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PNP transistor as a high side switch (2N3906) problem

Discussion in 'Electronic Basics' started by Paladiamors, Aug 27, 2004.

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  1. Paladiamors

    Paladiamors Guest

    Hi there,

    I have a problem using a PNP 2N3906 transistor as a high side switch. I'm
    plugging a 18V line to the emitter, with a 3.3k pull up resistor from the
    18V line from 2 9V batteries to the base of the transistor. The collector of
    the transistor is going to a 12V power regulator (so the transistor is
    acting as a high side switch). The base is eventually going to be pulled
    towards ground through a 1k resistor using a switch (later it's going to be
    connected to the serial port of a computer using an extra transistor to pull
    the base down).

    The problem I'm facing is that the base is some how being pulled down (I'm
    seeing a Vbe of about -0.5V) and there is about a 9V signal on the
    collector. When I short the base and the emitter together, I still see a 9V
    signal on the collector. When I pull the base down for a Vbe of about 0.8V I
    get the full 18 V on the collector. Has anyone experienced this problem
    before? Am I using bad transistor model (I tested 2 different transistors
    and got the same problem)? Is my circuit flawed?

    I've modeled the circuit in circuit maker and circuit maker is telling me
    that the theory is correct, that the transistor should work in this
    situation. Any advice would be appreciated.

    Thanks

    Paladiamors.
     
  2. CFoley1064

    CFoley1064 Guest

    Subject: PNP transistor as a high side switch (2N3906) problem
    First, check your wiring. Looking at the 2N3906 from the front (flat) part, it
    looks like this:

    .-----------.
    | |
    | |
    | |
    | 2N3906 |
    | |
    | |
    | |
    '-o---o---o-'
    | | |
    | | |
    | | |
    | | |
    | | |
    E B C
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

    You've probably either got your wiring mixed up, or hooked up something
    incorrectly before and fried the transistor. Make sure you've got a
    ground-referenced load on the collector of the PNP when you're testing. It
    should look something like this:

    2N3906
    .--------o----- ------------.
    | | V / |
    | | --- |
    | | | |
    | | o |
    | | | |
    --- .-. | |
    - 3.3K| | | |
    | | | | |
    --- '-' | .-.
    - | | R | |
    | '------o | |
    | | '-'
    | .-. |
    | | | |
    | 1K| | |
    | '-' |
    | | |
    | | |
    | o |
    | '\ |
    | \ |
    | o \ |
    | | |
    | | |
    '---------------o-------------'
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

    Good luck
    Chris
     
  3. Paladiamors

    Paladiamors Guest

    Hey, thanks for the suggestion, turns out that I had the transistor
    backwards :) How embarassing :p

    Paladiamors
     
  4. Jamie

    Jamie Guest

    well it only takes aprox -.6 V to bias the pnp
    try leaving the Base floating and check that.
    it could be your getting so current some where
    if that is the case maybe a diode to the base will
    help since it will take a min of .6 before it starts to
    conduct.
    you may want to consider a board leakage problem?
     
  5. John Fields

    John Fields Guest

     
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