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.
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.
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.
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.
On 14/09/2026 5:22 am, john larkin wrote:
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.
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.
I guess you've simulated such an arrangement, with or without a
saturable element.
Thing about inductors is that they encourage current to continue
to flow, no matter what voltage they need to develop on their
terminals.
It's an issue in most capacitive discharge pulse generators
that attempt a unipolar output. Also, if you see documentation
that does not include 'some' voltage/current reversal, feel free
to smell a rat.
The voltage reversal is needed to cause the current to reduce
from any established peak value. The current reversal is just
rounding off error, in any attemp at damping.
RL
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.
I guess you've simulated such an arrangement, with or without a
saturable element.
Thing about inductors is that they encourage current to continue
to flow, no matter what voltage they need to develop on their
terminals.
It's an issue in most capacitive discharge pulse generators
that attempt a unipolar output. Also, if you see documentation
that does not include 'some' voltage/current reversal, feel free
to smell a rat.
The voltage reversal is needed to cause the current to reduce
from any established peak value. The current reversal is just
rounding off error, in any attempt at damping.
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.
I guess you've simulated such an arrangement, with or without a
saturable element.
Thing about inductors is that they encourage current to continue
to flow, no matter what voltage they need to develop on their
terminals.
It's an issue in most capacitive discharge pulse generators
that attempt a unipolar output. Also, if you see documentation
that does not include 'some' voltage/current reversal, feel free
to smell a rat.
The voltage reversal is needed to cause the current to reduce
from any established peak value. The current reversal is just
rounding off error, in any attemp at damping.
RL
On 9/14/26 16:32, legg wrote:
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.
I guess you've simulated such an arrangement, with or without a
saturable element.
Thing about inductors is that they encourage current to continue
to flow, no matter what voltage they need to develop on their
terminals.
It's an issue in most capacitive discharge pulse generators
that attempt a unipolar output. Also, if you see documentation
that does not include 'some' voltage/current reversal, feel free
to smell a rat.
The voltage reversal is needed to cause the current to reduce
from any established peak value. The current reversal is just
rounding off error, in any attemp at damping.
RL
All that doesn't matter. All these inductor tricks are
too big and heavy.
I'd maybe get a finned heater element or something like it.
Those things laugh at a few kJ. Choose one to get a time
constant of 10 ms or so and reach 1V in a little over
half a second. Use an IGBT or power FET to switch it.
On 9/14/26 16:56, Jeroen Belleman wrote:
On 9/14/26 16:32, legg wrote:
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.
I guess you've simulated such an arrangement, with or without a
saturable element.
Thing about inductors is that they encourage current to continue
to flow, no matter what voltage they need to develop on their
terminals.
It's an issue in most capacitive discharge pulse generators
that attempt a unipolar output. Also, if you see documentation
that does not include 'some' voltage/current reversal, feel free
to smell a rat.
The voltage reversal is needed to cause the current to reduce
from any established peak value. The current reversal is just
rounding off error, in any attemp at damping.
RL
All that doesn't matter. All these inductor tricks are
too big and heavy.
I'd maybe get a finned heater element or something like it.
Those things laugh at a few kJ. Choose one to get a time
constant of 10 ms or so and reach 1V in a little over
half a second. Use an IGBT or power FET to switch it.
Did I say half a second? It's a little over 50 ms!
Jeroen Belleman
On 9/14/26 17:13, Jeroen Belleman wrote:
On 9/14/26 16:56, Jeroen Belleman wrote:
On 9/14/26 16:32, legg wrote:
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.
I guess you've simulated such an arrangement, with or without a
saturable element.
Thing about inductors is that they encourage current to continue
to flow, no matter what voltage they need to develop on their
terminals.
It's an issue in most capacitive discharge pulse generators
that attempt a unipolar output. Also, if you see documentation
that does not include 'some' voltage/current reversal, feel free
to smell a rat.
The voltage reversal is needed to cause the current to reduce
from any established peak value. The current reversal is just
rounding off error, in any attemp at damping.
RL
All that doesn't matter. All these inductor tricks are
too big and heavy.
I'd maybe get a finned heater element or something like it.
Those things laugh at a few kJ. Choose one to get a time
constant of 10 ms or so and reach 1V in a little over
half a second. Use an IGBT or power FET to switch it.
Did I say half a second? It's a little over 50 ms!
Jeroen Belleman
Sigh, I shouldn't try to make sense after a mountain hike...
