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rp181

Thu Jan 01 2009, 03:48AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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That would make more sense =p

badastronaut

Thu Jan 01 2009, 07:48AM

Registered Member #222 Joined: Mon Feb 20 2006, 05:49PM
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AC rated capacitors are also used for power factor correction and running motors and anywhere you want to shift the phase...

blackgrunge

Thu Jan 01 2009, 03:46PM

Registered Member #1889 Joined: Mon Dec 29 2008, 07:36AM
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!grr don't go scarin' him now

Barry

Thu Jan 01 2009, 06:14PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
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Most types of capacitors handle AC just great. It's just these lame polarized electrolytic capacitors that can be damaged by negative voltages, lol. But we love them because their price-per-joule is vastly better than other types of construction.

Still, I have my eye on a bank of polypropylene caps that would do the job so nicely and shrug off negative voltages without a care... They're the same ones that Braam Daniels used in his pulsed linear induction motor. Too bad they're frightfully expensive in kilojoule quantities.

Cheers, Barry
I know what irony is - It's like goldy and bronzy only it's made out of iron.

badastronaut

Thu Jan 01 2009, 09:50PM

Registered Member #222 Joined: Mon Feb 20 2006, 05:49PM
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If you have two identical electrolytic caps, you can wire them in such a way that they become unpolarized. That's how they make unpolarized electrolytic caps, two caps in a can.

Barry

Fri Jan 02 2009, 07:33AM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

badastronaut wrote ...

If you have two identical electrolytic caps, you can wire them in such a way that they become unpolarized. That's how they make unpolarized electrolytic caps, two caps in a can.

You may be right, but I've never heard of that type of capacitor construction before. It would need something in addition so each electrolytic element does not become reverse biased.

All the unpolarized capacitors I'm familiar with are constructed of insulators that do not rely on electrolyte. This can be a wide variety of materials, commonly including tantalum, ceramic, polypropylene and so forth. The "motor start" capacitors of modern times are frequently made with polypropylene insulation.

The "electrolyte" is a semi-liquid substance that forms an insulating barrier on an aluminum surface through chemical action. This barrier is very thin and therefore leads to high values of capacitance, significantly higher than physically manufacturing a thin insulating barrier.

However a reverse voltage causes the electrolyte's chemical reaction to degrade and the insulation to break down. The typical failure mode is for it to become a resistive connection, generating heat, which eventually boils or expands the liquid causing an explosion.

Hope this helps, Barry
www.coilgun.info

Myke

Fri Jan 02 2009, 10:15AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

I know that one way to make an unpolorized cap is the way that badastronaut described. The thing is, I don't think it's good for high voltages or powers. I think this method works because of the the leakage in electrolytic caps. I'm not completely sure though.

Barry

Fri Jan 02 2009, 04:58PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

Interesting, here is one spot

that explains it this way:

7-23-2003 Jim Meyer said:

It *IS* OK to put two polarized caps back-to-back (series) 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 symmetrical, 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.

You can guild the lilly with a couple of diodes. One diode across each cap in a direction that will prevent the cap's voltage from going more than a diode's drop reverse biased. The diodes are not necessary but may give you that belt and suspenders feeling of confidence.

Seems plausible, but I'm not sure I would rely on it unless I really had to.

Cheers, Barry
Barry answers the door and lets 2009 in, and gives her a hug.

blackgrunge

Fri Jan 02 2009, 06:42PM

Registered Member #1889 Joined: Mon Dec 29 2008, 07:36AM
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I'd really like to hear more about that. It seems like if you put diodes in however you'd be making a fancy voltage multiplier and your currents would spike to the maximum current allowed by the capacitors...

Seems a bit dangerous but I don't know enough about the mechanics of a capacitor to say that with any certainty

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