990, that it's impossible to fit a 300W
    power supply into 75 cubic inches.

    Some things have changed, but they probably still say you're not
    supposed to use a steel screw to fasten down a pot core. ;-)

    Ironically, the guy had a brass screw exactly the right size on hand.

  5. Pooh Bear

    Pooh Bear Guest

    Quite possible. You can only really regulate the DC voltages from one output
    meaningfully - or maybe cross-couple 2 perhaps.

    The output voltages on other windings will then be set by turns ratios ( and
    diode drop voltages).

    As it's just a 1 amp output it's probably simplest to drop a bit of voltage
    this way.

    I've seen designs that selectively use Schottkies with their lower voltage
    drop to get accurate voltage outputs too.

  6. SMPS supplies can have quite a high voltage/turn and fractional turns
    are a hassle.

    Best regards,
    Spehro Pefhany
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