Russell McMahon wrote:
>>> I have sometimes wondered about the use of polarized capacitors in
>>> places
>>> where they are just used to couple an AC signal to another circuit.
>>> Since
>>> the signal being coupled is AC, there really wouldn't be a whole
>>> lot of DC potential across it for any length of time.
>>>
>>> In this case, would it really matter what direction the capacitor's
>>> polarity was facing?
>
>> If I drive off a cliff, does it matter if the defroster is on?
>
> Olin is *probably* suggesting that the practice is a terminally fatal
> one because it will damage the capacitor.

Yes.  I was trying to point out that whatever polarity you chose, you are
still subjecting the capacitor to negative voltages and therefore operating
it outside of spec.  Worrying about polarity is therefore pointless.

Fortunately, if you are just trying to transfer an "AC signal", you can
probably arrange for the imput impedence to by high enough to that you can
use a low enough value cap where non-polarized technologies are readily
available.  For example, if the AC signal is going into a 10Kohm load then a
1.6uF DC blocking capacitor will cause a single pole high pass filter with a
rolloff of 10Hz.  That would work find for audio range signals, and 1.6uF or
a little higher is readily available in ceramic up to a few tens of volts.


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