Get a 2kW heater element for 240V. Resistance is 30 Ohms
or so and the time constant with your 200mF cap will be
6s. Wait for a little over 5 time constants to get down
to 1V, so 30-ish seconds.
Peak discharge current is a little over 6A, peanuts.
Adapt the recipe to taste.
Jeroen Belleman
On Mon, 14 Sep 2026 17:27:44 +0200, Jeroen Belleman
<jeroen@nospam.please> wrote:
On 9/14/26 17:13, Jeroen Belleman wrote:
On 9/14/26 16:56, Jeroen Belleman wrote:
On 9/14/26 16:32, legg wrote:
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.
I guess you've simulated such an arrangement, with or without a
saturable element.
Thing about inductors is that they encourage current to continue
to flow, no matter what voltage they need to develop on their
terminals.
It's an issue in most capacitive discharge pulse generators
that attempt a unipolar output. Also, if you see documentation
that does not include 'some' voltage/current reversal, feel free
to smell a rat.
The voltage reversal is needed to cause the current to reduce
from any established peak value. The current reversal is just
rounding off error, in any attemp at damping.
RL
All that doesn't matter. All these inductor tricks are
too big and heavy.
I'd maybe get a finned heater element or something like it.
Those things laugh at a few kJ. Choose one to get a time
constant of 10 ms or so and reach 1V in a little over
half a second. Use an IGBT or power FET to switch it.
Did I say half a second? It's a little over 50 ms!
Jeroen Belleman
Sigh, I shouldn't try to make sense after a mountain hike...
Get a 2kW heater element for 240V. Resistance is 30 Ohms
or so and the time constant with your 200mF cap will be
6s. Wait for a little over 5 time constants to get down
to 1V, so 30-ish seconds.
Peak discharge current is a little over 6A, peanuts.
Adapt the recipe to taste.
Jeroen Belleman
The customer, who has a dedicated safety staff, specified diascharge
to safe levels in two minutes.
A fast discharge, like using a giant heater element, needs a switch,
which needs logic, which adds a whole new set of hazards.
John Larkin
Highland Tech Glen Canyon Design Center
Lunatic Fringe Electronics
On 9/14/26 18:06, john larkin wrote:
On Mon, 14 Sep 2026 17:27:44 +0200, Jeroen Belleman
<jeroen@nospam.please> wrote:
On 9/14/26 17:13, Jeroen Belleman wrote:
On 9/14/26 16:56, Jeroen Belleman wrote:
On 9/14/26 16:32, legg wrote:
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.
I guess you've simulated such an arrangement, with or without a
saturable element.
Thing about inductors is that they encourage current to continue
to flow, no matter what voltage they need to develop on their
terminals.
It's an issue in most capacitive discharge pulse generators
that attempt a unipolar output. Also, if you see documentation
that does not include 'some' voltage/current reversal, feel free
to smell a rat.
The voltage reversal is needed to cause the current to reduce
from any established peak value. The current reversal is just
rounding off error, in any attemp at damping.
RL
All that doesn't matter. All these inductor tricks are
too big and heavy.
I'd maybe get a finned heater element or something like it.
Those things laugh at a few kJ. Choose one to get a time
constant of 10 ms or so and reach 1V in a little over
half a second. Use an IGBT or power FET to switch it.
Did I say half a second? It's a little over 50 ms!
Jeroen Belleman
Sigh, I shouldn't try to make sense after a mountain hike...
Get a 2kW heater element for 240V. Resistance is 30 Ohms
or so and the time constant with your 200mF cap will be
6s. Wait for a little over 5 time constants to get down
to 1V, so 30-ish seconds.
Peak discharge current is a little over 6A, peanuts.
Adapt the recipe to taste.
Jeroen Belleman
The customer, who has a dedicated safety staff, specified diascharge
to safe levels in two minutes.
A fast discharge, like using a giant heater element, needs a switch,
which needs logic, which adds a whole new set of hazards.
John Larkin
Highland Tech Glen Canyon Design Center
Lunatic Fringe Electronics
If the discharge resistor must be connected permanently, that
spec will result in some serious power dissipation. You can't
win.
Jeroen Belleman
Is there a limit on the peak discharge current the capacitors can handle?
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 [...]
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 [...]
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.
Also, be advised that inductors (particularly gapped structures) will
attempt to rearrange their physical shape into a less energetic form
when impressed into a high energy circuit. This also looks like an
explosion, but is really just a natural rearrangement of parts.
