Monday, January 25, 2010

mystery capacitor

Some years ago, I was given an apparent high voltage capacitor. Last weekend, I dragged it out to investigate it further.

It looks very old in terms of materials and construction, and it also looks like a one-off. It might be home-made, or it might have been custom made in the 1940s, it's hard to say.

mystery cap 1 web.jpg

That's a six inch ruler sitting on the paper in front of it.

The case is constructed of folded and soldered sheet brass and painted with black enamel. The case has considerable taper from the vertical on all four sides. The top is a thin sheet of phenolic laminate, fastened to the lips of the case with four small brass machine screws and nuts.

As is obvious from the photo, the connections are two wide copper straps.

Inside, the case is filled with an unknown wax:

mystery cap 2 web.jpg

I melted and burned a small sample, and it smells like neither paraffin nor beeswax. The wax is a dark amber color.

The capacitor measures about 25 nF at 1kHz and a low test voltage. I may try again at a higher charge voltage and frequency, but I don't expect the value to change much. No doubt the wax becomes lossy at very high frequencies, but this thing doesn't look like an RF capacitor, it looks like a pulse capacitor to me.

I have no idea what its maximum working voltage might be, although the construction and dimensions suggests it can't be good for more than a couple of tens of kilovolts.

I hope to get around to setting up a test rig this week to measure its breakdown voltage safely. The general idea with that is to limit the breakdown current to such a low value that no damage can possibly be done to the dielectric. We especially want to avoid carbon tracking through the wax. No arcs allowed!

Toward that end and others, I really ought to build myself a general purpose high voltage power supply some day.

The case being tapered (ie; it has draft) suggests that the builder might have been thinking of removing the molded capacitor once cooled. Hard to know.

I will post more data on this beast as I generate it.


c0rundumb0y said...

Those electrodes look fairly mighty, and solid copper. I'd not mess with the wax too much - health and safety wasn't a big thing in the '40s. In fact they were borderline cavalier.

Perhaps it was a defibrilator built for elephants. I'm stumped.

Bill Lemieux said...

Eventually I'll get around to doing the Bilstein test on that wax. I think I have an actual PCB test kit somewhere, but it's intended to work with oils, and I'll be darned if I'll melt any more of that wax than I have to, ventilation or no ventilation. (in fact, I'll probably wear a respirator while I'm doing the test)

The main thing to avoid with PCB is ingestion or inhalation. One can always wear gloves, too, just in case.

Say, is 'corundumb' in any way related to tungsten cowhide? (AKA Crapaloy)