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Is this capacitor polarized?

Discussion in 'Electronic Repair' started by mm, Feb 20, 2007.

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  1. mm

    mm Guest

    I have to replace a small capacitor that had a lead ripped out when
    the tv was dropped and the pcb broken.

    It's a cylinder about a half inch high, a quarter inch in diameter,
    with two leads at the bottom, and a narrowing near the bottom, like a
    waist line, but it gets back to full diameter at the bottom.

    It's black and says on it:

    extra info
    TL [in an elongated circle]
    50v4.7uF --

    CD71
    40/085/10
    N --

    There is lead coming out next to the N and next to the 4.7uF.

    Does the N mean that that is the negative lead?

    If not, does that mean the capacitor is non-polarized?

    BTW, what happens if I use a polarized cap where a non-polarized was
    intended, or vice versa? Will a reverse voltage puncture the
    dielectric? Even if it is less than 50 volts, like the rating here?

    Thanks.

    If you are inclined to email me
    for some reason, remove NOPSAM :)
     
  2. Arfa Daily

    Arfa Daily Guest

    A capacitor of this value and voltage rating could be either polarised, or
    not. Usually, a polarised type is quite clearly marked, but an alternative
    give-away is the PCB silk screening where it's located. If it is a polarised
    type, there is a 99% certainty that one of the legs will be marked.
    Sometimes the board will be marked with a " + " sign, and sometimes, there
    will be a circle with one side shaded. The shaded side normally indicated
    the " - " leg of the cap, and corresponds to a stripe on its body. The ESR
    figures for polarised and non-polarised electrolytics are usually quite
    different, which could cause a problem if substituting one type for the
    other, depending on the cap's circuit function. Also, if the cap is being
    subjected to a high level of AC across it, which is why non-polarised types
    are usually used, then a polarised type will probably not stand up to it for
    very long, although in saying that, standard polarised electrolytics are
    often used in the VFD supply multipliers in hifi's and although they do fail
    because of the reverse stresses across them, they do last quite a long time.

    Arfa
     
  3. Franc Zabkar

    Franc Zabkar Guest

    All my Googling suggests that "CD71" is a bipolar or non-polarised
    aluminium electrolytic. Yours seems to be a standard temperature type,
    ie -40C to +85C. The "10" may refer to the tolerance.

    - Franc Zabkar
     
  4. mm

    mm Guest

    Thanks. OK, there are no polarity markings on the pcboard, so I
    guess it is not.
    The cap is on the other side of a resistor that is near the flyback,
    and the other side of the cap goes to the pcb ground. I wish I could
    say more than "near the flyback" but it's very hard to see. Later
    today, I'll try to unscrew and move the pcb from the plastic frame
    it's mounted to.

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  5. mm

    mm Guest

    Thanks. I didnt' realize CD71 would be the part number.

    Since I need 4.7uF, I have two polarised 50v 2.2uF caps of the same
    appearance as the one I need to replace, that I was going to connect
    in parallel, with the negatives connected together.

    If instead I connected them in parallel with each negative connected
    to the other's positive, would that give me the equivalent of a
    non-polarised cap?

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    for some reason, remove NOPSAM :)
     
  6. Arfa Daily

    Arfa Daily Guest

    I guess that the only way that you're ever going to know for sure, is with a
    set of schematics and a reference number for the cap. No markings on the
    board would suggest a non-polarised type, but the fact that one leg goes to
    ground would suggest otherwise. As it's near the flyback, it could be
    decoupling for a rail, but its value seems a little small for that, and the
    value of the resistor would have to be pretty low if it was a flyback
    derived rail. Could be a pulse integrator for use by some other piece of the
    circuitry, but that also would not require the cap to be non-polarised. Bit
    of a mystery really.

    Arfa
     
  7. Arfa Daily

    Arfa Daily Guest

    To "make" a non polarised electrolytic, you have to connect two
    electrolytics of twice the required value - in your case that would be 10uF
    each - in inverse series. Usually, you connect the two ' - ' legs together
    leaving the two ' + ' legs to go to the outside world. Each cap need only be
    half the working voltage of the original non-polarised in theory, but it's
    not a bad idea to make each one the same as the original as, depending on
    what exactly is across the cap, you can't guarantee that the voltage will
    divide equally between them. However, all that said, non-polarised caps are
    readily available, so unless you are really stuck for obtaining one, it's
    better to fit what was originally there.

