Re: discharging caps

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Sujet : Re: discharging caps
De : bill.sloman (at) *nospam* ieee.org (Bill Sloman)
Groupes : sci.electronics.design
Date : 13. Sep 2026, 07:37:12
Autres entêtes
Organisation : A noiseless patient Spider
Message-ID : <1185gap$csfm$3@dont-email.me>
References : 1 2 3 4 5 6 7 8 9 10 11 12
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On 13/09/2026 5:43 am, john larkin wrote:
On Sun, 13 Sep 2026 02:53:12 +1000, Bill Sloman <bill.sloman@ieee.org>
wrote:
 
On 13/09/2026 2:12 am, john larkin wrote:
On Sun, 13 Sep 2026 01:20:46 +1000, Bill Sloman <bill.sloman@ieee.org>
wrote:
>
On 13/09/2026 12:54 am, john larkin wrote:
On Sat, 12 Sep 2026 16:24:19 +1000, Bill Sloman <bill.sloman@ieee.org>
wrote:
>
On 12/09/2026 7:28 am, john larkin wrote:
On Fri, 11 Sep 2026 17:08:00 +0100, Simon Simple
<nothanks@nottoday.co.uk> wrote:
>
On 10/09/2026 15:20, john larkin wrote:
>
<xxxx>>
2.4 KJ is about 16 firecrackers or five .45 cal bullets worth of bang.
>
If I've done the sums right, that's a capacitor bank bigger than six
beer cans storing about the same energy as one AAA cell!
>
People would notice if that were shorted.
>
Yes, I used to operate capacitor banks of maybe 2-4 times that physical
size charged to 1kV and discharged via a spark gap through a marine
sparker for seismic work. Long time ago before H&S was a thing.
>
I got one of the giant caps. It says that it's 30,000 uF but it
measures 50,000. So we'll have 4000 joules with four in parallel at
200v.
>
I think it's normal for elecs to have more initial capacitance than
the nameplate value.
>
https://www.dropbox.com/scl/fi/apargzu733abcq6ww7w3b/30mF_250v.jpg?rlkey=00osgeyqpxceowgqf7l32mlnr&raw=1
>
One of the more depressing things about electrolytic capacitors is the
need to reform them by keeping the rated voltage across them for a day
or so if they have been unused for a while.
>
The actual electrolyte is a layer of aluminium oxide on the electrode
surface, and that diffuses away slowly if the capacitor is stored
uncharged. In extreme cases you have apply the reforming voltage through
a big resistor because the thinned down layer can no longer stand off
the rated voltage. With the big resistor you can measure the reforming
current as the process proceeds.
>
Obviously the initial capacitance is higher than the rated capacitance.
That may have contributed to your initial 50mF - a +50% tolerance only
get you up to 45mF
>
I reformed a couple of electroltics many years ago when I bought some
military surplus electrolytics for my home built hi-fi. I didn't bother
to monitor the current.
>
I'm thinking that they overdo the capacitance to account for later-on
drying out or some electro-chemical effects, to ensure the guaranteed
minimum c over the (short) specified lifetime.
>
Not that I know of. Note that the tolerance printed on your can was -10%
+50%.
>
Hey, I saw that. It measured +66%.
>
Which does suggest that it had been left uncharged for long enough to
need  reforming.
>
 It charged to 100 volts with a power supply that was limited to 20 mA.
The rampup was very linear.
 
The tolerances are just arbitrary cut-offs on what's probably a Gaussian
distribution. Presumably the manufacturer samples individual capacitors
often enough to be confident  that the distribution is stable, but don't
measure every capacitor that they ship. That +66%  might just have been
an outlier.
>
The need to reform long unused electrolytic capacitors was well known
everywhere I worked. Nobody ever warned me about any "drying out", and
mythical electro-chemical effects weren't mentioned either. Back when I
was a graduate student in chemistry the electrochemists did get a hard
time,  but back then they didn't have good tools for looking at the
microscope detail of the very thin layers where the electrochemistry
took place.
>
Caps are tricky parts.
>
And electrolytic capacitors are even trickier than regular capacitors.
Charge soak isn't nice.
>
Charged to 100v and discharged for 2 seconds, that big cap went to
about 0.25 volts and then only recharged itself to 2 volts.
>
Not bad.
>
It's more of a worry in capacitor-based A/D converters. Dual ramp and
quad ramp schemes come to mind. Not all that clearly - that was back in
1979. Going from a Mylar to polycarbonate dielectric got the soak down
far enough to make it tolerable.
 I remember dual-slope ADCs. I designed a few. Very long ago. IC adc's
are dirt cheap now and super accurate. 24 bits for $2.
Sigma delta parts. Back when I last used one, in 1993, it spat out 24-bits - three bytes - but the accuracy spec was 20-bit. There were more precise versions around but they weren't dirt cheap.
Sloman A.W., Buggs P., Molloy J., and Stewart D. “A microcontroller-based driver to stabilise the temperature of an optical stage to 1mK in the range 4C to 38C, using a Peltier heat pump and a thermistor sensor” Measurement Science and Technology, 7 1653-64 (1996)

