Capacitor keeps developing a voltage....swap them around? - Page 1
Forum section: electronics / beginners

author: py-bb
author_rank: Regular Contributor
timestamp: January 20, 2023, 04:10:01 am
content: I've put this in beginners as it's a beginner practical question and I'm a theorist if anything.I've got 2 capacitors from a power supply I'm fixing:++++++SPRAGUEPOWERLYTIC(r)36D3000UF 85VDC8721L 18010180-2193+85 degrees CI discharged them (with a resistor - not an idiot) then the slowly develop a voltage again - not a big one (0.6 to 0.8v about the same for either)When I re-assemble the power supply should I swap these round? By swap them around I don't mean the polarity of course, but where they came from? Is this a sign of severe aging?Thanks.PS: Is it okay to measure them? My capacitance meter says "discharge fully before testing" - not sure it's a big deal

author: Kim Christensen
author_rank: Super Contributor
timestamp: January 20, 2023, 04:14:23 am
content: No need to swap them around. The little bit of voltage you see is the chemical reaction from the electrolyte and plates working a bit like a very weak battery. If their ESR, leakage, and capacitance measure fine, then don't worry about it.

author: DavidAlfa
author_rank: Super Contributor
timestamp: January 20, 2023, 04:15:28 am
content: https://www.robotroom.com/Capacitor-Self-Discharge-5.html

author: py-bb
author_rank: Regular Contributor
timestamp: January 20, 2023, 04:21:05 am
content: Quote from: Kim Christensen on January 20, 2023, 04:14:23 amThese things should be the better part of 40 years old easily - the capacitance measured is around 3500 uF for each, ESR 0.00 ohms and the loss is ~0.5%ish for each.Now they're supposed to be 3000 uF - this isn't the problem with the unit I'm just surprised to see that.

author: Kim Christensen
author_rank: Super Contributor
timestamp: January 20, 2023, 04:37:55 am
content: QuoteWell, 3500 uF is only +17% over 3000 uF which is probably well within it's rated tolerance...They sound like they tested OK, but at 40 years old, you might just want to replace them anyway depending on how important long term reliability is to you.

author: DavidKo
author_rank: Frequent Contributor
timestamp: January 20, 2023, 08:04:06 am
content: It was quite common to produce electrolytic capacitors in tolerances +100%. For exampleTesla TE672have tolerances -10%+50% or -10%+100%. One reason was, to have the capacity after long time usage.

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 20, 2023, 11:54:42 am
content: Stick them on a current limited power supply at their rated voltage, limited to a couple mA.Use a multimeter to measure the leakage current, it should be in the double-digit uA range for a good capacitor.If you stay in the mA range after a few minutes, they're bad.High leakage can sometimes cause an inflated capacitance measurement.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 20, 2023, 12:54:49 pm
content: This is a well known phenomena.It affects all types of caps.en.wikipedia.org/wiki/Dielectric_absorption

author: Siwastaja
author_rank: Super Contributor
timestamp: January 20, 2023, 12:58:32 pm
content: Don't worry about it, it's completely normal with all ages of capacitors and doesn't correlate with the capacitor's condition.If you want to measure performance, measure ESR to see if it's still within expected range.

author: MathWizard
author_rank: Super Contributor
timestamp: January 20, 2023, 07:24:04 pm
content: I'm sure I've seen several volts across some cap or caps, in circuit, like in big SMPS's, from the effects people are talking about. So what voltage can electrolytic caps get to out of circuit ? And it worse if they are run for longer or say hotter ?

author: Kleinstein
author_rank: Super Contributor
timestamp: January 20, 2023, 07:30:45 pm
content: Electrolytic capacitors have quite some dielectric absorbtion. After discharge and waiting a few minutes a several percent of the prior voltage can come back. This normal and not a sign of aging or defect.After longer discharge this gets slower and also a little less. It also depends on how long the capacitor was charged before.With high voltage caps (e.g. 400 V PFC stage) this may lead to a possibly even dangerous voltage.

author: py-bb
author_rank: Regular Contributor
timestamp: January 22, 2023, 04:25:43 am
content: TY for help. I'd appreciate some links to read about capacitor ageing and various types (so ideally PDFs with lots of graphs and equations). That wikipedia link is a start but if any of you have a good reference I'd love to see it.Cannot be too detailed!Thanks for the help again, I wasn't sure what to make of it.