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Discussion in 'Electronic Design' started by Dirk Bruere at NeoPax, Mar 12, 2007.

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  1. Something I never really thought about until just now.
    When electrolytics are used for AC coupling in (say) audio circuits I
    assume they survive because AC isn't polarising?

    Dirk - The UK's only occult talk show
    Presented by Dirk Bruere and Marc Power on ResonanceFM 104.4
  2. If there's a DC bias (and there often is) you wire them in that way.
  3. True, but the voltage swings both ways even with bias.

    Dirk - The UK's only occult talk show
    Presented by Dirk Bruere and Marc Power on ResonanceFM 104.4
  4. Should still be OK. They won't explode on a slight negative for less than
    50% of the time. But it would be better to bias them into the correct
  5. I was just casually wondering whether modern electrolytics are fairly
    immune to AC, since I have seen some AC electrolytics advertised.

    Dirk - The UK's only occult talk show
    Presented by Dirk Bruere and Marc Power on ResonanceFM 104.4
  6. DaveM

    DaveM Guest

    The electrolytics survive because the DC voltage impressed across the capacitor
    is much greater than the peak-to-peak levels of the AC being impressed. If you
    have a capacitor between a 20 VDC source and a 5 VDC source, the DC voltage seen
    by the capacitor is 15 VDC. Now, suppose the AC signal is 1VAC. the capacitor
    never becomes reverse biased, therefore, it stays polarized in the manner it was
    designed to be.
    If, however, the AC signal voltage is greater than the DC voltage across the
    capacitor, then you need to use a non-polarized unit, or a non-electrolytic

    Dave M
    MasonDG44 at comcast dot net (Just substitute the appropriate characters in the

    Life is like a roll of toilet paper; the closer to the end, the faster it goes.
  7. Phil Allison

    Phil Allison Guest

    "Dirk Bruere at NeoPax"

    ** Obviously, AC current is not polarising.

    The electro cap value needs to be large enough so very little SIGNAL voltage
    appears across it - maybe 100mV rms at the lowest operating frequency.

    ......... Phil
  8. Phil Allison

    Phil Allison Guest


    ** It will "survive" and function fine long as any reverse polarity
    imposed is limited to a low voltage - ie under 1 volt.

    Be even more careful with tantalum electros - as reverse current flow
    permanently damages them. If this is a possibility, then parallel any with
    a diode for protection.

    ** Better get this clear - in a correctly designed circuit, the AC voltage
    ACROSS a polarised electro is normally FAR less than the AC signal voltage
    it couples to the load.

    IOW do not use a polarised electro to create a low frequency roll off in a
    circuit with no polarising voltage for the cap.

    ....... Phil
  9. Robert Baer

    Robert Baer Guest

    Usually, an "AC electrolytic" is a non-polarized electrolytic, or it
    was derated by 50% to 75% on voltage.
  10. Eeyore

    Eeyore Guest

    You mean in zero-bias AC coupling of course. For a very long time, audio
    circuits were prediominantly and many still are single supply.

    But basically yes. Also, aluminium electrolytics will happily tolerate a small
    reverse voltage. Don't use tantalums for this, I gather they're less happy about

  11. Eeyore

    Eeyore Guest

    It turns out (I've measured them) that if you keep the reverse voltage below
    about -100mV, you don't even see any non-linearity. Beyond that, yes they start
    to add distortion.

    They're not going to 'explode' for small signal coupling anyway since we're
    talking typically about a mA or less of current. In loudspeaker crossover
    filters where criminals use electrolytics they're the back-to-back non-polarised

  12. Eeyore

    Eeyore Guest

    Motor start ? Those will be 'back-to-back' types.

  13. Eeyore

    Eeyore Guest

    Not true. But you need to be careful.

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