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Using transistors as SPDT switch?

Discussion in 'Electronic Design' started by Jon Danniken, Nov 26, 2006.

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  1. Jon Danniken

    Jon Danniken Guest

    Hello,

    I would like to build the switch in the following circuit using small signal
    transistors:

    5V +-------+ +---- GND
    | |
    | |
    | |
    o /o
    /
    /
    o
    |
    |
    .-.
    | |
    | | LOAD
    '-'
    |
    |
    |
    12V ---------+

    Any tips on how to go about this with a few discretes would be most
    appreciated.

    Thanks,

    Jon
     
  2. John  Larkin

    John Larkin Guest


    It's easier to visualize stuff like this if it's drawn with the
    current flowing down, namely with the +12 at the top.

    John
     
  3. Jon Danniken

    Jon Danniken Guest

    Sorry 'bout that. Thanks for the help; that is a very nice and simple
    solution.

    Thanks,

    Jon
     
  4. In this circuit, it looks like current passes through the load in both
    the ON and OFF positions. Assuming an N-channel enhancement mode FET:

    When switch is closed with application of Vg > Von to gate, FET does
    not have opportunity to conduct do to exponential characteristic of
    diode limiting drop to ground to approximately 0.7v, so current through
    load is [12-Vdiode)]/Rload.

    When switch is open with application of Vg < Von to gate, FET is off,
    and current through load is [12-Vdiode]/Rload.

    Assuming NPN BJT insted of NFET, you would also have to limit current
    through the base-emiitter diode.

    Depending on your load, which probably has relatively high resistance,
    I don't see what a FET with a low Rds(On) wouldn't work.

    -Le Chaud Lapin-
     
  5. Jim Thompson

    Jim Thompson Guest

    Just pretend you're a Physicist ;-)

    ...Jim Thompson
     
  6. John  Larkin

    John Larkin Guest

    When the fet is on, the diode is back-biased and out of the circuit.
    The fet saturates as well as it can, so current is (12-Vsat)/Rload or
    maybe 12/(Rload+Rdson).
    No, the current is (7-Vdiode) / Rload


    The 5 volt supply does need to be able to sink Iload.

    John
     
  7. I thought the same thing until I noticed that the OP has a GND
    separating the 5-volt branch from the rest of the circuit, which if
    meant to be present, would make that part of circuit irrelevant, hence
    chance for misinterpretation.

    I guess it would help to know whether the GND was meant to be there or
    not.

    -Le Chaud Lapin-
     
  8. John  Larkin

    John Larkin Guest


    Or an arabic engineer, with signals flowing from right to left!


    I like to draw with positive supplies high on the page, and current
    flowing down. That places PNP emitters up and NPN emitters down.
    Signals flow to the right, feedback to the left.


    John
     
  9. John  Larkin

    John Larkin Guest


    I think it's a font problem. I think he meant...


    5V +-------+ +---- GND
    | |
    | |
    | |
    o /o
    /
    /
    o
    |
    |
    .-.
    | |
    | | LOAD
    '-'
    |
    |
    |
    12V ---------+



    John
     
  10. Jon Danniken

    Jon Danniken Guest

    Yes, that is what I had meant to paste in originally, thanks.

    Jon
     
  11. Jim Thompson

    Jim Thompson Guest

    Same here, actually. I was just yanking your chain ;-)

    I'm really picky about schematic format... I like to draw so that it's
    quite obvious how signals flow and how circuits work.

    So I spend quite a lot of time cleaning up drawings to avoid clutter
    and confusion.

    ...Jim Thompson
     
  12. An n-channel enhancement-mode FET pesents a conducting path when a
    positive voltage is applied to its base and presents an open circuit
    when the applied voltage is zero.

    A p-channel depletion-mode FET presents an open circuit when a positive
    volage is applied to its base and presents a conducting path when the
    applied voltage is zero.

    Take these two devices, tie their drains and together and attach the
    drains to the oad. Tie their bases together to receive control signal.
    Tie one source to 5V, the other to GND.

    -Le Chaud Lapin-
     
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