    Arfa
     
  8. PeterD

    PeterD Guest

    In series, either positive to positive or negative to negative (back
    to back.) You need to compute the value that results (formulas and
    calculators abound on the web...) but with just two, you'd end up with
    half the capacitance so you need a couple of 10 uF caps (each rated at
    50 volts).

    However, series back to back capacitors are not (IMHO) a good
    solution--better to go out and get the right part instead.
     
  9. mm

    mm Guest

    I remember that now from 10 or 20 years ago, although I never knew the
    details you give below.

    Thanks to you and Peter.

    One more question for now. Like I said, the tv must have been droped,
    even though there is no damage on that part of the case. I'm
    soldering jumpers across all the broken traces.

    But my supply of spare caps is small, I guess because I only inherited
    a few and there are so many possible kinds to have.

    So it will take me a while to get this one little cap. Can I apply
    power to the tv without damaging anything if I have nothing where the
    capacitor under discussion should go?

    Or a bigger or smaller non-polarised one temporarily?



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  10. bz

    bz Guest

    Should there not also be a couple of diodes, one across each capacitor, to
    prevent applying reverse voltage to either?

    I seem to remember that electolytic do NOT like reverse voltage applied. It
    removed the 'insulating layer' that has been electroplated on one of the
    plates (that also serves as the dielectric). This tends to lead to high
    current flow and breakdown. Since there is another capacitor in series, the
    current will be limited, and the dielectric may reform when polarity is
    right again, but I would not expect it to be very reliable.

    http://yarchive.net/electr/electrolytic_caps.html
    has some interesting information.







    --
    bz 73 de N5BZ k

    please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
    infinite set.

    remove ch100-5 to avoid spam trap
     
  11. Arfa Daily

    Arfa Daily Guest

    Personally, without knowing the purpose of the cap, I wouldn't go ahead and
    apply power. If it was in some kind of snubber circuit for instance, the
    effect of it not being there might prove catastrophic to some following
    semiconductor

    Arfa
     
  12. mm

    mm Guest

    OK. Thanks.

    If you are inclined to email me
    for some reason, remove NOPSAM :)
     
  13. Arfa Daily

    Arfa Daily Guest


    In theory I guess, that works, but in practice, I don't think that it would
    prove to be an issue

    You are right that again, in theory, electrolytics do not like reverse
    polarity, but I think that the trick to this is the duration / duty factor
    of the reverse polarity. Electrolytics are subjected to AC across them in
    many circuit locations, and sometimes, in output stages for instance, the
    level of reverse voltage that they are subjected to can be quite
    substantial, but it does not seem to cause any long term problems, except
    when a cap with a poor ESR or insufficient basic voltage rating is used. One
    particular hifi that I used to repair a lot of, springs to mind in this
    regard. Both of it's output coupling caps used to fail open circuit, quite
    regularly. Once they had been replaced with low ESR 105 degree types with
    twice the voltage rating, there was never any further trouble. However, even
    though caps in this sort of position do suffer reverse voltage across them,
    the average duty factor is 50%, and the durations of the reverse excursions
    are relatively short, all of which, I think, contributes to the scheme
    working by and large, reliably.

    As I said elsewhere, another area where electrolytics are subjected to a
    severe reverse pounding, is in the voltage multipliers for VFD supplies. In
    these circuits, they have transformer derived line-power frequency AC of
    high levels across the multiplier input cap. Sometimes, ultimately, these do
    fail - it's very common in Aiwa's for instance - leading to a dark display.
    There also used to be a very popular Sony tape deck that was part of a
    stacking system, where two normal polarised electrolytics were in the VFD
    filament supply lines. These used to fail and wreck the display tube. Sony
    supplied a revised type that was brown with gold writing, but it was still a
    normal polarised type. However, the revised ones did not re-fail, so there
    doesn't seem to be any hard and fast rule that designers stick rigidly to in
    this regard.

    Arfa
     
  14. mm

    mm Guest

    Well, lucky me. This is one of 46 capacitors that Radio Shack sells,
    and it's even one that they normally keep in the store, AND according
    to the webpage, it is in 3 different stores near me.

    That's a good thing, because I was having a lot of trouble keeping
    myself from turning the tv on.

    There is a store I know has every such part, but it's 6 times as far
    away.

    If you are inclined to email me
    for some reason, remove NOPSAM :)
     
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