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.
Also, be advised that inductors (particularly gapped structures) will
attempt to rearrange their physical shape into a less energetic form
when impressed into a high energy circuit. This also looks like an
explosion, but is really just a natural rearrangement of parts.
Bill Sloman <bill.sloman@ieee.org> writes:
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 [...]
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 [...]
Assuming the capacitor discharge time is short compared to the time
constant of the heatsink(s), and that we?re talking about a single-event discharge rather than one every 10ms or something, to a good
approximation, you?re transferring all of the capacitor?s electrical
energy into the inductor as thermal energy during the discharge.
So I?d
think that it would matter less how thick the wire was than what the
total mass of the inductor was and how high a temperature it could
withstand.
A temperature rise of 100øC is roughly 100J/g with most
materials. Electrolytic capacitors charged to their rated voltage can sometimes store 20J/g, so I?d think the inductor mostly needs to be a
good fraction of the mass of the capacitor.
You should be able to use a thinner wire than you?d normally use for the current, but the wire thickness isn?t *completely* irrelevant, because
metals have a positive TCR. So the warmest spot in a thin enough wire becomes a ?voltage hog?, dissipating more and more of the power as it
heats up to the metal?s melting point. This is the dual of current
hogging by p-n junction hotspots, the phenomenon which causes second breakdown and which allows LEDs to handle much higher average current if they?re pulsed with a short duty cycle. Analogously, I?d expect it to
be less of an issue with a short enough pulse, but not a non-issue.
I?d think that this is a case where you?d sort of prefer to use not just
an ungapped core, but a solid iron core, so that as much as possible of
the power would be lost by eddy currents in the core, mostly because
iron is cheaper than copper.
Iron can also handle higher temperatures
than the copper or especially its insulation, but, if it were to come to that, the hottest part of the iron would be in direct contact with the copper, so I don?t think that would help much. Iron?s Curie point is
770ø, well above the Curie point of things like ferrite (pure magnetite
is 585ø) but both of those are well above the service temperature of
your insulation, unless you've sourced some of that exotic
ceramic-insulated wire Dalibor Farn? uses in his Nixie tubes.
None of the above is validated by me burning up any inductors, though,
or even doing FEM simulations; it?s purely based on my fallible
theoretical understanding. Corrections would be welcome, especially corrections based on actual measurement.
On Mon, 14 Sep 2026 15:53:50 +1000, Bill Sloman <bill.sloman@ieee.org>
wrote:
On 14/09/2026 5:22 am, john larkin wrote:
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.
Play with words all you like; I'd rather play with electronics.
It's a soft transition to turning the depletion fet on. Personally, I
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 some
simulations that we've seen here.
It takes more thinking to design simple circuits than it takes to
design complex ones.
On 15/09/2026 12:49 am, legg wrote:
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.
Also, be advised that inductors (particularly gapped structures) will
attempt to rearrange their physical shape into a less energetic form
when impressed into a high energy circuit. This also looks like an
explosion, but is really just a natural rearrangement of parts.
You must means an implosion. And I was specifically avoiding a gapped >structure. If you are going to let the core saturate, there's no point
in a gap. Making a more or less toroidal code out of two U-shaped core >elements does run the risk of a little deformation. A circular toroidal
core would move symmetrically (and even less).
On 15/09/2026 12:32 am, legg wrote:
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.
I guess you've simulated such an arrangement, with or without a
saturable element.
Thing about inductors is that they encourage current to continue
to flow, no matter what voltage they need to develop on their
terminals.
The inductor in a critically damped LCR does encourage the current to >continue to flow as the capacitor to get close to fully discharged.
Picking the L and R to match the capacitor means that the last current >completely empties the capacitor at the point where the inductor runs
out of stored energy. It's not a sharp cutoff, but there's less charge
left in the capacitor at a given time than you'd get with a pure exponential
It's an issue in most capacitive discharge pulse generators
that attempt a unipolar output. Also, if you see documentation
that does not include 'some' voltage/current reversal, feel free
to smell a rat.
Or careful design.
The voltage reversal is needed to cause the current to reduce
from any established peak value. The current reversal is just
rounding off error, in any attempt at damping.
You can provide enough damping to prevent current reversal. I'm not sure
if the no overshoot at all case is the fastest discharge curve, but it's >pretty close to it.
