Safely testing 22 kV capacitors

Nice!

Wow, you'd think they'd have someone a bit more technical doing their auction listings?

Well, the DC going to a CRT is in the 20KV range for color sets, I believe. Do you have a high voltage probe, and are you comfortable working with that sort of voltage? (one-hand technique and all that)

You'd want to discharge it through a resistor, in a carefully insulated circuit. Me, I'd pass, I can work with HV but I choose not to. Then again, nobody is going to buy untested caps. They're not PCB-era, are they? Because if they are, you just inherited someone else's problem.

This is kinda a sad commentary on the kind of people they'll hire at Fermilab these days, no?

I don't know the proper care and feeding of such high energy storage capacitors, but when I've seen people working with them they always carried a long (about 6') wooden stick with a shorting strap (about 1"x6") on top to always ensure the caps were discharged before they touched them.

Thanks Dave. I agree.,

Well, I suppose "it is not _their_ money".

I think that I do not have a 22kV rated probe, no.

I am aware that I cannot built a divider with regular resistors, and high voltage resistors might be hard to come by.

I think that I can easily test them at low voltage with a capacitor testing function of my multimeter.

Would testing them ALSO at HV be worthwhile?

Thanks Dave, yes, I am very apprehensive of HV.

i

Indeed.

OK, I think that we are up to something here.

The test setup that I may try is as follows:

- 9kV franceformer feeding the cap, with remote disconnect leading to the HV pole

- Wires from the cap connected to two junk steel flats, all set safely apart

- A shorting stick (also made of junk steel) with a wooden broomhandle

The procedure would be

- connect everything

- put on hearing protection

- turn on the franceformer

- disconnect franceformer with remote disconnect.

- turn face away, short the steel flats using broomhandle

- hear BOOM

- describe testing in the ebay ad

i

It'd look like these:

Yikes. OK, go to images.google.com and enter fluke high voltage probe

While I am aware of and respect your design skills and abilities, I don't think this is the time or application for a homebrew solution.

Sure, but that doesn't give you any evidence of the health of the electrolyte at high voltage. Wouldn't be "tested" from my perspective if I were a potential buyer.

I think so, yes. Certainly would get you better bids if you could show charge being held over time. Hell, give 'em a table of your test data. Buyers love to know that you've done your homework.

Respectful avoidance is the approach I take. I saw a guy get zapped by a defib he was fixing once. Didn't take off his ring, and he shorted from one side of the cap to ground. He didn't get anywhere near the full current, but it knocked him on his ass, hard, and we got a 12-lead EKG on him right away (this is when I was working biomed in a hospital). Nothing alarming on the EKG, but it was pretty exciting for all concerned. The cardiologist we had take a look at his waveforms was impressed, and it left a hell of a burn on his ring finger. Obviously he did at least two things wrong for this to happen, but he's a smart guy, just made a mistake. So please be careful. A HV probe can probably be bought on eBay for not too terribly much, and you can probably resell it for about the same later when you're done. Apparently, you're better at the "selling" part than I am, I usually buy something with the intent to sell it when I'm done with it, and then never get to that selling part.

Dave

Don't complain. The best bargains on E-bay come from poorly described/misdescribed items.

I worked at Fermilab circa 20 years ago, BTW. In general to any surplus property/stockroom person, the presence or absence of a little letter like "k" in "kV" doesn't mean much.

DOE budgets aren't as big as they used to be but I'm sure some interesting stuff is gonna show up from Fermilab on E-bay sooner or later!

I remember in the 90's when the Vandenberg shuttle launch site was being shut down and equipment surplused. I always wanted my own shuttle pad!

Tim.

Something in the dim recesses of my mind is telling me that a "boom" discharge would be bad for caps. I'd rather, as a buyer, see a one, five, and ten minute voltage check, and know that it was discharged through a power resistor. Those, I can help you with.

Yes, I had one like this, but, alas, I sold it.

Also a good point. Maybe I will just buy a Fluke HV probe on ebay.