The caps that people can make inside ICs are about the best caps that
are possible.
If a little too small for you application.
--
Bill Sloman. Sydney

Date Sujet#  Auteur
7 Sep 26 * discharging caps162john larkin
7 Sep 26 +* Re: discharging caps7Simon Simple
7 Sep 26 i`* Re: discharging caps6john larkin
7 Sep 26 i +- Re: discharging caps1Simon Simple
8 Sep 26 i `* Re: discharging caps4legg
8 Sep 26 i  +- Re: discharging caps1john larkin
8 Sep 26 i  `* Re: discharging caps2Phil Hobbs
8 Sep 26 i   `- Re: discharging caps1john larkin
7 Sep 26 +* Re: discharging caps12ehsjr
7 Sep 26 i`* Re: discharging caps11john larkin
7 Sep 26 i +* Re: discharging caps2Liz Tuddenham
7 Sep 26 i i`- Re: discharging caps1john larkin
8 Sep 26 i `* Re: discharging caps8Jan Panteltje
8 Sep 26 i  `* Re: discharging caps7john larkin
8 Sep 26 i   `* Re: discharging caps6Jan Panteltje
8 Sep 26 i    `* Re: discharging caps5john larkin
9 Sep 26 i     `* Re: discharging caps4Jan Panteltje
11 Sep11:54 i      `* Re: discharging caps3Simon Simple
11 Sep12:01 i       +- Re: discharging caps1Jan Panteltje
11 Sep14:59 i       `- Re: discharging caps1Phil Hobbs
7 Sep 26 +* Re: discharging caps2Bill Sloman
7 Sep 26 i`- Re: discharging caps1john larkin
7 Sep 26 +* Re: discharging caps87chrisq
7 Sep 26 i+* Re: discharging caps84john larkin
8 Sep 26 ii+* Re: discharging caps62Bill Sloman
8 Sep 26 iii+* Re: discharging caps4john larkin
8 Sep 26 iiii`* Re: discharging caps3Bill Sloman
8 Sep 26 iiii `* Re: discharging caps2john larkin
9 Sep 26 iiii  `- Re: discharging caps1Bill Sloman
8 Sep 26 iii`* Re: discharging caps57john larkin
8 Sep 26 iii `* Re: discharging caps56Bill Sloman
8 Sep 26 iii  `* Re: discharging caps55john larkin
9 Sep 26 iii   `* Re: discharging caps54Bill Sloman
9 Sep 26 iii    `* Re: discharging caps53piglet
9 Sep 26 iii     `* Re: discharging caps52john larkin
9 Sep 26 iii      +- Re: discharging caps1Bill Sloman
9 Sep 26 iii      `* Re: discharging caps50Jeroen Belleman
9 Sep 26 iii       +* Re: discharging caps9Bill Sloman
9 Sep 26 iii       i+* Re: discharging caps6john larkin
9 Sep 26 iii       ii`* Re: discharging caps5Bill Sloman
9 Sep 26 iii       ii `* Re: discharging caps4john larkin
10 Sep 26 iii       ii  `* Re: discharging caps3Bill Sloman
10 Sep 26 iii       ii   `* Re: discharging caps2john larkin
10 Sep 26 iii       ii    `- Re: discharging caps1Bill Sloman
9 Sep 26 iii       i`* Re: discharging caps2chrisq
9 Sep 26 iii       i `- Re: discharging caps1Bill Sloman
9 Sep 26 iii       `* Re: discharging caps40Bill Sloman
9 Sep 26 iii        +* Re: discharging caps37Jeroen Belleman
10 Sep 26 iii        i`* Re: discharging caps36Bill Sloman
13 Sep01:27 iii        i `* Re: discharging caps35Waldek Hebisch
13 Sep01:38 iii        i  +* Re: discharging caps20Lane W
13 Sep04:11 iii        i  i`* Re: discharging caps19Waldek Hebisch
13 Sep15:50 iii        i  i `* Re: discharging caps18john larkin
13 Sep16:14 iii        i  i  +* Re: discharging caps3Lane W
13 Sep17:18 iii        i  i  i`* Re: discharging caps2Bill Sloman
13 Sep17:28 iii        i  i  i `- Re: discharging caps1Lane W
