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Topic Title: How or when to use Polarized and non polarized capacitors?
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Created On: 29 April 2010 11:27 AM
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 29 April 2010 11:27 AM
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weikychin

Posts: 14
Joined: 02 February 2004

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.
 29 April 2010 01:42 PM
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amillar

Posts: 1918
Joined: 28 May 2002

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.

-------------------------
Andy Millar CEng MIET CMgr MCMI

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

"The aim of argument, or of discussion, should not be victory, but progress." Joseph Joubert
 07 May 2010 05:04 PM
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saridgway

Posts: 148
Joined: 07 May 2002

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!)

-------------------------
Steve Ridgway MIET
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