film cap test

Fun! That's quite a lot lower than I would have expected, and a useful wrench for the toolkit.

Of course the null position just demonstrates that some series-LC someplace in a more complicated model has that inductance--the external leads, banana jack, and so on will contribute as normal.

That 8 nH is reasonably believable because the conduction current in adjacent foils goes in opposite directions and the capacitive displacement current will be more or less radially symmetric, so that the B field tends to cancel there too. (Inductance goes as the volume integral of |B/I|**2.)

Cheers

Phil Hobbs

Could it be a stacked-foil item, instead? Mechanically, those have windings, but the leadwires contact in two spots on each layer. They're always radial, not axial (I think...).

I really want a high-res xray machine. I guess I could whack this cap with a hammer and get an idea of the construction.

I just tested one of these

and it was remarkable. It would be cool to see what's inside.

John Larkin wrote: ===============

** Did you read the maker's spec? Says <1nH per mm of body and lead length.

It's a bog standard, Ilinois Capacitor: MMR series, metallised polyester.

.... Phil

The usual rule-of-thumb for wire is 20 nH per inch (800 pH/mm for the imperially challenged). However, that cap is 26 mm long, not even counting the banana connectors, which ought to be above 20 nH.

Cheers

Phil Hobbs

No, I tested a few sample caps for another engineer, to check his power supply Spice model and see if we'll have a problem with heating from ripple current at 250 KHz. I'm better equipped to measure these things.

I like to measure (and explode) parts. I wonder what will happen if I put kilovolts into this one. Any guesses?

The conductive path, end to end, is probably many concentric cylinders of foil, which I guess could have very low inductance. In my measurement, the leads themselves seem to dominate L.

The frequency null computes to 8 nH against the 6.8 uF capacitance. Things may not be that simple, but the tiny net impedance seems real.

I have a cool LCF program if anyone is interested.

I connected it to my trusty ole (it has one tube!) Kepco HV supply. I was expecting a kilovolt at least, but at 250 volts it made popping frying noises and got warm. But IR is still off the scale on my Fluke DVM and it's still a capacitor around its original value.

It appears to be a metalized film spiral that has big metal (flame sprayed?) electrodes on both ends.

We'll use lots of these so it's good to understand them.

I think the rule of thumb takes into account a return path for a straight wire separated by a PCB thickness.

The test jig illustrated has terminals mounted on

0.75 inch centers.

If that is being 'nulled', then you'd get a different measurement, not the full loop.

The rolled film cap construction has a larger width than a straight wire. This gives the path a reduced L/W ration, reducing both R and L.

RL

<snip>

. . . . . called 'schoopage'.

RL

Maybe they manufacture them shorted and use the self-clearing effect to remove the shorts.

And maybe they test them to arc-over, ditto.

Fun--named after a Dutch guy named Schoop.

Inquiring minds and all--is that "scoopadge" or "shoopadge" or "showpadge" or some *tertium quid*?

Cheers

Phil Hobbs

Nah, the 20 nH per inch number is for wire flapping in the breeze, per the ARRL Handbook et al. It does depend slightly on the gauge.

Yup. Also, while the field outside doesn't depend on the radial distribution of current, the field inside does, and the spiral construction forces it to be pretty well uniform.

Cheers

Phil Hobbs

Never mind, he was Swiss, so I expect that means it's roughly "scope-age", with a bit of "soup" in the 'o' sound.

Cheers

Phil Hobbs

I am puzzled where the huge inductance, so nicely visible on your scope shots, comes from as the layers are all shorted on each side.

John Larkin wrote: ==================

** Bet you will, polyester film caps are not rated for tolerating high frequencies with high applied voltages. ** I have actually done that. 250VAC* at 10 to 20kHz will smoke nearly any such cap in seconds.

• 50watt audio amp driving cap and 1 to 5mH air core choke in series at resonance - big fun.

** Polyester caps are quite fragile, even at 50/60Hz they will fail over time below rated AC voltage.

The issue is internal trapped air pockets and corona discharge across same. Class X1 and X2 types are "double wound " so act as 2 in series to stop this. Seems winding in vaccuo is just too damn hard.

..... Phil

8 nH isn't huge. Extending the wire leads up an extra half inch triples the inductive spikes.

The leads aren't even zero length on those Pomona adapters. Inductance would be even lower if the cap were soldered onto a pcb's power pours.

I should try that, just for fun. The caps are plenty good enough for our power supply uses.

At our voltages, we need films because ceramic caps will lose all their C.

We did some testing today. One of these caps runs cold at 48 VDC and half an amp RMS ripple curent at 250 KHz. And we'll run four in parallel.

John Larkin wrote: ==============

** So the ripple voltage is TINY !! It's the di-electric loss in the film that heats.

...... Phil