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In Reply to: RE: Importance of the "polarity" of a film capacitor posted by Lew on July 25, 2016 at 08:22:25
The importance of "polarity" depends on the use of the cap and impedance of the circuit. For example, a coupling cap should conduct well and have very little differential across it at most frequencies. Whatever noise appears on either lead will appear on the other. A bypass cap, however, should always have the outer foil connected to ground. By doing that, the "active" side of the cap where signal is present will be shielded by the grounded foil. Even in this situation, the degree to which polarity is important will vary depending on circuit impedance. Low-Z applications, such as cathode bypass, will probably not benefit much from this technique. As far as determining polarity, you might consider cutting one of the caps open. They're probably all the same in terms of the direction of printing on the body and the foil-connected lead.
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Follow Ups:
I've only got four of these 10uF/100V polystyrenes, and I need exactly four. They seem to be No Longer Available anywhere that I have looked. (Not merely this exact product but 10uF polystyrenes rated at > 50V in general. I bought them 4-5 years ago.) So, cutting one open is not an option. Like Steve says, I too have found inconsistency in using the direction of the label as a guide.
However, thanks very much for your analysis of when it might be critical to orient the outer foil and when not.
Not necessarily unless specifically stated. Some are random (K40s). Depends on brand.
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