discharging caps

Not that I know of. Note that the tolerance printed on your can was -10%

The need to reform long unused electrolytic capacitors was well known everywhere I worked. Nobody ever warned me about any "drying out", and mythical electro-chemical effects weren't mentioned either. Back when I was a graduate student in chemistry the electrochemists did get a hard time, but back then they didn't have good tools for looking at the microscope detail of the very thin layers where the electrochemistry took place.

And electrolytic capacitors are even trickier than regular capacitors. Charge soak isn't nice.

Hey, I saw that. It measured +66%.

Charged to 100v and discharged for 2 seconds, that big cap went to about 0.25 volts and then only recharged itself to 2 volts.

Not bad.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Which does suggest that it had been left uncharged for long enough to need reforming.

The tolerances are just arbitrary cut-offs on what's probably a Gaussian distribution. Presumably the manufacturer samples individual capacitors often enough to be confident that the distribution is stable, but don't measure every capacitor that they ship. That +66% might just have been an outlier.

It's more of a worry in capacitor-based A/D converters. Dual ramp and quad ramp schemes come to mind. Not all that clearly - that was back in

It charged to 100 volts with a power supply that was limited to 20 mA. The rampup was very linear.

I remember dual-slope ADCs. I designed a few. Very long ago. IC adc's are dirt cheap now and super accurate. 24 bits for $2.

The caps that people can make inside ICs are about the best caps that are possible.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

You ignore simple rule of thumb: energy is more efficiently stored in air. So to get energy storage needed for critical damping you need inductor as big or bigger than air cored inductor. You need core because otherwise winding resistance is likely be too big for critical damping.

It seems that inductor with EI core with the following dimensions could satisfy the needs:

Surface of the central column: 200 cm^2. Side of cental column: 14.14 cm. Height of cental column: 21.21 cm. Width of winding window: 7.07 cm. Total height of core: 35.35 cm Total width of core: 42.42 cm Total volume of core: 16962.87 cm^3 Weight of the core: 129426.74 g Air gap: 1cm

To get L = 5H needs 1727 turns. I get 1.43 Ohm as winding resistance.

So I guess that if one really needed such an inductor it would be practical. But it is bulky. I am not sure if it is bigger than the capacitor bank, but I expect it to be heavier.

Which one of these specs represents the radius of the inductor? That would be a great help in visualizing it.

I need that in surface mount.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Imagine rectangular box of dimensions 43x36x30 (all 3 in centimeters).

The power companies do that all the time...big box mounted to a concrete slab...surface of the earth mount! :-)

Efficiency is just the ratio of your solution to an ideal solution.

You haven't indicated what you are comparing.

Energy is more simply stored in an iron cored inductor because can get a higher inductance in a given volume - the iron eventually saturates which complicates life, and the iron would act as a shorted turn if you gave it half a chance.

That doesn't follow.

We can all dream of superconducting wire, but all the versions I know of stop being super-conducting at a high enough magnetic field. I once got to clamber around the Nijmegen University's super-conducting magnet so I know that that can be a pretty high field.

You haven't specified the core material. My guess is that you would have to glom it together from rectangular lumps of ferrite. With that much air-gap, the exact material wouldn't matter much.

I'd be more interested in a toroidial core wound out of iron ribbon.

The data sheets aren't exactly helpful.

Just like the capacitor?

Sigma delta parts. Back when I last used one, in 1993, it spat out

Sloman A.W., Buggs P., Molloy J., and Stewart D. “A microcontroller-based driver to stabilise the temperature of an optical stage to 1mK in the range 4C to 38C, using a Peltier heat pump and a thermistor sensor” Measurement Science and Technology, 7 1653-64 (1996)

If a little too small for you application.

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).

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.

Approximate formula for maximal energy stored in inductor with a gap is:

E = S*B_max^2*(l_i/mu + l_g)/(2*\mu_0)

where E is the energy, B_max is maximal possible induction in the core, S is surface area of the perpendicular cut trough the core, l_i is average length of magnetic path in the core, l_g is effective path trough the gap, \mu is relative magnetic permeability of the core, \mu_0 is magnetic permeability of the vacuum.

The formula above assumes that you can pass whatever current is needed trough the winding and the only limit to current is due to core saturation. As you can see better magnetic permeability _decreases_ maximal possible energy, simply core will saturate at lower current and gap significantly increases possible energy storage.

For comparison, formula for inductance is:

L = N^2*S*\mu_0/(l_i/\mu + l_g)

where N is number of turns and the other are as above. So design for high energy will by neccessity have lower inductance. Winding resistance is proportional to N^2, so if you need higher ratio of inductance to resistance you need to go for bigger inductor or lower stored energy.

The formula above shows this clearly: removing core allows bigger B and increases l_g term. Of course, once gap it too big approximation is rather poor, but trend is clear.

Yes. And we dream of superconducting wire which needs no refrigeration.

I assumed iron. What matter is maximal allowed induction. Actually AFAICS going slightly into saturation does not hurt, so I assumend operation slightly above normal limits.

As I explained, main trouble is core saturation. The approximate formula applies to toroids too.

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

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Cool. I wouldn't need a resistor. 130 KG of iron and copper could absorb a lot of joules.

The inductor was a great idea, Bill.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Bah chalk it up as another failure for Google AI. This is the second major error I have encountered, the other being a visualization involving a SIMPLE cube.

Waldeck Hebisch may have designed an inductor, though he hasn't bothered to post a link to the data sheet for the EI cores he has in mind, and his claim that it needs an airgap isn't justified by any kind of argument.

Toriodal cores made by winding iron tape (or thin strips of other high-permeability ferromagnetic material) offer more microHenries per turn,and saturate at higher magnetic fields that the ferrites he appears to have in mind.

At the moment using an inductor to speed up the discharge process is looking like a bulky solution, but the capacitors you need to discharge quickly aren't exactly surface mount parts either.

We all know about putting an air-gap in the magnetic path to increase the energy stored at the expense of the inductance you can get out of a given core.

That depends on the energy you are trying to store.If you can store enough energy without saturating the core, the inductor can be a lot smaller than an air-cored inductor

If you need to saturate the core, and it is beginning to looks as if John Larkin would have to.

That depends on the application. There are jobs that can pay for the refrigeration. Research magnets are the original examples, but there's going to be a magnetic resonance imaging system in a hospital near you. I've got one just down the street.

High temperature super-conductors might work with just liquid nitrogen, which is cheap enough, but nobody wants to keep a rack of electronics submerged in liquid nitrogen.

But you didn't spell out that crucial detail.

You didn't explain that at all in your original post, and you certainly didn't specify the saturation field you had in mind.