Re: discharging caps

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Sujet : Re: discharging caps
De : jl (at) *nospam* glen--canyon.com (john larkin)
Groupes : sci.electronics.design
Date : 13. Sep 2026, 15:41:11
Autres entêtes
Organisation : A noiseless patient Spider
Message-ID : <4addaltet2mi7d6hlo1371g2kpgcaveag2@4ax.com>
References : 1 2 3 4 5 6 7 8
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On Sun, 13 Sep 2026 08:15:01 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:

john larkin <jl@glen--canyon.com> wrote:
>
On Tue, 8 Sep 2026 08:45:44 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
 
john larkin <jl@glen--canyon.com> wrote:
>
On Mon, 7 Sep 2026 22:26:09 +0100, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
 
john larkin <jl@glen--canyon.com> wrote:
>
On Mon, 7 Sep 2026 21:18:48 +0200, Lasse Langwadt <llc@fonz.dk> wrote:
 
On 9/7/26 17:18, 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.
 
>
is a relay (or two) controlled by the AC not dumb enough?
>
maybe use a PTC self limiting heater to dump the energy in
>
>
 
We get DC power, and I wouldn't like the relay to fail anyhow and
start a fire.
>
The load-dump resistor of a fairly large CNC milling machine was
controlled by a power FET; twice in two years the FET failed and the
resistor, which was in a cage on the top of the control cabinet, ran
red hot.  A temperature sensor inside the cabinet eventually shut the
machine down.
>
>
A PTC might work. It would sit there and get hot all the time and go
sorta constant-power as the caps discharge. Maybe some PTCs and some
series resistors, to not spike to a zillion amps at startup.
 
The DC power supply might not start up if it had a true
constant-power load.
>
Could you have a current-operated relay in the incoming supply with
normally-closed contacts that bring in a contactor for the discharge
current?  The contactor could be supplied by the power it is
discharging and would automatically drop out when the voltsge reached
a safe level.
 
Maybe. But any failure mode could start a fire.
>
Connect the incoming supply with a short length of solder wire close to
the resistor.
 
Or buy a real fuse.
 
>
 
Steady-state, the cap charging current can be zero.
>
If the circuit is fed with DC, without access to the incoming AC supply,
how will you detect mains failure?   If you have half a volt to spare
and you put a diode in the supply line, then you could use a voltage
relay as a detector on the supply side of the diode.  You could actually
power a normally-closed contactor directly off the supply.
>
The circuit then becomes extremely simple (and less error-prone),
needing only a contactor, a diode, a solder fuse and a resistor.
>
http://www.poppyrecords.co.uk/other/Discharger.gif
 
Something like that would work. I'd need a big diode with a heat sink,
but that's not a show stopper. The power supply is
programmable/variable, so the contactor would have to work over the
voltage range. That's managable too.
>
For redundancy, a three-pole relay with each contact rated at 10A could
be used; with DC on the coil, the voltage range from pull-in to drop-out
is very wide.  Each contact could control a single 1 kW heating element
from almost any domaestic appliance (kettle, cooker, hair dryer,
toaster).
>


If the relay coil, or its driver, fails, the box catches fire.



A 1kW element for 240v supply draws about 4 amps, so all three would
draw 12 amps with a resistance of 20 ohms.  The time constant with 0.2F
would be about 4 seconds with triple-redundancy in case of a resistor or
contact failing.
>
If the charging does not need to be particularly rapid, the control
circuit could detect a constant current of 4 amps or more and go into
'blip' mode, which would guard against the contactor failing with a
contact closed.  If that is not possible, running a length of solder
across all three elements wouldn't be difficult to arrange as a thremal
cut-put.

John Larkin
Highland Tech Glen Canyon Design Center
Lunatic Fringe Electronics

Date Sujet#  Auteur
7 Sep 26 * discharging caps165john 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 Sep 26 i      `* Re: discharging caps3Simon Simple
11 Sep 26 i       +- Re: discharging caps1Jan Panteltje
11 Sep 26 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 caps89chrisq
7 Sep 26 i+* Re: discharging caps86john 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 Sep 26 ii+* Re: discharging caps3Bill Sloman
17 Sep08:19 ii`* Re: discharging caps6Cydrome 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 caps9legg

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