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PS linear regulator issues

Discussion in 'Electronic Basics' started by Jon Slaughter, May 11, 2007.

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  1. A PS I build a while back was screwed up yesterday when I was messing
    around. Its a variable split PS using 317's with, I think, about +-35V that
    it handles(20 somethign for the regulators).

    I'm not exactly sure what happened(well, I think I know) but in any case
    what happened after was that the voltage would read either about 2 voltage
    when the turned down or around 25+ volts when up.

    I think I fixed it as I replaced one of the protection diodes(the one from
    that goes from output to input). (I figured it was the diode since thats
    what there for). In any case it seems to be working but I'm not sure if any
    other damage happened.

    When I built the circuit I didn't design it to be able to remove components
    easily(didn't have the room in the box). I thought that the protection
    diodes were there to protect the 317(even though it has its own protection)
    but I didn't know it would destroy the diodes in the process. Is this
    normal? (if it did I would have made it easier to replace them).

    Also, in the case that the protection diodes do go does that completely
    eliminate(99%) any other things in the regulator circuit from being ruined.
    It wouldn't be fun to check the other protection diode I have or the 317.
    (since I probably would have to take them out of circuit to be completely

    I'm worried about the other diode but really don't want to try and take it
    out to check it and since the PS looks like its working.

    (BTW, I think I essentially shorted the negative supply to ground and that
    is what caused it... but on the positive side ;/)

    What do you guys think I should do?

  2. Chris

    Chris Guest

    Hi, Jon. From what you're saying, I'll bet you've got the D1s but not
    the D2s shown in the ASCII diagram below (view in fixed font or M$

    | .---------|<-----------------.
    | | D1 |
    | | .-------------. |
    | | | | |
    | V+o----o---o o-----o----o
    | | | + Regulator | | |
    | + | | | + | |
    | C --- | | C --- D2-
    | --- | | --- ^
    | | '------o------' | |
    | | | | |
    | GNDo----o----------o------------o----o
    | | | | |
    | + | .------o------. + | |
    | C --- | | C --- D2-
    | --- | | --- ^
    | | | | | |
    | | | - Regulator | |
    | V-o----o---o o-----o----o
    | | | | |
    | | '-------------' |
    | | D1 |
    | '--------->|-----------------'
    (created by AACircuit v1.28.6 beta 04/19/05

    A lot of funny things can happen with dual supplies on turn-on and
    short circuit conditions. Many of these have to do with wiring
    inductance causing unexpected things to happen, some with the
    transformer at the input.

    Just put a couple of 1N5402 6 amp barrel diodes across the outputs of
    your power supply (put 'em right at the banana jacks, inside the box),
    and this should keep your problem from recurring. If your D1s are
    1N400X, you could also upgrade those to 1N540X, also. Just clip the
    plastic body of the diode, and solder the heavier 1N5402 diode lead to
    what remains of the 1N400X lead on the circuit board. Be sure to use
    a clip on the smaller lead to prevent the solder under the circuit
    board from melting and spoiling your quick fix. Remember, your output
    caps aren't current limited to 1.5A like the ICs. Your power supply
    will be a lot more loyal if it knows you've spared no expense to
    protect it from itself.

  3. No, I have both(which I mentioned but maybe wasn't clear). I haven't
    checked them both though. There are a total of 4 diodes but I was just
    refering to one side of the supply since the other seemed to work fine.
    Thanks, I'm designing another power supply that has a bit more current to it
    and I'll try to keep this stuff in mind next time I do it.

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