# Capacitor Polarity

Discussion in 'The Projects Forum' started by Rockhound, Sep 25, 2009.

1. ### Rockhound Thread Starter New Member

Sep 25, 2009
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Hello. I am using a large capacitor instead of a battery in a 12 v motorcycle charging system. The recommended capacitor http://www.trailtech.net/040-CAP27.html appears to have a polarity. Can I use an AC motor starting capacitor instead and do they also have a polarity? I was not aware that there was any difference between an AC and DC capacitor or does an AC capacitor just have a diode built in?

Thank you.

2. ### Wendy Moderator

Mar 24, 2008
21,839
3,051
Electrolytic capacitors have a chemical basis in how they work. Non polarized capacitors are usually electrolytic capacitors back to back, one cap takes the strain while the other relaxes. Twisted, but it works. They do this to get large capacitance values in a small space, if you were using straight forward means the result would be extremely large physically.

If you connected a battery backwards the chemistry would cause it to spew it's innards all over, possibly catching on fire. Same thing happens with caps.

3. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
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That cap is 27,000uF. I have never seen a non polar cap that big, but maybe they exist. You should be able to use a non polar if it is big enough value.

4. ### Audioguru Expert

Dec 20, 2007
11,251
1,350
I think you will need a big trailer to haul 100 motor start capacitors that add up to 27,000uF.

5. ### Rockhound Thread Starter New Member

Sep 25, 2009
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Good explanations guys. So if I understand correctly, I should be using a large DC capacitor for a 12V DC circuit and leave the motor starting capacitor to the AC circuits. Seems simple after hearing a little more info.

Thanks.

6. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
Not so fast!

The "rule of thumb" for caps is to use those that are rated for twice the maximum voltage expected in the circuit.

Nominal voltage for an automotive system is 13.8v after the engine has been running a while and the battery has been charged back up. So, 13.8v x 2 = ?

You might sneak by with caps rated for 25v, but you would be much better off with 35v or 50v rated caps. The higher voltage rated caps are more bulky and expensive due to the increased insulation required, but they will last far longer. Using lower voltage rated caps will cause them to generate heat due to internal leakage, waste power (and gasoline), and fail much sooner.
[eta]
You don't have to use one single large capacitor. You can solder multiple smaller caps together, using very heavy gauge wire or copper bars. Make certain to insulate them well using something like RTV silicone, and anchor them so they won't bang around in the battery box.

7. ### bountyhunter Well-Known Member

Sep 7, 2009
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I would make sure the cap is rated for at least 18V. You might look into those monster caps the audio nuts use for their subwoofer amps. They pack a lot of capacitance in a small size.

8. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
Typically, electrolytic capacitor voltage ratings go right from 16v to 25v. 16v is too low, as the leakage rate would likely be pretty high.
Interesting idea, but those are generally rated at 1 Farad or more. That might put quite a load on the alternator and regulator when the engine is first started; it'll look like a dead short.