13 Sep16:28 iii        i  i  `* Re: discharging caps14Bill Sloman
13 Sep20:03 iii        i  i   `* Re: discharging caps13john larkin
13 Sep23:54 iii        i  i    +* Re: discharging caps9Simon Simple
14 Sep01:12 iii        i  i    i`* Re: discharging caps8john larkin
14 Sep07:05 iii        i  i    i `* Re: discharging caps7Bill Sloman
14 Sep16:21 iii        i  i    i  `* Re: discharging caps6john larkin
15 Sep09:28 iii        i  i    i   `* Re: discharging caps5Bill Sloman
15 Sep18:23 iii        i  i    i    `* Re: discharging caps4john larkin
16 Sep09:39 iii        i  i    i     `* Re: discharging caps3Bill Sloman
16 Sep11:21 iii        i  i    i      `* Re: discharging caps2john larkin
16 Sep12:16 iii        i  i    i       `- Re: discharging caps1Bill Sloman
14 Sep06:59 iii        i  i    `* Re: discharging caps3Bill Sloman
14 Sep16:22 iii        i  i     `* Re: discharging caps2john larkin
15 Sep09:31 iii        i  i      `- Re: discharging caps1Bill Sloman
13 Sep01:59 iii        i  +* Re: discharging caps3john larkin
13 Sep05:21 iii        i  i+- Re: discharging caps1wmartin
13 Sep07:27 iii        i  i`- Re: discharging caps1Bill Sloman
13 Sep07:24 iii        i  `* Re: discharging caps11Bill Sloman
13 Sep14:36 iii        i   `* Re: discharging caps10Waldek Hebisch
13 Sep17:05 iii        i    `* Re: discharging caps9Bill Sloman
13 Sep19:40 iii        i     `* Re: discharging caps8John R Walliker
13 Sep20:08 iii        i      +* Re: discharging caps4john larkin
14 Sep07:18 iii        i      i`* Re: discharging caps3Bill Sloman
14 Sep16:24 iii        i      i `* Re: discharging caps2john larkin
15 Sep09:36 iii        i      i  `- Re: discharging caps1Bill Sloman
14 Sep07:11 iii        i      `* Re: discharging caps3Bill Sloman
14 Sep16:56 iii        i       `* Re: discharging caps2john larkin
15 Sep09:50 iii        i        `- Re: discharging caps1Bill Sloman
10 Sep 26 iii        `* Re: discharging caps2john larkin
10 Sep 26 iii         `- Re: discharging caps1Bill Sloman
8 Sep 26 ii+* Re: discharging caps14chrisq
8 Sep 26 iii+- Re: discharging caps1chrisq
8 Sep 26 iii`* Re: discharging caps12john larkin
8 Sep 26 iii +* Re: discharging caps3chrisq
8 Sep 26 iii i`* Re: discharging caps2chrisq
9 Sep 26 iii i `- Re: discharging caps1john larkin
8 Sep 26 iii +* Re: discharging caps5Bill Sloman
8 Sep 26 iii i`* Re: discharging caps4john larkin
9 Sep 26 iii i `* Re: discharging caps3Bill Sloman
9 Sep 26 iii i  `* Re: discharging caps2john larkin
9 Sep 26 iii i   `- Re: discharging caps1Bill Sloman
8 Sep 26 iii +- Re: discharging caps1Bill Sloman
14 Sep17:44 iii `* Re: discharging caps2Buzz McCool
14 Sep18:15 iii  `- Re: discharging caps1john larkin
11 Sep07:23 ii+* Re: discharging caps3Bill Sloman
17 Sep08:19 ii`* Re: discharging caps4Cydrome Leader
8 Sep 26 i`* Re: discharging caps2Jan Panteltje
7 Sep 26 +* Re: discharging caps20Lasse Langwadt
8 Sep 26 +- Re: discharging caps1legg
9 Sep 26 +- Re: discharging caps1Jeremiah Jones
9 Sep 26 +* Re: discharging caps17someone
9 Sep 26 +* Re: discharging caps6Joerg
13 Sep19:52 `* Re: discharging caps8legg

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