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This should be easy ???

Discussion in 'Electronic Basics' started by TimW, Aug 13, 2007.

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  1. TimW

    TimW Guest

    Hello:

    I need advise on what kind of capacitor I could use to store a 12v
    signal in the following application. I have been a lifelong tinkerer
    but know nothing about electronics.

    I have a piece of audio video equipment (let's call it "A") which
    outputs a 12 vdc signal to another piece of AV equipment (called "B").
    When "B" senses the power signal it turns on.

    During the normal startup sequence for "A", the 12 v signal is
    interrupted for ten to fifteen seconds causing "B" to cycle on - off -
    back on again. It would seem to me I ought to be able to insert a
    capacitor (something from Radio Shack) into the 12vdc positive to
    maintain the signal for 20 seconds or so. I doubt we're talking even
    1/10th of an amp. I have no problem if there is a slight delay for
    capacitor to build up charge initially or a delay later when "A"
    disconnects the signal before "B" shuts down.

    Any suggestions would be most welcome. Thank you.

    Tim W., Wheaton, IL
     
  2. Mikkel Lund

    Mikkel Lund Guest

    TimW skrev:
    We really need to know how much current B draws. Also we need to know
    what the input circuit is, to be able to determine the high/low voltage
    threshold.
     
  3. John Fields

    John Fields Guest

    ---
    View in Courier:

    The simplest way to accomplish your goal would be to do this:


    .. +---[DIODE>]--+------+
    .. |+ |+ |+
    .. [A] [BFC]
    .. | | |
    .. +-------------+------+


    The problem that arises is that for the BFC to supply 100 mA to "B"
    during "A"'s 15 second shutdown and to keep the input voltage droop
    at "B" from falling more than 1V during that 15 seconds, the BFC
    needs to be:


    Idt 0.1A * 15s
    C = ---- = ------------ = 1.5F
    dv 1V

    which, depending on how long the first "ON" of the ON-OFF-ON
    sequence lasts and the amount of current "A" can supply during that
    time might make it impossible to fill the BFC up with enough charge
    to last for 15 seconds.

    BUT...

    It may be that if the ON_OFF_ON sequence is signal only (not power)
    the 100mA is way overkill. Why don't you measure it and post back
    with what you find and that way we may be able to come up with
    something more sensible.

    Some spec's on your AV equipment (or links to data sheets) would
    also be helpful.
     
  4. Eeyore

    Eeyore Guest

    And you'll probably need a diode to stop any 'reverse flow' too.
    Even 100mA results in a fairly unrealistic value of storage cap. You need to
    measure the current drawn.

    Graham
     
  5. TimW

    TimW Guest

    Mikkel/JF/Graham

    Thank you all for replying to my post. I will measure the current draw
    and get back to you. I need to retrieve my meter and it's not at my
    house so it may take me a few days to get back to you.

    I know the signal is definitly not power. Per the AV equipment specs
    (and there's not much to them) unit "B" requires a 5vdc input. Unit
    "A" is quoted as delivering a 12vdc trigger signal. So even if the 12v
    were to drop off during the 15 seconds below a nominal 12v the signal
    should still be seen by "B" as long as it remains above 5v. Both units
    are of course 120vac plug-in for actual power.

    Hope this info helps and like I said I will try to get actual current
    draw measurement. The wiring going into unit "B" is very small guage,
    perhaps #28 or even smaller. Thanks again for your collective
    assistance.

    Tim
     
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