Electrolytic Cap (reverse voltage effects?)

Is it safe to operate an aluminum electrolytic cap at 10% reverse voltage? The cap is 1000uF at 35 volts and charges with about 1 mA of current at slow rates of 1 minute or less, and reaches 2 volts reverse v on each half cycle. Seems to work ok, but I'm wondering about the long term effects? I don't think there will be a heating problem with only 1mA of current. Will the capacitance change over time?

-Bill

Reply to
Bill Bowden
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It might. As far as I know, electrolytics are okay for a few volts reverse; you can probably filter 3.3VDC with caps of either direction and never notice anything wrong.

But then, what are you using an electrolytic for if the capacitance matters? :-)

Tim

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Reply to
Tim Williams

That might be asking for trouble. It may work in the short term, but reduce the life of the cap.

Anything over a volt or two of reverse potential (and you appear to be on the threshold of that) could damage the oxide layer and result in gas buildup and/or a shorted cap. There are non-polarized electrolytic caps (basically back to back polarized caps) that might suit your app better.

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Paul Hovnanian  paul@hovnanian.com
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Reply to
Paul Hovnanian P.E.

I think you will be just fine, but would contact Spraque or another capacitor company if you want a better opinion. Could you put a diode in parallel with the capacitor?

Dan

Reply to
dcaster

Neither reverse voltage nor even zero voltage is good, in long-term use, for an electrolytic. Anything more than a volt in the reverse direction will cause some (slow) chemical erosion. Spikes are OK, but if it's a repeating event, consider a clamp diode.

It isn't about a percentage of the rated standoff voltage, but about the chemical stability of the materials of the capacitor.

Reply to
whit3rd

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I did some work on this kind of thing a while ago by turning a polarized
electrolytic into a non-polarized one by carefully applying DC current
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Reply to
John Fields

Possibly a slick application for a single JFET, and one source resistor (or none, depending on the FET).

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Dave Platt                                    AE6EO
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Reply to
Dave Platt

...or use a diode for reverse protection? Too bad the old Sprague TE series are no longer available as they would take up to 50 percent reverse rating for short periods and were reasonable to use as a non-polarized at 10 percent of rating.

Reply to
Robert Baer

=A0 =A0 =A0 =A0 =A0 =A0 =A0 Dan

Yes, I thought about back to back caps with diodes in parallel to block reverse voltage, but think I'll go ahead with the single cap to see how it works out. There is a pot to adjust the RC time and may be all that's needed if capacitance changes. Thanks for all the ideas.

-Bill

Reply to
Bill Bowden

A reverse biased electrolytic is a diode. One of the first rectifiers was the 'Electrolytic Rectifier'.

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Reply to
Michael A. Terrell

Well, yes, but in some cases the cap acts like a poor diode..

Reply to
Robert Baer

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