UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Components and Circuits

Notices

Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

Reply
 
Thread Tools
Old 8th Oct 2018, 2:46 pm   #1
m3vuv51
Tetrode
 
Join Date: Feb 2018
Location: Horncastle, Lincolnshire, UK.
Posts: 95
Default Cap leakage current.

Hi folks, just wondered what level of of leakage current would be considered too much on low voltage caps <50V and at what point would they be looked at as u/s?, Just wondered as I'm building a leakage tester into my homebrew ESR meter and would like some idea of when a cap is bad as regards leakage.

Regards, Paul M3VUV.
m3vuv51 is offline   Reply With Quote
Old 8th Oct 2018, 3:06 pm   #2
GrimJosef
Nonode
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 2,158
Default Re: cap leakage current

Since it'll be in an ESR meter I assume you're only interested in testing electrolytics ? I confess most of my experience has been with higher voltage capacitors, but the two points I will make are (perhaps this is obvious) firstly the leakage current will depend on the capacitance - higher value capacitors tend to leak more than lower value ones, roughly in proportion to the value of the capacitance. Secondly the leakage can be nonlinear with voltage. So a capacitor might hardly leak at all at 80% of its rated voltage but might be leaking pretty seriously at 90%. And if the capacitor has run for a long time at a certain voltage it may be fine up to that voltage but then misbehave pretty badly at just a few percent above it - a sort of 'memory effect', if you like.

Cheers,

GJ
__________________
http://www.ampregen.com
GrimJosef is offline   Reply With Quote
Old 8th Oct 2018, 3:59 pm   #3
broadgage
Octode
 
Join Date: Jan 2010
Location: North Somerset, UK.
Posts: 1,131
Default Re: cap leakage current

Yes, electrolytic capacitors after prolonged use at a much reduced voltage tend to "get used" to this voltage and then to fail if subjected to a higher voltage that is still within the rating.

This is a particular problem with "multi voltage" electronic lamp ballasts. After prolonged use on a 120 volt circuit they may go bang on a 240 or 277 volt circuit, despite being rated for operation from 100 volts up to 300 volts.

When still reasonably new they may be switched between voltages with impunity.
broadgage is offline   Reply With Quote
Old 8th Oct 2018, 4:03 pm   #4
m3vuv51
Tetrode
 
Join Date: Feb 2018
Location: Horncastle, Lincolnshire, UK.
Posts: 95
Default Re: cap leakage current

I thought that might be the case, I just stuck my meter in series with my bench PSU first with a bad cap 10uF @ 25V and then a new one of the same value. The bad one was leaking 1.6 micro amp and the new one 0.1 micro amp. Maybe the way to go is test like for like new and old?

Cheers M3VUV.
m3vuv51 is offline   Reply With Quote
Old 8th Oct 2018, 4:15 pm   #5
Station X
Moderator
 
Station X's Avatar
 
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 13,800
Default Re: Cap leakage current.

I wouldn't let 1.6 microamps leakage worry me.

For higher voltage capacitors I reckon 1mA per 30 uF is fine.
__________________
Graham. Forum Moderator

Keep the soldering iron hot.
Station X is offline   Reply With Quote
Old 8th Oct 2018, 5:14 pm   #6
G8HQP Dave
Dekatron
 
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,459
Default Re: Cap leakage current.

A leakage specification is often given in capacitor datasheets. However, how much is too much depends almost entirely on how the capacitor is used in a circuit - assuming it is too small to cause significant heating and pressure build-up.
G8HQP Dave is offline   Reply With Quote
Old 9th Oct 2018, 6:04 am   #7
Diabolical Artificer
Dekatron
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 3,761
Default Re: Cap leakage current.

I looked into this and have also noticed a few things when testing for leakage. After looking at a few datasheet's and some technical paper's from cap manufacturer's I found cap dialetric is typically X megaohm's, sorry, can't remember exact figure, therefore as the resistance should be constant and linear, if you test a cap @ 100v, leakage of a good cap will be around 1 or 10uA or whatever sub multiple of one. If it's 25 say, 36, 95.6 or whatever, then leakage is wrong.

Not sure that help's design a test, some sort of comparison, logic doodad? The way I test is to use a voltage source, current limited, apply V to the cap through an amp meter whilst monitoring V at the cap. If said CUT doesn't charge pretty quick and I is off as above, then CUT is suspect.

The problem with a current limited V supply is it can't deal with big value electrolytics. Not sure how this help's you, but check out how other's have done it. Someone recently built a cap tester using a PIC or similar I think in the homebrew section and Mr Carlson off Utube built a low voltage cap leakage tester; you have to stump up cash to see the plan's though.

