Author Topic: Failure modes of electrolytics under heavy pulse load  (Read 814 times)

Offline Twospoons

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Failure modes of electrolytics under heavy pulse load
« on: November 19, 2020, 11:59:24 PM »
A colleague is trying to replace film caps with electrolytics in a heavy pulse application (electric fence shock driver).
Does anyone know what failure modes are likely for an electrolytic driven well beyond its ripple current rating? These caps are not pulse rated AFAIK.

Pulsing is 1 to 3 pulses of ~50usec ~50A pk, from 5uF, every few minutes.  I'm concerned the very high discharge current may cause internal damage that is not immediately obvious. 

Offline davekni

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Re: Failure modes of electrolytics under heavy pulse load
« Reply #1 on: November 20, 2020, 03:20:53 AM »
Your instinct is accurate.  Electrolyte will dry out and ESR rise.  Cap life will be brief with that degree of abuse.
David Knierim

Offline klugesmith

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Re: Failure modes of electrolytics under heavy pulse load
« Reply #2 on: November 20, 2020, 03:24:10 AM »
A sort of similar case is the photoflash application.

Are "single use" film cameras still a thing, like for parties and receptions?
They have (or had) very low cost xenon-flashlamp circuits,
that are tested and re-used with new film, battery, etc.
Electrolytic capacitors at 300 V get discharged many dozens of times in 100 amp pulses.

Offline Twospoons

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Re: Failure modes of electrolytics under heavy pulse load
« Reply #3 on: November 20, 2020, 03:48:39 AM »
There's no doubt they can work under heavy pulse loads, its just the lifetime that suffers. 
This application requires in excess of 50k pulses.  Supposedly the engineer has tested the circuit for 100k pulses with electrolytics, but I have my doubts about his assessment that it works. I don't think there's been any proper before/after comparison of the capacitor  parameters.


Offline davekni

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Re: Failure modes of electrolytics under heavy pulse load
« Reply #4 on: November 20, 2020, 06:20:24 AM »
Photoflash capacitors are designed specifically for that use, and generally do have limited life.

Perhaps the 100k life is reasonable for a normal cap.  I reread your initial question - was mistakenly thinking pulses every few seconds, not minutes.  That will keep the average cap temperature low, so limit degradation.
David Knierim

Offline Twospoons

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Re: Failure modes of electrolytics under heavy pulse load
« Reply #5 on: November 20, 2020, 06:53:56 AM »
So you are not aware of any other potential failure mechanisms, apart from electrolyte loss through heating?

Offline plasma

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Re: Failure modes of electrolytics under heavy pulse load
« Reply #6 on: November 20, 2020, 07:02:28 AM »
With Ac and large changes for DC there is vibration fitgue ,higher voltage and capatence the more the effect.
Electylight uses thin Ali or Thantium plates that would work harden.

Offline johnf

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Re: Failure modes of electrolytics under heavy pulse load
« Reply #7 on: November 20, 2020, 07:43:42 AM »
Here n NZ our specs for electric fences would not allow the use of electrolytics as the internal ESR of a good one is such that the pulse width maximum (Which you have guarantee for the life of the unit) could go outside spec due to the esr causing injurious problems to people with heart problems. Most manufactures here use polypropylene or polycarbonate.
Designing for as 10 year life (NZ minimum for these units under consumer guarantee act) would equal 315.36 million pulses.
 a film camera might do 10,000 pics in its life and half of htose probably without the flash yes 5000 shots of some length would be easily capable from an electrolytic in this case rising esr has little effect and there would be no heating of the capacitor internals

consider motor start electrolytics most have a rating as to how many starts per hour some as low as 25

you get what you pay for in this area of electronics

Offline Mads Barnkob

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Re: Failure modes of electrolytics under heavy pulse load
« Reply #8 on: November 20, 2020, 03:32:19 PM »
A colleague is trying to replace film caps with electrolytics in a heavy pulse application (electric fence shock driver).
Does anyone know what failure modes are likely for an electrolytic driven well beyond its ripple current rating? These caps are not pulse rated AFAIK.

Pulsing is 1 to 3 pulses of ~50usec ~50A pk, from 5uF, every few minutes.  I'm concerned the very high discharge current may cause internal damage that is not immediately obvious.

It seems that noone else has mentioned reverse-voltage ratings, these are also very important in pulse applications where you could expect a oscillation to occur in the load being driven by the current pulse. Most pulse-rated capacitors are only rated for 30-70% reverse-voltage, so take a wild guess at how much a polarized electrolytic capacitor is rated for :)
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Offline John123

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Re: Failure modes of electrolytics under heavy pulse load
« Reply #9 on: March 04, 2021, 11:32:28 PM »
take a wild guess at how much a polarized electrolytic capacitor is rated for :)

Its about 1.5v isn't it?  :D Or at-least I've read somewhere that's how they often survive as audio input coupling capacitors.

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Re: Failure modes of electrolytics under heavy pulse load
« Reply #9 on: March 04, 2021, 11:32:28 PM »

 


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