Hi,

I was involved in filter design recently. I came across the use of polarized and non-polarized capacitor in active filter design and bypass capacitor. This is one of the questions that I to ask but never remember to.

When do we choose to use polarized and we do we use non polarized? What is the difference, apart from the polarity?

Please advice.

In my eyes, it's all about size.

If you need some pico or nano Farad capacitor, these are generally only available in non polarised versions.

If you need a bigger capacitor, more like micro Farad then the only option available is a polarised cap (electrolytic, tantalum etc).

Take a look at the RS catalogue: www.rswww.com and see what various capacitors are available on the shelf (values, materials, polarisation)

Ideally we would never use polarised capacitors, but, as the above poster says, sometimes they are all that is available.

The problem is that polarized capacitors must never have a voltage applied in reverse, so in a filter or AC coupling circuit you must always provide a DC bias to keep the positive end higher than the negative end even with the largest expected reverse AC voltage across it (hope that makes sense).

Sometimes you are stuck with needing a high value non-polarized electrolytic, for example maybe in a loudspeaker 'crossover filter'. There are such capacitors (which are, as far as I know, two electrolytics connected 'back to back') but these are quite expensive.

Normally in an active filter you can use non-polarised capacitors as the impedances in the circuit are so high the the capacitance values you need are small.

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Andy Millar CEng MIET MCMI

http://www.linkedin.com/in/millarandy

Douglas Self provides some information on measured non-linearities introduced by different capacitor types (including electrolytics) in his book Small Signal Audio Design. If you are interested in achieving very low distortion levels it is probably worth a read. As someone who used to dabble in audio design back in the 1970s, I found it quite interesting. (I also found it somewhat heartening to learn that, unlike the digital world, not much appears to have changed in analogue audio, i.e. .001% of me may not be obsolete!)

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Steve Ridgway MIET