Andy.
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is offline   Reply With Quote
Old 12th Oct 2018, 1:54 am   #8
Maarten
Octode
 
Join Date: Dec 2007
Location: Haarlem, Netherlands
Posts: 1,921
Default Re: Cap leakage current.

Electrolytics shouldn't get used to the voltage they're operating on and should stay formed when operated above a certain percentage of their rated working voltage. I think in the order of 10-20%. I'm sure I've read that in some manufacturers data but cannot reproduce that unfortunately.

That said, a capacitor that is either severely worn from usage or one that is stocked too long without reforming, is more likely to blow up at a higher voltage than at a lower.
Maarten is offline   Reply With Quote
Old 12th Oct 2018, 11:30 am   #9
boxdoctor
Heptode
 
boxdoctor's Avatar
 
Join Date: Sep 2010
Location: Ripley, Derbyshire, UK.
Posts: 585
Default Re: Cap leakage current.

I have found when testing N.O.S. electrolytics for E.S.R. that the value measured may sometimes be quite an amount higher than the value measured after the cap. has been sujected to its working voltage for only a few minutes and then re-tested.
Leakage current I have not checked in the same way, but the above experience leads me to expect a noticeable difference after the cap. has had a "dose of volts" (and been discharged afterwards, of course) before re-testing. The chemical changes even a brief exposure to a polarising voltage causes appears to affect the E.S.R., so probably may also similarly affect the leakage current.
I am talking about caps. that are new, and have never been in service. Tony.
__________________
The job "couldn't be done".
He didn't know that,
So he did it.

Last edited by boxdoctor; 12th Oct 2018 at 11:35 am.
boxdoctor is offline   Reply With Quote
Old 12th Oct 2018, 12:20 pm   #10
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 3,855
Default Re: Cap leakage current.

Any capacitor that requires reforming is going to have a memory effect. The reforming process replenishes any lost oxide insulation but only as it conducts. If it is reformed at 50V and 100V is applied to it, it will leak and restart the reforming process. This was a particular issue with wet electrolytics.
PJL is offline   Reply With Quote
Old 12th Oct 2018, 12:36 pm   #11
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 6,409
Default Re: Cap leakage current.

Are current-production electrolytics always 'formed' during manufacture, or is this left to happen at first-use? I could see a significant production-cost-reduction if the chemistry could be arranged to permit formation-through-use.
G6Tanuki is offline   Reply With Quote
Old 12th Oct 2018, 12:57 pm   #12
boxdoctor
Heptode
 
boxdoctor's Avatar
 
Join Date: Sep 2010
Location: Ripley, Derbyshire, UK.
Posts: 585
Default Re: Cap leakage current.

If what you suggest is in fact the case, G6Tanuki, and it seems a feasible possibility, then measuring the leakage resistance of a new electrolytic is even more pointless than I implied in my post.
If such an economy is practicable, then I would expect it will be already being implemented (at least in China, if nowhere else). Tony
__________________
The job "couldn't be done".
He didn't know that,
So he did it.
boxdoctor is offline   Reply With Quote
Old 12th Oct 2018, 2:16 pm   #13
G8HQP Dave
Dekatron
 
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,459
Default Re: Cap leakage current.

I understand that electrolytics are formed at the factory (the aluminium is treated before the capacitor is wound) and so should show lowish initial leakage provided they have not sat on a distributor's shelf for too long (e.g. years). Whether the completed component then has a voltage put across it to complete the forming I don't know. Some datasheets specify a minimum time for applied voltage, before which the capacitor may not meet its leakage spec. My guess is that cheaper ones (and fake ones) are more likely to cut corners.
G8HQP Dave is offline   Reply With Quote
Old 12th Oct 2018, 11:14 pm   #14
Maarten
Octode
 
Join Date: Dec 2007
Location: Haarlem, Netherlands
Posts: 1,921
Default Re: Cap leakage current.

Quote:
Originally Posted by G6Tanuki View Post
Are current-production electrolytics always 'formed' during manufacture, or is this left to happen at first-use? I could see a significant production-cost-reduction if the chemistry could be arranged to permit formation-through-use.
They are factory formed, usually at quite a bit higher voltage than the rated WV. I don't think there's a reasonable alternative chemistry that would permit a form-as-you go and still be reliable after some use.
Maarten is offline   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 8:42 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2018, vBulletin Solutions, Inc.
Copyright ©2002 - 2018, Paul Stenning.