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Re: Electrolytic Capacitor Substitute

Discussion in 'General Electronics' started by Dan Fraser, Jul 19, 2003.

  1. Dan Fraser

    Dan Fraser Guest

    A non polarized capacitor of the same capacitance and voltage rating can
    replace a polarized capacitor in almost every application with NO
    modifications.

    In fact the diode trick won't work because current has to flow both in
    and out of a capacitor.

    However, why bother as polarized caps are far cheaper than unpolarized.

    --
    Dan Fraser

    From Costa Mesa in sunny California
    949-631-7535 Cell 714-420-7535

    Check out my electronic schematics site at:
    http://www.schematicsforfree.com
    If you are into cars check out www.roadsters.com
     
  2. Rob Judd

    Rob Judd Guest

    Look in the audio section under "crossover caps". That's generally where
    they're kept.

    Rob
     
  3. DarkMatter

    DarkMatter Guest

    There ya go!
     
  4. Don't electrolytic caps require a bias voltage to keep them healthy? Seems
    like wiring two back to back would cause problems with this.

    Regards,
    Bob Monsen
     
  5. DarkMatter

    DarkMatter Guest

    Non-polarized electrolytics ARE physically two caps of the given
    value in a back to back configuration, in a single package.
     
  6. The common gets charged to a negative or positive voltage.
    Absolutely correct. They look identical to any other electrolytic
    inside. It's all in the processing of the foil to create the oxide
    layer before they are assembled. Cut one apart and see (I have).


    Best regards,
    Spehro Pefhany
     
  7. Some details: A normal polarized electrolytic capacitor has two foils,
    anodic and cathodic, separated by paper impregnated with electrolyte. The
    anodic foil has an oxide layer which is the dielectric of the capacitor. The
    thickness of the oxide layer is propotional to the voltage applied when the
    foil is manufactured. There is also an oxide layer on the cathodic foil, but
    it is very thin, about 1 Volt.

    The oxide layer can only support voltage in one direction.

    If the voltage is applied correctly it vill be across the anodic foil and
    all is well. If applied backwords the anodic foil will be shorted out, a
    large current will pass and the cathode foil will start to grow its own,
    thicker oxid layer. This will lead to gas and heat being produced, and the
    capacitor will explode if the availible current is big enough.

    In a nonpolarized capacitor there are two anodic foils so it does not matter
    which of the foils gets the plus or minus connection, one one the foils will
    always be connected correctly.

    Regards
    Stig Carlsson
     
  8. Jim Meyer

    Jim Meyer Guest

    This comes up about once every two or three years.

    It *IS* OK to put two polarized caps back-to-back to make an AC
    cap out of them. The reason is that there is a tiny bit more leakage
    in the reverse biased cap. That leakage charges the "properly" biased
    cap in the right direction to keep it happy when the current reverses.
    Since the circuit is symetrical, each cap's leakage protects the
    other.

    Do a test with real capacitors (or even a SPICE simulation). You
    will find that after only a couple of cycles that each cap will have a
    DC voltage across it that is of a magnitude and polarity such that it
    will be completely protected from reverse voltages.

    You can guild the lilly with a couple of diodes. One diode
    across each cap in a direction that will prevent the cap's voltage
    from going more than a diode's drop reverse biased. The diodes are
    not necessary but may give you that belt and suspenders feeling of
    confidence.

    Jim
     
  9. I just tried to do your experiment, since it seemed absurd to me (how could
    one side charge without passing charge the wrong way through to the other
    side?) 14v of AC (off of a step down transformer) across two back to back
    (positive side out) 100uF 25v polarized capacitors.

    I should have been wearing safety goggles. I was just about to hook up the
    oscilloscope to check out this mysterious DC bias, when one of the 100uf
    capacitors exploded right in front of me. I've never seen that before! Its
    amazing how much stuff sprayed out all over the place. They were both brand
    new. The electrolyte got all over my new tek probes and superstrip!

    Regards,
    Bob Monsen
     
  10. Actually, once I started cleaning up the mess from the explosion, I realized
    that BOTH had exploded. The reason I didn't notice is that one only sprayed
    electrolyte. The CAN exploded on the other one. This is NOT a good way to
    hook up capacitors, and will serve as a lesson to me to be careful with
    polarized electrolytics during circuit failure modes. It took less than 10
    seconds to blow it up.

