Cursitor Doom <cd@notformail.com> wrote:

[...]CD.

PS: the aforementioned caps are 47000uF 16V Vishay ones - and I have a
brand new spare that's also testing as "leaky"!

If an electrolytic capacitor hasn't been used for some time, it is a
good idea to charge it up from a variable-voltage power supply, starting
at a very low voltage with an ammeter in series. If it is leaky the
initial charging current surge will settle down to a steady leakage
which will decrease in time. Then the voltage can be raised a bit and
the leakage current will increase for a while, then reduce again.

By doing this slowly over several hours you re-form the insulating layer
in the capacitor. If you apply the full voltage straight away, the
leakage current will be high and can cause further damage. It can also
boil the electrolyte and cause the capacitor to explode. Modern
capacitors have a 'safe' explosion arrangment built-in (a pattern of
weak points in the top of the casing) but older ones don't - and the
damage from flying debris can be quite spectacular (I have seen a CRT
cut in half when a capacitor can went through it).


--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

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On Mon, 4 Mar 2024 09:40:20 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:

Cursitor Doom <cd@notformail.com> wrote:

[...]CD.

PS: the aforementioned caps are 47000uF 16V Vishay ones - and I have a
brand new spare that's also testing as "leaky"!

If an electrolytic capacitor hasn't been used for some time, it is a
good idea to charge it up from a variable-voltage power supply, starting
at a very low voltage with an ammeter in series. If it is leaky the
initial charging current surge will settle down to a steady leakage
which will decrease in time. Then the voltage can be raised a bit and
the leakage current will increase for a while, then reduce again.

By doing this slowly over several hours you re-form the insulating layer
in the capacitor. If you apply the full voltage straight away, the
leakage current will be high and can cause further damage. It can also
boil the electrolyte and cause the capacitor to explode. Modern
capacitors have a 'safe' explosion arrangment built-in (a pattern of
weak points in the top of the casing) but older ones don't - and the
damage from flying debris can be quite spectacular (I have seen a CRT
cut in half when a capacitor can went through it).

Quite so. I blew up a little 10uF 10V cap the other day and the bang
wasn't far off that of a starting pistol. Mind you I did use a massive over-voltage reverse polarity to purposely get a bigger bang out of
it. I don't often get bored, but that's what happens once in a while.

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Cursitor Doom <cd@notformail.com> wrote:

[...]

But then how do you determine - given that electrolytics come in all
sorts of votlage and temperature ratings, capacitance values etc - how
much leakage current in each case is "too much" leakage current
rendering the cap unsuitable for use?

According to the Post Office Elelctrical Engineers' Journal (July 1962
p120) the leakage should be less than 5,000 ohm-farads for electrolytic capacitors to be accepted for use in undersea cable repeaters. If your capacitor is 47,000 microfarads (0.047F), that leakage current would be equivalent to about 100k i.e. 0.1 milliamps at 10v.

For general domestic use, leakage figures at least 10 times greater than
that are usually acceptable.


--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

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