Electrolytic Cap (reverse voltage effects?)
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From: John Fields on 26 Feb 2010 21:07 On Wed, 24 Feb 2010 15:29:34 -0800 (PST), Bill Bowden <wrongaddress(a)att.net> wrote: >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? --- 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
From: Dave Platt on 26 Feb 2010 22:26 In article <obugo5p045k05enof6ica76salihsenci4(a)4ax.com>, John Fields <jfields(a)austininstruments.com> wrote: >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. Possibly a slick application for a single JFET, and one source resistor (or none, depending on the FET). -- Dave Platt <dplatt(a)radagast.org> AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads!
From: Robert Baer on 27 Feb 2010 03:16 Paul Hovnanian P.E. wrote: > Bill Bowden wrote: > >> 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 > > 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. > ....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.
From: Bill Bowden on 27 Feb 2010 22:40 On Feb 26, 1:11 pm, "dcas...(a)krl.org" <dcas...(a)krl.org> wrote: > On Feb 24, 11:29 pm, Bill Bowden <wrongaddr...(a)att.net> wrote: > > > 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 > > 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 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
From: Michael A. Terrell on 28 Feb 2010 00:04
Bill Bowden wrote: > > On Feb 26, 1:11 pm, "dcas...(a)krl.org" <dcas...(a)krl.org> wrote: > > On Feb 24, 11:29 pm, Bill Bowden <wrongaddr...(a)att.net> wrote: > > > > > 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 > > > > 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 > > 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. A reverse biased electrolytic is a diode. One of the first rectifiers was the 'Electrolytic Rectifier'. -- Greed is the root of all eBay. |