If you figure in component tolerances, "no reversal" may be difficult to >guarantee without a test and adjust stage, but I don't think that
anybody would bother.
On 15/09/2026 12:49 am, legg wrote:
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.
Also, be advised that inductors (particularly gapped structures) will
attempt to rearrange their physical shape into a less energetic form
when impressed into a high energy circuit. This also looks like an
explosion, but is really just a natural rearrangement of parts.
You must means an implosion. And I was specifically avoiding a gapped >structure. If you are going to let the core saturate, there's no point
in a gap. Making a more or less toroidal code out of two U-shaped core >elements does run the risk of a little deformation. A circular toroidal
core would move symmetrically (and even less).
On Tue, 15 Sep 2026 17:53:12 +1000, Bill Sloman <bill.sloman@ieee.org>
wrote:
On 15/09/2026 12:49 am, legg wrote:
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.
Also, be advised that inductors (particularly gapped structures) will
attempt to rearrange their physical shape into a less energetic form
when impressed into a high energy circuit. This also looks like an
explosion, but is really just a natural rearrangement of parts.
You must means an implosion. And I was specifically avoiding a gapped
structure. If you are going to let the core saturate, there's no point
in a gap. Making a more or less toroidal code out of two U-shaped core
elements does run the risk of a little deformation. A circular toroidal
core would move symmetrically (and even less).
Homogenous material in the magnetic field is important, as you say, to
avoid it. Torus is good, but I have seen some that want to pretzel
into a folded-over infinity symbol.
Most typically, you'd be fooling with surges more commonly found in terrestrial nature. It's only weapons or biomed nuts that go out
farther than time scalesa of a ~hundred milliseconds.
Small equipment applications that incidentally store large energy,
without an intended operational use (an obvious discharge method is
just to run the thing without accumulation)are the result of poor
vision.
Someone should correct them, before they dig a deeper hole.
On 9/14/26 17:13, Jeroen Belleman wrote:
On 9/14/26 16:56, Jeroen Belleman wrote:
On 9/14/26 16:32, legg wrote:
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.
I guess you've simulated such an arrangement, with or without a
saturable element.
Thing about inductors is that they encourage current to continue
to flow, no matter what voltage they need to develop on their
terminals.
It's an issue in most capacitive discharge pulse generators
that attempt a unipolar output. Also, if you see documentation
that does not include 'some' voltage/current reversal, feel free
to smell a rat.
The voltage reversal is needed to cause the current to reduce
from any established peak value. The current reversal is just
rounding off error, in any attemp at damping.
RL
All that doesn't matter. All these inductor tricks are
too big and heavy.
I'd maybe get a finned heater element or something like it.
Those things laugh at a few kJ. Choose one to get a time
constant of 10 ms or so and reach 1V in a little over
half a second. Use an IGBT or power FET to switch it.
Did I say half a second? It's a little over 50 ms!
Jeroen Belleman
Sigh, I shouldn't try to make sense after a mountain hike...
On 15/09/2026 9:55 am, Kragen Javier Sitaker wrote:
Bill Sloman <bill.sloman@ieee.org> writes:
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 [...]
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 [...]
Assuming the capacitor discharge time is short compared to the time
constant of the heatsink(s), and that we?re talking about a single-event
discharge rather than one every 10ms or something, to a good
approximation, you?re transferring all of the capacitor?s electrical
energy into the inductor as thermal energy during the discharge.
The resistance is in the wire, not the inductor, and the heat has to
diffuse into iron, which is a process that has it's own thermal time >constant.
So I?d
think that it would matter less how thick the wire was than what the
total mass of the inductor was and how high a temperature it could
withstand.
It the wire melts and loses structural strength before the iron has
heated up, the temperatue of the iron core doesn't matter.
A temperature rise of 100?C is roughly 100J/g with most
materials. Electrolytic capacitors charged to their rated voltage can
sometimes store 20J/g, so I?d think the inductor mostly needs to be a
good fraction of the mass of the capacitor.
You should be able to use a thinner wire than you?d normally use for the
current, but the wire thickness isn?t *completely* irrelevant, because
metals have a positive TCR. So the warmest spot in a thin enough wire
becomes a ?voltage hog?, dissipating more and more of the power as it
heats up to the metal?s melting point. This is the dual of current
hogging by p-n junction hotspots, the phenomenon which causes second
breakdown and which allows LEDs to handle much higher average current if
they?re pulsed with a short duty cycle. Analogously, I?d expect it to
be less of an issue with a short enough pulse, but not a non-issue.
