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Backwards cap

Discussion in 'Electronic Design' started by D Yuniskis, Dec 6, 2010.

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  1. D Yuniskis

    D Yuniskis Guest


    I was reverse engineering the charger for an
    air mouse and noticed a 1u (50V) electrolytic
    installed *backwards* at the input. I've checked
    this twice to make sure I wasn't tracing foils
    improperly. I don't know, for sure, what it is
    *intended* to see across it though the battery
    in question is ~3V and the line is protected
    with a 15V TVS -- I'm guessing around 6V based
    on other components in the signal path.

    Is there any *intended* reason for this? Or,
    is it just a stuffing error?
  2. Did you actually measure the voltage across it?

    Probably just a stuffing error.

    A 50V e-cap could last forever at -3V but I wouldn't bet the farm on
  3. D Yuniskis

    D Yuniskis Guest

    Hi Spehro,

    It sees whatever the input voltage (wall wart)
    supplies to the charger. I.e., it is *at*
    the input (a diode drop below the input voltage).
    Since I don't *know* what the input voltage is
    supposed to be (other than the constraints imposed
    by the other components in the circuit), I can't
    guess as to what is "supposed" to be across the
    That's what I was thinking. But, wondered if there
    wasn't some bizarre "trick" involved...
    Well, it will see at least that much as the battery it
    is driving (looks like a simple series resistor) is
    about 3V. I'm guessing it has 5-6 volts across it
    (it currently does as that is what I am using for
    a wall wart currently). But "never" more than ~15
    (owing to the TVS).

    <shrug> It's lasted this long, I'll just leave it
    there instead of risking breaking the case trying
    to get it open again.
  4. Sjouke Burry

    Sjouke Burry Guest

    Well, wont any electrolytic cap reverse its polarity, when
    you feed it reversed very gently for a long time,limiting
    current to a trickle?
    There will of course be a period, where it does not act as a cap....
  5. D Yuniskis

    D Yuniskis Guest

    Sjouke Burry wrote:

    [attributions elided]
    Directly across the output of the wall wart hardly seems
    like it will see "a trickle"... :>
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