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Capacitor polarity -- how to tell with confusing markings?

Discussion in 'Electronic Basics' started by larry moe 'n curly, Jan 30, 2006.

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  1. Phil Allison

    Phil Allison Guest

    "larry moe 'n curly"

    ** Tantalums actually ......

    ** Either next to the coloured dot or to the RHS of it.


    All those caps are fucking antiques.

    ......... Phil
  2. Don Bruder

    Don Bruder Guest

    Point the first: Those look a lot more like tantalum caps than any
    electrolytic I've ever encountered. Point the second: According to how
    I learned to count, there are *FIVE* caps in that picture.
    Look for the "longer" lead when the dots are confusing. If there isn't
    one obviously longer, look for the one that's "straightest" coming out
    of the body of the cap - Notice how on the three right-most ones, the
    marked lead is significantly more "looped" than the un-marked lead?

    Which makes me think that the best first-guess for the second from the
    left would be the rightmost lead. The leftmost cap I can't really tell -
    It's cut off too short to try to "read" the leads. However, it *APPEARS*
    that the leftmost lead is going to be the one that's "supposed to be"
  3. John Fields

    John Fields Guest

    Yes, but electrolytics nonetheless.

    What 'electrolytic' denotes is that the dielectric is formed by an
    electrolyte, just as it is in aluminum electrolytics. That's the
    reason why electrolytics are polar; the electrolyte chemically
    forms the non-conducting dielectric film on the 'plates' when
    voltage is applied between them in one direction and removes it when
    applied in the other direction, shorting out the plates.
  4. Chris

    Chris Guest

    Good morning, Dr. Howard, Dr. Fine, and Dr. Howard. The other posts
    have given good advice. However, if all else fails, you can do a
    leakage test like this:
    | .------.
    | | |
    | | 1K 2W| 10K
    | +| .-. ___
    | 12V --- | |<-o--|___|--o--.
    | - | | | | |
    | | '-' | | o+
    | | | | _ |
    | | | | / \ | DUT
    | | | '--( V )--'
    | | | \_/ o-
    | '------o----------------'
    (created by AACircuit v1.28.6 beta 04/19/05

    This test setup works on the concept that, if the Device Under Test has
    the correct polarity in the fixture, leakage current will be very small
    up to the rated voltage of the cap. However, if you plug in the cap
    backwards, leakage current starts increasing rapidly once you get past
    a couple of volts. The 10K resistor limits current to 1.2mA max, which
    should keep the cap from damage, and also provides a means of measuring
    current with the voltmeter.

    Use a high impedance DVM set to the 20V range. Starting at 0V, connect
    the DUT and slowly bring up the voltage at the wiper of the pot. Look
    for the voltage across the 10K resistor to increase if the cap is
    hooked up with the polarity reversed.

    Be careful not to raise the applied voltage any higher than necessary
    to see significant reverse leakage current across the 10K resistor (1V
    or so). While the 10K resistor limits reverse current to no more than
    1mA or so, tantalums generally don't like reverse voltage, so don't
    crank up the pot any higher than necessary. Turn the pot voltage down
    before removing the DUT.

    If you get leakage current going both ways, the cap is bad, and should
    be thrown away (thos caps look pretty ancient).

    Don't try this without the 10K resistor, or you'll end your test by
    explosively determining that the cap is _now_ bad.

    For Duty And Humanity!
  5. what are these called?? Frog caps?
  6. Phil Allison

    Phil Allison Guest

    "John Fields"
    "Phil Allison"

    ** But "tantalum " is the distinguishing NAME used in the trade.

    The ones in the pic are "solid tantalum" ( aka "bead tantalum" ) types which
    have a dry electrolyte ( manganese dioxide) unlike "wet" tantalums or
    aluminium electrolytic capacitors.

    .......... Phil
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