I?d think that this is a case where you?d sort of prefer to use not just
an ungapped core, but a solid iron core, so that as much as possible of
the power would be lost by eddy currents in the core, mostly because
iron is cheaper than copper.
A solid iron core probably wouldn't be a good idea. Winding a toroidal
core out of a thin ribbon of iron or some other high permeability alloy
is a better idea, which is why you can buy them off the shelf (if from
only a small number of specialist suppliers, many of them in China).
Iron can also handle higher temperatures
than the copper or especially its insulation, but, if it were to come to
that, the hottest part of the iron would be in direct contact with the
copper, so I don?t think that would help much. Iron?s Curie point is
770?, well above the Curie point of things like ferrite (pure magnetite
is 585?) but both of those are well above the service temperature of
your insulation, unless you've sourced some of that exotic
ceramic-insulated wire Dalibor Farn? uses in his Nixie tubes.
None of the above is validated by me burning up any inductors, though,
or even doing FEM simulations; it?s purely based on my fallible
theoretical understanding. Corrections would be welcome, especially
corrections based on actual measurement.
They would be very desirable, but expensive. John Larkin is the only
poster who is getting paid for his work, but he seems to have been >frightened by a coil winding machine when young and prefers
off-the-shelf woundl components.
On Tue, 15 Sep 2026 18:07:43 +1000, Bill Sloman <bill.sloman@ieee.org>
wrote:
On 15/09/2026 9:55 am, Kragen Javier Sitaker wrote:
Bill Sloman <bill.sloman@ieee.org> writes:
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 [...] >>>> 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 [...]
Assuming the capacitor discharge time is short compared to the time
constant of the heatsink(s), and that we?re talking about a single-event >>> discharge rather than one every 10ms or something, to a good
approximation, you?re transferring all of the capacitor?s electrical
energy into the inductor as thermal energy during the discharge.
The resistance is in the wire, not the inductor, and the heat has to
diffuse into iron, which is a process that has it's own thermal time
constant.
So I?d
think that it would matter less how thick the wire was than what the
total mass of the inductor was and how high a temperature it could
withstand.
It the wire melts and loses structural strength before the iron has
heated up, the temperatue of the iron core doesn't matter.
A temperature rise of 100øC is roughly 100J/g with most
materials. Electrolytic capacitors charged to their rated voltage can
sometimes store 20J/g, so I?d think the inductor mostly needs to be a
good fraction of the mass of the capacitor.
You should be able to use a thinner wire than you?d normally use for the >>> current, but the wire thickness isn?t *completely* irrelevant, because
metals have a positive TCR. So the warmest spot in a thin enough wire
becomes a ?voltage hog?, dissipating more and more of the power as it
heats up to the metal?s melting point. This is the dual of current
hogging by p-n junction hotspots, the phenomenon which causes second
breakdown and which allows LEDs to handle much higher average current if >>> they?re pulsed with a short duty cycle. Analogously, I?d expect it to
be less of an issue with a short enough pulse, but not a non-issue.
I?d think that this is a case where you?d sort of prefer to use not just >>> an ungapped core, but a solid iron core, so that as much as possible of
the power would be lost by eddy currents in the core, mostly because
iron is cheaper than copper.
A solid iron core probably wouldn't be a good idea. Winding a toroidal
core out of a thin ribbon of iron or some other high permeability alloy
is a better idea, which is why you can buy them off the shelf (if from
only a small number of specialist suppliers, many of them in China).
Iron can also handle higher temperatures
than the copper or especially its insulation, but, if it were to come to >>> that, the hottest part of the iron would be in direct contact with the
copper, so I don?t think that would help much. Iron?s Curie point is
770ø, well above the Curie point of things like ferrite (pure magnetite
is 585ø) but both of those are well above the service temperature of
your insulation, unless you've sourced some of that exotic
ceramic-insulated wire Dalibor Farn? uses in his Nixie tubes.
None of the above is validated by me burning up any inductors, though,
or even doing FEM simulations; it?s purely based on my fallible
theoretical understanding. Corrections would be welcome, especially
corrections based on actual measurement.
They would be very desirable, but expensive. John Larkin is the only
poster who is getting paid for his work, but he seems to have been
frightened by a coil winding machine when young and prefers
off-the-shelf woundl components.
