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small amp

Discussion in 'Electronic Design' started by Flygel, May 12, 2005.

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  1. Flygel

    Flygel Guest

    HI all

    I have made circut B at this scheme.
    http://ourworld.compuserve.com/homepages/Bill_Bowden/printer2.gif

    it somewhat works, and when there is current on base of transistor 1 (from
    the left)
    the output is 9 volts.. witch is great.

    BUT, when there is no base on trans 1 , there is a output of 4 volts..
    what causes this ?

    it makes the relay hit when there is base current, but not release when the
    base current ends :(

    Thanks for any help

    Chris
     
  2. Guest

    Hi, there is nothing wrong with the circuit as published, you need to
    check your wiring.
     
  3. Rich Grise

    Rich Grise Guest

    See what happens if, rather than simply leaving the base lead open,
    add another resistor, say 4.7K to 10K, from the base of the 2N3904
    to ground. I think what is happening is that the collector-emitter
    leakage (with the base open) of the 2N3904 is sinking enough current
    from the base of the 2N2907 to cause what you're seeing. The 2N2907
    can have a beta of 300, so it wouldn't take much, especially if it's
    a sensitive-coil relay.

    A 4.7K to 10K from the base of the 2N2907 to the +12 rail wouldn't
    hurt, either.

    Good Luck!
    Rich
     
  4. Chris,

    If no base current means an open input, the floating base does not block the
    collector emitter current. Not reliably. The base should be tied to ground
    to make the transistor block. Like in all digital circuits an open input
    always should be avoided. If the current to the base of the transistor is
    disconnected by a switch or a plug you can introduce a pulldown resistor
    between de base of the first transistor and ground. it's value depends on
    the available input voltage. I guess 22k for the upper and 47k for the lower
    circuit may be a good starting point.

    petrus bitbyter
     
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