On Mon, 14 Sep 2026 15:53:50 +1000, Bill Sloman <bill.sloman@ieee.org>But if you don't know what words mean, you aren't going to "design" (another word whose meaning you play fast and lose with) all that well.
wrote:
On 14/09/2026 5:22 am, john larkin wrote:Play with words all you like; I'd rather play with electronics.On Mon, 14 Sep 2026 03:16:21 +1000, Bill Sloman <bill.sloman@ieee.org>>
wrote:
>Thinking about John Larkin's problem of discharging a capacitor fast in>
an LCR network, I was reminded of a scheme that I lucked onto where an
inductor that was mostly saturated did what was needed in the brief
intervals it was out of saturation.
>
John's problem is that discharging through a resistance is purely
exponential, and thus too slow.
>
My suggestion was to add an inductor and chose the resistance and
inductance to set up a critically damped circuit which has a shorter tail.
>
An inductor that won't get saturated by the peak current is big, but if
we choose a smaller inductor that will saturate early in the discharge
we can probably live with with the consequences - even a simple RC will
lose a lot of energy early on. Once the current has dropped to the point
where it doesn't saturate the inductor, you will have a critically
damped LCR circuit which would then give the critcally damped discharge,
but only at the end of the discharge where it would get rid of the last
of the energy rather faster than a simple RC would.
>
The wire still has to be heavy enough to carry the peak discharge
current, so it still has to be a bulky inductor, but we can use an
ungapped high permeability core and get the desired inductance - say 5H
- with fewer turns than you'd need on a gapped core - and in a smaller
volume.
>
It would need to be very high permeability core - Waldek Hebisch seems
to have had an iron core in mind and his core got 5H with a 1cm air gap
with just 1727 turns. Without the airgap he would have needed fewer
turns, so we could probably get the 5H with a few less turns on a
somewhat smaller core.
>
The aim has to be to get the inductor coil resistance high enough to
damp the LCR - perhaps somewhere around 10R - with enough wire in the
inductor that 4kJ won't get it hot enough to soften the insulating
enamel on the wire.
My solution was to use wirewound bleeder resistors with depletion fets
kicked in near the end to squash the exponential tail.
>
Four of five such circuits for redundancy.
>
No switching involved.
Extra depletion FETs kicked in? That's switching, no matter how you do it.
It's a soft transition to turning the depletion fet on. Personally, IYuck. I can see why you have this enthusiasm for keeping an LED on all the time, but it does waste a lot of power all the time in order let you drain the capacitors relatively fast when the power does go off.
call that a nonlinearity, not a switch.
https://www.dropbox.com/scl/fi/w86g1m4si37dhadi9hw5u/P200_Discharge_1.jpg?rlkey=e1gyhxtivgjqzi85izlixk6sf&raw=1
The real point is that it's simple and reliable. Unlike someHow would you know that? You do seem to be able to evolve designs until they more or less work, and sometimes they do end up complicated, but thinking doesn't really seem to come ito it.
simulations that we've seen here.
It takes more thinking to design simple circuits than it takes to
design complex ones.
--
Bil Sloman, Sydney
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