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.
The caps that people can make inside ICs are about the best caps that
are possible.
John Larkin
Highland Tech Glen Canyon Design Center
Lunatic Fringe Electronics
| Date | Sujet | # | Auteur | |
| 7 Sep 26 |  discharging caps | 162 | john larkin | |
| 7 Sep 26 |  Re: discharging caps | 7 | Simon Simple | |
| 7 Sep 26 |   Re: discharging caps | 6 | john larkin | |
| 7 Sep 26 |   Re: discharging caps | 1 | Simon Simple | |
| 8 Sep 26 | Re: discharging caps | 4 | legg | |
| 8 Sep 26 | Re: discharging caps | 1 | john larkin | |
| 8 Sep 26 | Re: discharging caps | 2 | Phil Hobbs | |
| 8 Sep 26 | Re: discharging caps | 1 | john larkin | |
| 7 Sep 26 | Re: discharging caps | 12 | ehsjr | |
| 7 Sep 26 | Re: discharging caps | 11 | john larkin | |
| 7 Sep 26 | Re: discharging caps | 2 | Liz Tuddenham | |
| 7 Sep 26 | Re: discharging caps | 1 | john larkin | |
| 8 Sep 26 | Re: discharging caps | 8 | Jan Panteltje | |
| 8 Sep 26 | Re: discharging caps | 7 | john larkin | |
| 8 Sep 26 | Re: discharging caps | 6 | Jan Panteltje | |
| 8 Sep 26 | Re: discharging caps | 5 | john larkin | |
| 9 Sep 26 | Re: discharging caps | 4 | Jan Panteltje | |
| 11 Sep11:54 | Re: discharging caps | 3 | Simon Simple | |
| 11 Sep12:01 | Re: discharging caps | 1 | Jan Panteltje | |
| 11 Sep14:59 | Re: discharging caps | 1 | Phil Hobbs | |
| 7 Sep 26 | Re: discharging caps | 2 | Bill Sloman | |
| 7 Sep 26 | Re: discharging caps | 1 | john larkin | |
| 7 Sep 26 | Re: discharging caps | 87 | chrisq | |
| 7 Sep 26 | Re: discharging caps | 84 | john larkin | |
| 8 Sep 26 | Re: discharging caps | 62 | Bill Sloman | |
| 8 Sep 26 | Re: discharging caps | 4 | john larkin | |
| 8 Sep 26 | Re: discharging caps | 3 | Bill Sloman | |
| 8 Sep 26 | Re: discharging caps | 2 | john larkin | |
| 9 Sep 26 | Re: discharging caps | 1 | Bill Sloman | |
| 8 Sep 26 | Re: discharging caps | 57 | john larkin | |
| 8 Sep 26 | Re: discharging caps | 56 | Bill Sloman | |
| 8 Sep 26 | Re: discharging caps | 55 | john larkin | |
| 9 Sep 26 | Re: discharging caps | 54 | Bill Sloman | |
| 9 Sep 26 | Re: discharging caps | 53 | piglet | |
| 9 Sep 26 | Re: discharging caps | 52 | john larkin | |
| 9 Sep 26 | Re: discharging caps | 1 | Bill Sloman | |
| 9 Sep 26 | Re: discharging caps | 50 | Jeroen Belleman | |
| 9 Sep 26 | Re: discharging caps | 9 | Bill Sloman | |
| 9 Sep 26 | Re: discharging caps | 6 | john larkin | |
| 9 Sep 26 | Re: discharging caps | 5 | Bill Sloman | |
| 9 Sep 26 | Re: discharging caps | 4 | john larkin | |
| 10 Sep 26 | Re: discharging caps | 3 | Bill Sloman | |
| 10 Sep 26 | Re: discharging caps | 2 | john larkin | |
| 10 Sep 26 | Re: discharging caps | 1 | Bill Sloman | |
| 9 Sep 26 | Re: discharging caps | 2 | chrisq | |
| 9 Sep 26 | Re: discharging caps | 1 | Bill Sloman | |
| 9 Sep 26 | Re: discharging caps | 40 | Bill Sloman | |
| 9 Sep 26 | Re: discharging caps | 37 | Jeroen Belleman | |
| 10 Sep 26 | Re: discharging caps | 36 | Bill Sloman | |
| 13 Sep01:27 | Re: discharging caps | 35 | Waldek Hebisch | |
| 13 Sep01:38 | Re: discharging caps | 20 | Lane W | |
| 13 Sep04:11 | Re: discharging caps | 19 | Waldek Hebisch | |
| 13 Sep15:50 | Re: discharging caps | 18 | john larkin | |
