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Ask an Expert Forum Why is D5 failing?
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Why is D5 failing?

inavitech
inavitech over 1 year ago

image

Project I am working on. I'm not a board designer. Box integration is more my thing. This circuit is needed for providing a 13VDC output. Input power is 28VDC. When 28VDC is applied the power supply voltmeter drops to 6VDC, and it goes into current limiting mode. Current output is maxed out at 5 amps. D5 starts to look like a well done hotdog on the grill and begins to smoke. There were other parts of this circuit in the original design that are not being utilized, but as I said I'm not a board guy. I think with the removal of the unnecessary portion, the current is too high, thus R15's value is too low now. Figured maybe one of you on the forum might have the answer. So how this works is the tester utilizes 28VDC to test a battery pack. All of the other functions work properly when removing this part of the circuit from the equation. There is a test for a particular circuit in the battery that requires 13VDC to verify proper operation, so I can't ditch this portion of the tester. Maybe there is a better way to do this?

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  • inavitech
    inavitech over 1 year ago +4
    Well, I have it figured out. It is Q1. I looked at the spec sheet again and looked at it's installation on the prototyping board and the collector and base are swapped. So that is probably why the current…
  • dougw
    dougw over 1 year ago +3
    Was this circuit ever working, or was it just assembled and never worked? The connections for U1 don't make sense, but are not likely the problem. If the collector and emitter of Q1 are reversed, The…
  • JWx
    JWx over 1 year ago +1
    could Q1 be damaged (C-B shorted for example)?
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  • dougw
    0 dougw over 1 year ago

    Was this circuit ever working, or was it just assembled and never worked?

    The connections for U1 don't make sense, but are not likely the problem.

    If the collector and emitter of Q1 are reversed, The emitter could breakdown and cause excessive current through the base and D5, if this high current burnt D5 to a short, you could be left with the breakdown voltage of the emitter, and high current....

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  • dougw
    0 dougw over 1 year ago

    Was this circuit ever working, or was it just assembled and never worked?

    The connections for U1 don't make sense, but are not likely the problem.

    If the collector and emitter of Q1 are reversed, The emitter could breakdown and cause excessive current through the base and D5, if this high current burnt D5 to a short, you could be left with the breakdown voltage of the emitter, and high current....

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  • JWx
    0 JWx over 1 year ago in reply to dougw

    good diagnosis - Ueb of TIP122 is about 5V, which explains 6V observed...

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  • dang74
    0 dang74 over 1 year ago in reply to dougw

    You're instincts were spot on.

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