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Switch off unneeded periperals - how?

Discussion in 'Electronic Basics' started by Heinz Liebhart, Feb 15, 2005.

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  1. Hi Folks,

    I'm looking for a solution to switch off not used peripheral components in a
    portable uP system.

    My first thought was ULNxxxx, but, cause they have a rather hi C-E-drop
    voltage, they are not suitable.

    Does anybody have a suggestion how to do this?
    Current is about 20mA...

    I'm thinking on something with a (mos)fet, but I'm not really fit with that
    semi-analog stuff.

    TIA for any hints,

    Heinz
     
  2. If the current was less, you could simply power it from a PIC pin...
    I've done that before. They will source or sink up to 20mA.

    A PNP transistor will give you a pretty good 'high side' switch,
    particularly for such a low current.

    VCC
    +
    |
    1k |
    ___ |<
    PIC -|___|--| PNP like 2N4403 or 2N3906
    |\
    |
    | Output
    '-------------

    Drop is about 0.2V, which is Vce(sat) for the transistor.

    (created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

    Turn it *off* by setting the pin high. Turn it on by setting the pin to
    ground.

    You can use a MOSFET as well. In that case, you don't need a gate
    resistor, and it's possible the V drop won't be as severe. However, make
    sure you get a 'logic level' MOSFET. Otherwise, you won't be able to
    turn it all the way on, and the drop across it will be bigger. Logic
    level mosfets turn on at a Vgs of less than 5V, as specified in the
    datasheet.

    An N-channel mosfet will go between ground and your circuit, a P-channel
    mosfet will go between VCC and your circuit. N or NPN are both active
    high, whereas P and PNP are active low.

    Reed switches may also work, but I don't have any experience with them,
    so I can't help there.

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
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