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caps back to back

Discussion in 'Electronic Design' started by Deefoo, Nov 21, 2005.

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  1. Deefoo

    Deefoo Guest

    Probably a silly question, but when you connect two electrolytic capacitors
    back to back to make a non-polarised capacitor does it matter if you connect
    the positive plates together or the negative plates? If so, why?

    -- DF
  2. Guest

    No not really, it will fail just as quick either way.
  3. Guest

    the classic arangement of doing that uses two diodes, one in series
    with each cap. this adds a few vulgarities to the circuit, and I
    wouldnt use it for audio!

    Steve Roberts
  4. Mark

    Mark Guest

    that can't be correct, one of the fews rules I know that is ALWAYS

    a cap and a diode in series (with nothing else) is always wrong...

  5. john

    john Guest

    ive done this in a old tv years ago
    to replace a cap, i think i put the 2
    negitves to the centre.
  6. No it does not matter although I seem to always put the positives
    I would like to know why other responders think this will not work
    properly or will fail?
    I have done this many times with no problems in AC and DC circuits and
    as long as the
    voltage rating is not exceeded this will work the same as a NP cap.
  7. An aluminium electrolyte capacitor, depends on the forward bias, to keep
    the aluminium oxide layer on the plates. If you reverse bias such a
    capacitor, the oxide dissolves into the electrolyte. Depending on the
    current flowing, and the duration, this can cause the capacitor to go
    short circuit. You have probably 'got away' with it, because with AC, with
    the current limited by the other capacitor, there is not time for the
    layer to be completely destroyed. However you are 'living on borrowed
    time', especially if voltages are present for longer periods...
    It will also shorten the overall life of the capacitor.

    Best Wishes
  8. Mark

    Mark Guest

    the interesting question that we should get to is.....

    if you place two caps back to back.....+ to + or - to -

    is the value of the combination 1/2 each or not.....

    i.e. take 2 10 uF caps back to back, is it 10uF or 5 uF

    its not a simple question as it seems...

  9. Kevin White

    Kevin White Guest

    There are a couple of ways of connecting up polarized caps to make a
    bi-polar one:

    1) Just put them in series back to back (doesn't matter which way).
    Usually an electrolytic cap will have a higher leakage in the reverse
    direction and act a bit like a diode (they used to have electrolytic
    rectifiers that relied on this action). This will tend to give a DC
    bias on the capacitor so both caps are biased in the correct direction.
    The effective capacitance will be that of two caps in series i.e. 5uF
    in your example.

    2) As someone suggested you can augment the basic circuit with a couple
    of diodes to ensure that the caps don't get reverse biased - the diodes
    are not just put in series with the capacitors though, they are put
    across each cap in the appropriate direction - this improves the
    rectification to guarantee the bias on the capacitor. The effective
    capacitance will be the same as above.

  10. Guest

    When you use the diodes, two 10 uF caps in series will give you the
    equivalent capacitance of 10 uF, not 5 uF.

  11. Al

    Al Guest

    What happenes to the capacitance if the voltage applied is below the
    forward voltage threshold of the diodes?

    Does the capacitance jump from 10 to 5 to 10 as the diodes conduct and
    then reverse bias?

  13. Phil Hobbs

    Phil Hobbs Guest

    Yes, this scheme is nonlinear at small swings. On the other hand, we
    generally use wet electrolytics as coupling capacitors only when their
    reactances are small enough to be negligible, because they're crappy
    otherwise--nonlinear and time dependent.

    I usually prefer to use a big resistor to one of the power supplies from
    the middle of the capacitor string, to keep them both at a few volts of
    bias in the right polarity. That keeps their characteristics much more
    stable and makes them more linear, whereas the diode trick makes them
    less linear.

    Usually one or other side of the cap string is low enough in impedance
    that this doesn't cause any loss problems.


    Phil Hobbs
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