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Back-to-Back Nonpolarized Caps
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PostPosted: Sun Sep 13, 2009 7:46 am    Post subject: Back-to-Back Nonpolarized Caps
Subject description: How do they work?
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We all know about that trick of putting two electrolytic caps in series, with their negative terminals tied together. This makes a non-polarized cap of half the value. The quesion is: why does it work?

I've asked around in prior workplace(s) about this and nobody seems to know why it works, and why half the value. What's your guess?

Les

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Blue Hell
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PostPosted: Sun Sep 13, 2009 8:27 am    Post subject: Reply with quote  Mark this post and the followings unread

Half the value is expected for two caps in series, but why it becomes unpolarized was never clear to me either.
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DrJustice



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PostPosted: Sun Sep 13, 2009 7:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

The only real attempt at explaining this that I could find is in this discussion, 8th posting down :

"It *IS* OK to put two polarized caps back-to-back to make an AC
cap out of them. The reason is that there is a tiny bit more leakage
in the reverse biased cap. That leakage charges the "properly" biased
cap in the right direction to keep it happy when the current reverses.
Since the circuit is symetrical, each cap's leakage protects the
other.

Do a test with real capacitors (or even a SPICE simulation). You
will find that after only a couple of cycles that each cap will have a
DC voltage across it that is of a magnitude and polarity such that it
will be completely protected from reverse voltages."

Better read the following messages too.

BTW, both common + or - will work.

BTW(2), if it's for audio decoupling, designers commonly use a single electrolytic cap and get away with it even in expensive eqiupment. This has never seemed quite right to me...

DJ
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Uncle Krunkus
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PostPosted: Sun Sep 13, 2009 10:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

Nice bit of research DJ Wink
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daverj



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PostPosted: Mon Sep 14, 2009 10:58 am    Post subject: Reply with quote  Mark this post and the followings unread

My understanding was that this is reliable with tantalum caps only, but that with aluminum caps you need to put a bias resistor on the point where the positive nodes meet.

At least that's what I was told, many many moons ago.
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PostPosted: Mon Sep 14, 2009 12:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

Well, some say that tantalum caps should never be used in this application. However, e.g. Kemet mentions this a perfectly fine application in one of their applications notes. They also mention that they sell prepackaged back-to-back pairs of tantalum caps for convenience. The same goes for aluminium electrolyte caps. That's good enough for me... but I'd rather spend on some non polar, non electrolytic caps (e.g. polypropylene) for audio AC coupling to max out the quality.

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