I had a toroid winding machine when I was a teenager. And I've
designed maybe a hundred inductors and transformers since then.
My laser driver will have about 15 pounds of capacitors. And inductors
are way worse than caps for energy storage. So the estimate of over
100 kilograms for the inductor is in the ballpark.
I'd rather use a depletion fet.
On 16/09/2026 4:53 am, john larkin wrote:
On Tue, 15 Sep 2026 18:07:43 +1000, Bill Sloman <bill.sloman@ieee.org>
wrote:
On 15/09/2026 9:55 am, Kragen Javier Sitaker wrote:
Bill Sloman <bill.sloman@ieee.org> writes:
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 [...] >>>>> 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 [...]
Assuming the capacitor discharge time is short compared to the time
constant of the heatsink(s), and that we?re talking about a single-event >>>> discharge rather than one every 10ms or something, to a good
approximation, you?re transferring all of the capacitor?s electrical
energy into the inductor as thermal energy during the discharge.
The resistance is in the wire, not the inductor, and the heat has to
diffuse into iron, which is a process that has it's own thermal time
constant.
So I?d
think that it would matter less how thick the wire was than what the
total mass of the inductor was and how high a temperature it could
withstand.
It the wire melts and loses structural strength before the iron has
heated up, the temperatue of the iron core doesn't matter.
A temperature rise of 100?C is roughly 100J/g with most
materials. Electrolytic capacitors charged to their rated voltage can >>>> sometimes store 20J/g, so I?d think the inductor mostly needs to be a
good fraction of the mass of the capacitor.
You should be able to use a thinner wire than you?d normally use for the >>>> current, but the wire thickness isn?t *completely* irrelevant, because >>>> metals have a positive TCR. So the warmest spot in a thin enough wire >>>> becomes a ?voltage hog?, dissipating more and more of the power as it
heats up to the metal?s melting point. This is the dual of current
hogging by p-n junction hotspots, the phenomenon which causes second
breakdown and which allows LEDs to handle much higher average current if >>>> they?re pulsed with a short duty cycle. Analogously, I?d expect it to >>>> be less of an issue with a short enough pulse, but not a non-issue.
I?d think that this is a case where you?d sort of prefer to use not just >>>> an ungapped core, but a solid iron core, so that as much as possible of >>>> the power would be lost by eddy currents in the core, mostly because
iron is cheaper than copper.
A solid iron core probably wouldn't be a good idea. Winding a toroidal
core out of a thin ribbon of iron or some other high permeability alloy
is a better idea, which is why you can buy them off the shelf (if from
only a small number of specialist suppliers, many of them in China).
Iron can also handle higher temperatures
than the copper or especially its insulation, but, if it were to come to >>>> that, the hottest part of the iron would be in direct contact with the >>>> copper, so I don?t think that would help much. Iron?s Curie point is
770?, well above the Curie point of things like ferrite (pure magnetite >>>> is 585?) but both of those are well above the service temperature of
your insulation, unless you've sourced some of that exotic
ceramic-insulated wire Dalibor Farn? uses in his Nixie tubes.
None of the above is validated by me burning up any inductors, though, >>>> or even doing FEM simulations; it?s purely based on my fallible
theoretical understanding. Corrections would be welcome, especially
corrections based on actual measurement.
They would be very desirable, but expensive. John Larkin is the only
poster who is getting paid for his work, but he seems to have been
frightened by a coil winding machine when young and prefers
off-the-shelf woundl components.
I had a toroid winding machine when I was a teenager. And I've
designed maybe a hundred inductors and transformers since then.
Using the work "design" very loosely, as John is prone to do.
My laser driver will have about 15 pounds of capacitors. And inductors
are way worse than caps for energy storage. So the estimate of over
100 kilograms for the inductor is in the ballpark.
My point about letting the inductor saturate was that you don't need to >store all the energy initially present in the inductor, you only have to >store enough to clean out the last of the charge left in the capacitor
more quickly than a simple RC would. The estimate of 130 kilograms was
for a gapped inductor that was big enough not to saturate.
I'd rather use a depletion fet.
Of course you would. It only costs $0.43. It can only handle tiny
amounts of current, so it isn't going to discharge the capacitor fast,
but if you use it right, it too can get rid of the last of the charge
left in the capacitor faster than a simple RC, but a couple of orders of >magnitude slower than a saturating inductor.
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