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 : 11. Sep 2026, 16:59:18
Autres entêtes
Organisation : A noiseless patient Spider
Message-ID : <11818h6$31htb$1@dont-email.me>
References : 1 2 3 4 5
User-Agent : Mozilla Thunderbird
On 12/09/2026 12:45 am, john larkin wrote:
On Fri, 11 Sep 2026 16:23:29 +1000, Bill Sloman <bill.sloman@ieee.org>
wrote:
 
On 8/09/2026 4:52 am, john larkin wrote:
On Mon, 7 Sep 2026 19:33:35 +0100, chrisq <syseng@gfsys.co.uk> wrote:
>
On 9/7/26 16:18, john larkin wrote:
Suppose we have a box with some big caps inside, for example 0.2
farads that run at about 200 volts. When AC power is off, we want to
discharge them for several reasons.
>
Putting, say, a 1K resistor across them dissipates 40 watts and has a
200 second time constant. It will take many tau before the voltage
gets low enough for people to poke around inside.
>
The ideal discharger would be a constant-current or even better a
constant-power load, all the way down to zero volts. It would be dumb,
not switched by some decision circuit or anything fancy like that.
>
And of course we need several LEDs as warnings that the thing is hot.
>
>
What sort of design needs 0.2 Farad cap, at 200 volts ?. If you are
working at that level, put in a cheap relay and a rated heatsink
wirewound resistor to dump the energy, when the power goes off.
Cheap, and lossless as well. Motor drive inverters often have
big resistors to brake the motor.
>
>
John Larkin
Highland Tech Glen Canyon Design Center
Lunatic Fringe Electronics
>
It's a 1500 amp laser driver.
>
So it's a bunch of modules which you have glommed together.
>
The 0.2F of capacitance is a lot of smaller capacitors in parallel.
 Six, 30mF 200v each, in parallel. The caps are bigger than beer cans.
 
>
You are trying to design in safety as an after-thought.
 It's not my design, but I thought I should try to keep people from
being killed.
Always a good idea. You are still trying to design in safety as an after thought, but at least it is somebody else's fault

If you put a discharge circuit into each module and broadcast a
"discharge" signal to all the modules, the problem would be a lot easier
to solve.
 There are no "modules", and complex safety schemes can fail and kill
people.
So can simple ones. The idea is to keep as simple as is practical, but excessively simple schemes have their own risks.
Putting a discharger on each capacitor might not be a bad idea. At least it keeps each discharge current loop as compact as possible,and if one of six fails, at least the other five won't be a menace.

The input to our box is DC, from an external power supply.
>
I do want a circuit that's foolproof, that always discharges the caps
but doen't often go up in flames.
It shouldn't ever go up in flames. There were liquid filled capacitors when I started in the business, but if the liquid was a halo-carbon they didn't burn - they put out fires. Some of the chloro-carbon fluids were pretty nasty but the fluoro-carbons weren't as bad.

A resistor makes an exponential decay which could be a very long time
to get down to safe levels.
And a series inductor could let you make the discharge path critically damped, which does offer faster decay.
Even one inductor per 30mF capacitor is going to need big inductors, but one big inductor per beer can - a toroid that was dropped over each capacitor - might allow neat packaging, if probably not the packaging that was originally intended,
I thought the group might like a circuit design problem once in a
while, a break from politics.
A good thought. Pity you took so long to make it clear what the problem was.

This looks more like a dumb newbie afterthought question, with the usual
dumb newbie fault of insufficient background information, released
incidentally half-way down the thread.
 It's a discussion group, not a lecture platform.
But the more detail you put in up-front, the better the discussion

You might introduce an interesting topic some day too. In full detail.
I'm applying for a provisional patent on a neater way of making a non-progressively wound toroidal inductor. Obviously I can't talk about that until the provisional patent has been registered.
I've certainly posted the occasional detailed question here in the past, but I'm retired - whether I like it or not - and that does means that I don't get introduced to other peoples interesting problems all that often.
--
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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