    Regards,
    Bob Monsen
     
  11. DarkMatter

    DarkMatter Guest


    That would be one high sodium intake!
     
  12. Dave Platt

    Dave Platt Guest

    They will do this in DC circuits if installed backwards,
    I'd guess that it wasn't a matter of overvoltage - it was a matter of
    ripple current, and the resulting heat dissipation.

    Running a large AC current through a 'lytic cap is generally a bad
    idea. Aluminum electrolytics have a relatively high dissipation
    factor... their equivalent series resistance is fairly high and
    they'll heat up rapidly if high currents are run through them.

    I'd guess that the OP didn't puncture the dielectric (as an
    overvoltage would have done) - he boiled the electrolyte into steam.
     
  13. Jeff

    Jeff Guest

    I just tried the same "test" with the same value caps
    and a 5 ohm resistor it has been sitting at 11V AC RMS
    for a lot longer than 10 seconds. (varriac)
    Very little current draw and nothing getting hot or
    exploding, RMS voltage across each cap ~5.5VAC
    Now for the surprise, DC reading across each cap
    6-7VDC polarized + and- with each capacitor.
    Jeff
     
  14. One of the earlier posters suggested that there is some kind of leakage
    effect that pumps the junction between the capacitors up to a high dc level,
    high enough to prevent the AC voltage from causing a reverse bias.

    Given a diode over one of the capacitors pointing towards the junction, I
    can see that it would get pumped up to the highest voltage, minus the .6
    drop on the diode. If the diode was pointed away from the junction, then the
    opposite would be true, ie, the junction would drop to the lowest voltage +
    the .6 PN drop.

    If there is asymetrical leakage of current, then this would also happen
    without the diode, one way or another. If the asymetric leakage is toward
    the junction, then the junction would pump up to the highest voltage. If its
    away from the junction, then the junction would drop to the lowest voltage.

    Given the above mentioned posting, my guess was that it will leak current
    from - to + more readily than + to -. Thus, I put on goggles, and tried the
    experiment again, this time joining the + leads rather than the - leads.
    Also, I used caps with a higher voltage (50 VDC) rating. The maximum voltage
    pp at any one time is about 40v, so I guessed that it should be better able
    to handle the stress.

    The first experiment was with a diode from the - to + terminal of one of the
    caps. The DC voltage between the caps grew to 16vdc, as expected.

    Next, I discharged the caps, and tried it again without the diode. The caps
    did not explode, but the dc voltage seemed to land at a random place
    initially, and then slowly climb. Thus, its probably true that the leakage
    is greater from - to + than from + to -, at least with these new caps.
    However, it seems to take a long time for these 100uf caps to charge up to a
    value that will protect them from being charged with incorrect polarity
    (without the diode, anyway). I let the caps charge for 30 seconds, and it
    looked like they were charging at about 2v per minute. Thus, the 'bad'
    polarity charging would occur in my test setup for about 7 minutes, albeit
    with smaller and smaller negative voltages all the time.

    I did not repeat the experiment with the caps connected in the other
    direction, although by symmetry it should simply charge the negative
    junction to a large negative value over time. However, I'm still pulling cap
    stuffing out of my hair from the last experiment, so I though I'd leave
    things as they stand... Any brave souls who want to give it a try might
    email me the results at rcmonsen @ comcast.net

    I'm guessing that the explosion from before occured because the leakage
    current wasn't fast enough to prevent a big negative overvoltage of the
    caps, or perhaps one of the caps had a big leak in the wrong direction.

    I'd say that in order to replace nonpolarized caps with polarized caps, I'd
    use a diode to quickly charge the junction between them up (or down). Point
    the diode from the - to the + terminal over one of the caps. Not using the
    diode risks an explosion, or at the least degradation of the caps over time
    due to reverse voltages for a time while the system charges up.

    The other point is that since the caps are in series, the total value is
    C1*C1/(C1+C2), so not only do you need two caps, you need twice the value
    that the original non-polarized cap had (in addition to the diode). Also,
    both of these caps need to have a dc rating at least twice the peak to peak
    AC voltage expected.

    Regards
     
  15. Fred Abse

    Fred Abse Guest

    You must be one of the newer fellas :)

    When I was (much) younger, I and a few of the guys used to amuse
    ourselves with a piece of pipe closed at one end, a few electrolytic
    capacitors, and the AC supply. We called it the "Cap Gun"
     
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