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
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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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.
-- Paul Hovnanian paul@hovnanian.com ---------------------------------------------------------------------- Have gnu, will travel.
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
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.
--- 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 through it in the reverse direction and growing an oxide coating on the cathode.
As I recall the capacitance changed, as one would expect, because of the increase in the thickness of the dielectric, but I've lost the data.
It takes a while to do, but if there's any interest I can run the experiment again over the weekend and post the data on Monday.
Or, anyone else who wants to can do it, since it's easy.
Basically, all you do is monitor the reverse current into the cap and increase the voltage across it until the current gets to be around 10mA or so, then just wait until the current drops into the microamps as the oxide layer is formed.
Once that happens, increase the voltage again, and just keep repeating the process until the reverse voltage is what you want and the current is within the leakage current spec of the cap.
I guess a slicker way to do it would be to run a constant reverse current through the cap, keeping it small enough not to heat the cap too much, and then toward the end, once the voltage was somewhat higher than was needed, to go constant voltage and make sure the leakage current was OK.
JF
Possibly a slick application for a single JFET, and one source resistor (or none, depending on the FET).
--
Dave Platt AE6EO
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...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.
=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
A reverse biased electrolytic is a diode. One of the first rectifiers was the 'Electrolytic Rectifier'.
-- Greed is the root of all eBay.
Well, yes, but in some cases the cap acts like a poor diode..
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