| 13 Sep16:14 | Re: discharging caps | 3 | Lane W | |
| 13 Sep17:18 | Re: discharging caps | 2 | Bill Sloman | |
| 13 Sep17:28 | Re: discharging caps | 1 | Lane W | |
| 13 Sep16:28 | Re: discharging caps | 14 | Bill Sloman | |
| 13 Sep20:03 | Re: discharging caps | 13 | john larkin | |
| 13 Sep23:54 | Re: discharging caps | 9 | Simon Simple | |
| 14 Sep01:12 | Re: discharging caps | 8 | john larkin | |
| 14 Sep07:05 | Re: discharging caps | 7 | Bill Sloman | |
| 14 Sep16:21 | Re: discharging caps | 6 | john larkin | |
| 15 Sep09:28 | Re: discharging caps | 5 | Bill Sloman | |
| 15 Sep18:23 | Re: discharging caps | 4 | john larkin | |
| 16 Sep09:39 | Re: discharging caps | 3 | Bill Sloman | |
| 16 Sep11:21 | Re: discharging caps | 2 | john larkin | |
| 16 Sep12:16 | Re: discharging caps | 1 | Bill Sloman | |
| 14 Sep06:59 | Re: discharging caps | 3 | Bill Sloman | |
| 14 Sep16:22 | Re: discharging caps | 2 | john larkin | |
| 15 Sep09:31 | Re: discharging caps | 1 | Bill Sloman | |
| 13 Sep01:59 | Re: discharging caps | 3 | john larkin | |
| 13 Sep05:21 | Re: discharging caps | 1 | wmartin | |
| 13 Sep07:27 | Re: discharging caps | 1 | Bill Sloman | |
| 13 Sep07:24 | Re: discharging caps | 11 | Bill Sloman | |
| 13 Sep14:36 | Re: discharging caps | 10 | Waldek Hebisch | |
| 13 Sep17:05 | Re: discharging caps | 9 | Bill Sloman | |
| 13 Sep19:40 | Re: discharging caps | 8 | John R Walliker | |
| 13 Sep20:08 | Re: discharging caps | 4 | john larkin | |
| 14 Sep07:18 | Re: discharging caps | 3 | Bill Sloman | |
| 14 Sep16:24 | Re: discharging caps | 2 | john larkin | |
| 15 Sep09:36 | Re: discharging caps | 1 | Bill Sloman | |
| 14 Sep07:11 | Re: discharging caps | 3 | Bill Sloman | |
| 14 Sep16:56 | Re: discharging caps | 2 | john larkin | |
| 15 Sep09:50 | Re: discharging caps | 1 | Bill Sloman | |
| 10 Sep 26 | Re: discharging caps | 2 | john larkin | |
| 10 Sep 26 | Re: discharging caps | 1 | Bill Sloman | |
| 8 Sep 26 | Re: discharging caps | 14 | chrisq | |
| 8 Sep 26 | Re: discharging caps | 1 | chrisq | |
| 8 Sep 26 | Re: discharging caps | 12 | john larkin | |
| 8 Sep 26 | Re: discharging caps | 3 | chrisq | |
| 8 Sep 26 | Re: discharging caps | 2 | chrisq | |
| 9 Sep 26 | Re: discharging caps | 1 | john larkin | |
| 8 Sep 26 | Re: discharging caps | 5 | Bill Sloman | |
| 8 Sep 26 | Re: discharging caps | 4 | john larkin | |
| 9 Sep 26 | Re: discharging caps | 3 | Bill Sloman | |
| 9 Sep 26 | Re: discharging caps | 2 | john larkin | |
| 9 Sep 26 | Re: discharging caps | 1 | Bill Sloman | |
| 8 Sep 26 | Re: discharging caps | 1 | Bill Sloman | |
| 14 Sep17:44 | Re: discharging caps | 2 | Buzz McCool | |
| 14 Sep18:15 | Re: discharging caps | 1 | john larkin | |
| 11 Sep07:23 | Re: discharging caps | 3 | Bill Sloman | |
| 17 Sep08:19 | Re: discharging caps | 4 | Cydrome Leader | |
| 8 Sep 26 | Re: discharging caps | 2 | Jan Panteltje | |
| 7 Sep 26 | Re: discharging caps | 20 | Lasse Langwadt | |
| 8 Sep 26 | Re: discharging caps | 1 | legg | |
| 9 Sep 26 | Re: discharging caps | 1 | Jeremiah Jones | |
| 9 Sep 26 | Re: discharging caps | 17 | someone | |
| 9 Sep 26 | Re: discharging caps | 6 | Joerg | |
| 13 Sep19:52 | Re: discharging caps | 8 | legg |
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