What causes electrolytic's to gain in capacity. - Page 1
Forum section: electronics / general

author: G7PSK
author_rank: Super Contributor
timestamp: November 10, 2012, 10:38:41 pm
content: I have just been replacing the capacitors in a tape recorder made in 1960. When I measured some of the electrolytic's they were up to 4 times the original marked capacity other than the ones that were open or short circuit or way under capacity.I can see how they would short or go open and even dry up and go low but how do they go high, the plates would have to either get larger or closer together.

author: Psi
author_rank: Super Contributor
timestamp: November 10, 2012, 10:41:37 pm
content: Some caps have a tolerance of +80%, but i've never heard of more than that and i cant see why the capacity should increase from age.

author: tom66
author_rank: Super Contributor
timestamp: November 10, 2012, 11:14:45 pm
content: The dielectric "constant" reduces which is similar to making the plates closer together.This leads to an increase in capacitance, but there is no such thing as a free lunch, so the voltage rating decreases quite substantially.Often ESR increases too, but sometimes it can remain within spec.

author: DRT
author_rank: Regular Contributor
timestamp: November 11, 2012, 12:09:50 am
content: Quote from: tom66 on November 10, 2012, 11:14:45 pmCapacitance is proportional to permittivity (i.e. it would have to rise not fall)

author: tom66
author_rank: Super Contributor
timestamp: November 11, 2012, 12:11:23 am
content: Quote from: DRT on November 11, 2012, 12:09:50 amYeah, sorry, that's what I meant. I couldn't think of the word. I have found capacitors with 2x original capacitance and high ESR in power supplies. Something causes them to fail like that, and that explanation was the only one I could find from brief research.

author: sorin
author_rank: Frequent Contributor
timestamp: November 11, 2012, 12:33:28 am
content: see this video

author: AndyC_772
author_rank: Super Contributor
timestamp: November 11, 2012, 09:04:02 am
content: I bet they're just leaking, and the effective parallel resistance is confusing the meter.What happens if you measure their dc resistance and wait for the reading to stabilise?

author: G7PSK
author_rank: Super Contributor
timestamp: November 11, 2012, 10:50:28 am
content: I tested all the capacitors with my Peak ESR60 meter as well as a cheap LSR meter I have plus my Uni T61E and an Anprobe 240 I have they all pretty much agreed on the capacitance. One (a 3uf) reads over 100uf and has an esr of 1.7.Another 30uf rated cap reads 164.8uf at 2.2 esr. the third one is open circuit I have just tested with the ohms range of a meter and it charges up as one would expect the shows OL reverse the leads and you get it discharging into the meter and recharging in reverse.I reckon that they had to have been made way over range as I cannot think of any physics that would cause such a rise in capacity.

author: SeanB
author_rank: Super Contributor
timestamp: November 11, 2012, 11:41:34 am
content: If the oxide layer has degraded to a ultra thin film ( no more than a 1V withstand or so, only a few atoms thick ) then the capacitance will be very high.Try applying the rated voltage to them via a current limiting resistor and leave them for a few hours to reform, or until the current drops to under1% of it's original value then retest them.

author: G7PSK
author_rank: Super Contributor
timestamp: November 11, 2012, 09:21:19 pm
content: I did what Sean B suggested and connected a cap up to my power supply, I did not bother with the current limit resistor though after a few minuets I tested the cap and whereas it had been reading 164.8 uf it now reads 83.4 uf. I don't think I will put it back though as it is 52 years old.

author: PA4TIM
author_rank: Super Contributor
timestamp: November 12, 2012, 12:04:15 pm
content: The dielectic is the oxide layer. They etch the aluminium foil to get a bigger surface and increase capacitance. So a thinner layer should give a lower capacitance.But the peak and many multimeters use DC to measure bigger capacitance and a not formed cap eith a to thin layer often leaks to much current so appearant capacitance is high while true capacitance could be low.Multimeters use often two modes Cp and Cs. If loss is high they use Cp. This value is not the value you want. If D is low Cp= Cs but if Cp is high it is not. (1 + D^2 ) x Cp = Cs but in that case your reading will be much lower.The Peak ( I have) is designed to use down to 10 uF. It will meaure lower but can be way of in that case.C meters often measure |Z| so ESL and ESR can disturb measurements. The wires of your meters are zero' d for R but the inductors they form are still there. At 1KHz not a problem but some meters use 100 KHz.A cap on a power supply with no current limiting has two options, first die because the leakage is to high, it heats up and water from the electrolyte gets away. Or you are lucky and there is just enough isolation left to reform again.I ha one that had double capacity but also to much leakage. This was a rare capacitor, not easy to replace, but after a carefull graduate 24 hour reforming it was perfect again.

author: BravoV
author_rank: Super Contributor
timestamp: November 12, 2012, 12:15:47 pm
content: Quote from: PA4TIM on November 12, 2012, 12:04:15 pmHow ? Just as simple as applying rated voltage ?

author: PA4TIM
author_rank: Super Contributor
timestamp: November 12, 2012, 12:41:30 pm
content: http://www.pa4tim.nl/?p=1385more about reforming