# Capacitor question - poloarized vs. non

• posted

I have a few questions about capacitors.

First of all, what makes a capacitor polarized vs. non-polarized?

I'm aware a polarized capacitor can't be used as a bypass - as in: being in series with an AC source and the input of an op-amp. But what if I used a non-polarized on the input and/or output of a voltage regular?

Also, why is a polarized capactior used as a filter to ground in a power supply since current is going back and forth - doesn't it defeat the purpose of having polarity?

Thanks

• posted

There are various kinds of polarized capacitors. What you are alluding to is probably electrolytic capacitors. In that case, high capacitance per volume is achieved by electrolyticly producing a very thin film. Typically, an aluminum foil immersed in a pasty electrolyte is made an anode by making it positive. As a consequence a thin film is generated by oxidizing aluminum wherever current flows. Try looking up the subject in Wikipedia.

As a filter for a power supply, there should be no voltage reversal on the anode side of the capacitor. As long as the electrode carrying the thin aluminum oxide film is positive, the film will not be destroyed.

Bill

-- Fermez le Bush--about two years to go.

• posted

| Also, why is a polarized capactior used as a filter to ground in a power | supply since current is going back and forth - doesn't it defeat the | purpose of having polarity?

A capacitor can be more efficiently designed when assumptions about the polarity are valid. This is a valid assumption after AC is rectified to pulsating or rippling DC. Then the capacitor smooths out the DC. Current does flow in and out of the capacitor, but as long as the voltage does not completely reverse polarity, it can meet the design criteria.

• posted

OK

If the capacitor can stand rated voltage in both polarities it is non- polarized, if it always has to have a positive voltage to the positive terminal it is polarized.

Aside: usually the question has to do with electrolytic capacitors which cannot take a reverse voltage without damage. The solution is to put two polarized caps in series so both wires are to the + terminal of the given capacitor. Basically what happens is that as the voltage builds in a certain direction on capacitor is wired backwards and more or less shorts out while the other one is wired correctly for polarity and stands off the voltage. If you reverse the polarity the same thing happens only which capacitor is shorted and which one if standing off the voltage changes.

Capacitors that use dielectrics that are not sensitive to polarity (ceramic, mylar, mica etc.) are always non-polarized though the name is not usually used with these. The only thing you have to watch is that on lead in say a tubular cap will go to the outside foil which can act as a shield for the whole capacitor. In some applications if you connect wrong lead the capacitor can pick up hum etc.

You can do that. the only problem is (as seen above) you are installing two capacitors but only one of them is doing work! Plus, in addition to space and cost issues, one of the two capacitors is always shorted and this can create reliability issues.

No, doesn't go "back and forth"! Voltage goes from some point above ground to a higher point above ground. The voltage never tries to force current backward through the capacitor.

A place where non-polarized electrolytic capacitors are common is in speaker crossover networks. (especially cheapo ones!) There the signal is AC and actually does reverse. Plus since the speaker impedance is low you need lots of capacitance and electrolytic capacitors do that cheaply. Most Radio Shack speakers go this way! The only problem is that at low voltages the switch from one cap shorted to the other cap shorted is less than linear and it creates distortion. All quality speaker networks would use naturally non- polar caps like say mylar, which do not have the cross-over distortion. But they are much larger, and much more expensive than non- polarized electrolytic capacitors.

Hope this helps.

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