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Transistor dead again

Discussion in 'Electronic Basics' started by [email protected], Apr 23, 2007.

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  1. Guest

    I am trying to convert a pushbutton to a toggle switch. The circuit is
    working fine for a few tests, then I turn off the battery, come back
    the next day and it's not working any more. Replacing the transistor
    Q1 fixed it once but now it's dead again which makes me think that
    there is some underlying problem.

    The light bulb is draining 60mA at 12.5V.

    | .-. | |
    | | |10k .-. --- +13V
    Q2 | | | ( X ) -
    \| '-' '-' |
    2N2222 |-------o---------------------o | |
    <| | ___ | | ===
    | | 2N2222 .--|___|----)------o GND
    | \| | | |
    | |-------o 330k | |
    /+\ Volt <| Q1 | | ||-+
    ( )meter | | | ||<-
    \-/ | | '---||-+ NTE67
    | | | |
    | .-. .-. |
    === | | | | ===
    GND 4M| | | |10k GND
    '-' T '-'
    | --- |
    o--o o----'
    --- 220nF
    (created by AACircuit v1.28.6 beta 04/19/05
  2. john jardine

    john jardine Guest

    Is the sketch right?. Seems it powers up and the bulb must come ON.
    The 4meg and "T" in the right place?.
  3. Ross Herbert

    Ross Herbert Guest

    I am not sure if the circuit is correct, but assuming it is, then Q1
    is not likely to take a hit, but Q2 is - if that voltmeter connection
    were to experience a momentary short circuit.
  4. Guest

    Oops. There are mistake here. Q1's emitter goes to ground and the 4.7M
    is connected to the collector instead. It's based on the third circuit

    The voltmeter is actually a voltage comparator, but I ran into
    problems just connecting a 10k in its place. Initially the FET is off
    and thus minimal current goes through the lamp. Briefly toggling Q1
    then raises the gate voltage and turns the light on. At least that's
    what it did yesterday but not any more..
  5. john jardine

    john jardine Guest

    It's a nice circuit arrangement and will work fine.
    There's no way that Q1 can be blown by overvoltage or overcurrent. Sounds
    like there's a wire shorting somewhere.
  6. Could it be happing when you remove the power? It seems that if its working
    while the power is on and if theres no power then it can't fail(except maybe
    caps but doubtful) then it must happen when the power is changed.

    You might try running it with the power on for a while and put it through
    some stress test and see what happens. If it lasts for a long time then try
    turning the power off for a split second and then put it through another
    test for a few mins. Repeat this a few times. Then turn it off for a longer
    period of time and try. You'll be able to determine which stage the problem
    is comming from.
  7. Guest

    Never mind. It wasn't the transistor that was dead (again) but a
    partially broken trace causing the problem.
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