How to make bipolar caps?

[jmartinson]

First post on this forum... I spent about 30 minutes searching and reading but didn't find the answer... also, I'm a Unix admin w/almost no electronics background, so please forgive my ignorance of basic electronic components. This project is definitely a learning experience for me.

I am trying to put together a cheap passive crossover setup for my car until I have to time and energy to build the 24db/octave active crossover project on ESP.

There seems to be no one in Houston who stocks bipolar capacitors of useful sizes or power ratings.

I have been told different things by different people about how to make a "bipolar cap" from two polar caps.

The first set of instructions was to connect two caps in series, positive to positive, and that will yield a bipolar with total capacitance equalling the sum of the two caps you used.

The second set was to connect two caps in parallel but with the leads on one reversed, so it goes positive to negative on each end. These instructions did not indicate how to figure the final capacitance of the circuit.

Two questions:

1. Is it really possible to make a bipolar cap from two polar caps?

2. If so, how do you wire it up, and how do you figure the correct (final) capacitance of the bipolar circuit?

Any information or links are greatly appreciated!!!

Thanks!

-Jacob

 

[gammelmalle]

Hi! I dont know if this is hifi but I've seen it in stereos in guitar amps in car speakers etc:
Take two identically caps lets say 22uF hook them up in serie, BUT one the other way around. (+ to + or - to -) and you'll have a cap of 11uF.....
easy ah!?

cheers,
gammelnalle

 

[KevinLee]

Hello Jacob,

I have made bipolar caps from polar caps only once.

What you do is take the two (-) ends and connect them. You now have a cap with two ends (the original two + ends).

Your new capacitance will be 1/4 of the sum of the two original caps.

Example: make (2) 470uf polar caps bipolar as above and you will end up with about 235uf.

I believe voltage rating stays the same.

Thanks

KevinLee

 

[MarcelvdG]

I agree with gammelmalle and KevinLee: you can indeed make a bipolar electrolytic capacitor from two normal ones with the double capacitance connected in anti-series, + to + or - to -. The total capacitance is given by the equation for series connections of capacitors, Ctot=C1*C2/(C1+C2). The voltage rating of the electrolytic capacitors should be greater than or equal to the peak to peak signal voltage, which for a sine wave is 2*sqrt(2) times the RMS voltage.

There are several disadvantages compared to a proper bipolar capacitor intended for cross-over networks. To name a few, firstly, the effective series resistance could be somewhat higher, especially for low capacitance values. Secondly, the distortion generated by a normal electrolytic capacitor is higher than that generated by a bipolar capacitor. Thirdly, normal electrolytics are usually less accurate.

 

[jmartinson]

Muchas gracias...

I would say thanks in Dutch, but it's been a while.. .

I am going to just put some electrolytics together for now... this is all temporary until I get a new head unit, another amp, and build a real crossover-
http://sound.westhost.com/project09.htm

Thanks again!

-Jacob

<off topic>
a friend of mine moved to holland with his dutch girlfriend. he felt bad about his difficulty in learning dutch.

one day as he was walking through the park a bum asked him for some money in dutch. realizing my friend couldn't speak dutch, he asked again, but in english.

this was truly demoralizing...
</off topic>

 

[theChris]

why in series? are electrolytic caps wired backwards like a short circut or what?

 

[Lisandro_P]

A good idea is adding a small polyestyer/poly cap in parallel to the "fake" bipolar one to improve the ESR. It helps a lot, specially if you're using them for crossovers.

 

[MarcelvdG]

Quote from theChris:
"why in series? are electrolytic caps wired backwards like a short circut or what?"

I'm no capacitor expert, but as far as I know, electrolytic capacitors start conducting when the backward voltage is too high (higher than 2V or so). When this happens continuously at high current levels, they may explode, which is why you should be careful with the way you connect them, especially in power supplies. When the negative sides of two electrolytics are connected together in an anti-series connection, the voltage at that node automatically becomes equal to the most negative peak voltage, due to the rectifying effect.

In fact, I've been told that electrolytics were originally intended as rectifiers. Their parasitic capacitance turned out to be so high that they were more useful as capacitors than as diodes...