Electrolytic Cap (reverse voltage effects?)
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From: Bill Bowden on 24 Feb 2010 18:29 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
From: Tim Williams on 25 Feb 2010 10:54 "Bill Bowden" <wrongaddress(a)att.net> wrote in message news:ded78e91-8116-47d9-9a4e-9ef081914ff7(a)u5g2000prd.googlegroups.com... > 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? 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 -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: Paul Hovnanian P.E. on 26 Feb 2010 14:21 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. -- Paul Hovnanian paul(a)hovnanian.com ---------------------------------------------------------------------- Have gnu, will travel.
From: dcaster on 26 Feb 2010 16:11 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
From: whit3rd on 26 Feb 2010 17:24
On Feb 24, 3:29 pm, Bill Bowden <wrongaddr...(a)att.net> wrote: > Is it safe to operate an aluminum electrolytic cap at 10% reverse > voltage? 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. |