Very scary. You convinced me, I will buy a HV probe. Another question is safe discharge. I just posted a message with some homebrew plan, but now I am not so sure that it is the way to go.

i

Thanks Dave. I will go this route, then... I would try to get a save resistor...

i

Fermilab is kind of like the military, no one is interested in getting highest possible prices. It is not "their money". The military, though, has a greater volume of items, if I compare Great Lakes GovLiquidation vs. Fermilab. This lot, though, was hard to pass up.

i

Unmless you are going to test them to 80% of their maximum voltage rating or to their full working voltage, I wouldn't describe them as tested for resale. You'd be better off checking them at say 1KV and then selling them as untested but belived working with a (short) warrenty.

I wouldn't even want to ship or store them without shorting straps accross each one.

If you do decide to do the high voltage testing, a TV LOPT commonly operates at up to about 27KV but the tube has typically 1/100 of the capacitance. A TV EHT probe or meter is easily capable of measuring these voltages safely, though personally I'd want to set it up in a grounded clamp stand so I wasn't hand holding it.

Discharging them safely is going to be a real problem. The obvious approach is a series chain on high voltage high power resistors in an insulating tube that is good to at least 30KV. Maybe a chain of bleeder resistors from microwave ovens? However I'd want specialist advice myself. Why not ask on sci.electronics.repair. Do you have a drum of suitably rated EHT cable? If not you cant really make ANY connections safely to them.

--
Ian Malcolm.   London, ENGLAND.  (NEWSGROUP REPLY PREFERRED)
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It's really, really fun to drop them in a bucket of water while fully charged.

Actually, it doesn't do much, but you get a nice crack and a cloud of steam each time.

We did that at the Transitron lab because the engineering department was too cheap to spring for a high voltage bleeder array. Not that it would've cost much, but they didn't want to use up "valuable bench space".

Now... dropping a wet paper towel across the terminals -- THAT's fun!

LLoyd

Thanks. What I normaly do is describe the testing method. For instance, I would say, hypothetically:

``I charged them to 9,090 volts, and after 10 minutes, they showed charge of 9,034 volts. Will ship with a shorting strap. Untested beyond that.''

Great point.

Ian, I would try to go the high voltage resistor route...

i

After looking at HV resistors on ebay, I realized that I may have a couple in my junk pile (pieces from a radar test set). Will check tonight. There are probably 5 kV rated, but I could put 2 in series.

i

If you want to test them closer to their actual rating, you could charge two caps to 9kv each, and then hook them in series across a single, giving you

18 kv. Doing it all with broomsticks, of course. For discharge, you could point a steam hose in their vicinity. Again, with a long stick, and from a good distance.

I would start by inventing remotely operated aligator clips.

Last time I did a boom with a capacitor, it broke the "broomhandle" AND the capacitor terminal leaving a nice leak hole... And that was ONLY 300V, but 10uF. I think the resistor way is safer.

ok... what would be a good resistor... I figure, 1 megohm or so should be just about right, RC of these 1 uF caps would be about 1 second. Discharging most of about 40 joules in a couple of seconds.

i

Why hasn't anyone mentioned yet that if you DO get a HV probe for your meter it will serve as a resistor to discharge those caps.

It will take a while, but you can observe the progress on the meter it's hooked up to and when the voltage gets down to something that won't jump out and grab you, an easier to obtain lower voltage rated resistor can be used to pull them the rest of the way down quickly.

MAKE SURE you watch out for "dielectric adsorption" which is the property of some capacitors to "recharge themselves" when left open circuited after you've discharged them. Needless to say, that effect can make for some nasty suprises. To play it safe, I'd leave a shorting link or low value resistor connected across them after they're discharged.

Know anyone with a small Van DeGraf generator you could borrow? One of those could probably charge up those caps slowly enough for you to watch the voltage rise on the meter, and you could turn it off when you got to

20 KV.

Just my .02,

Jeff

--
Jeffry Wisnia

(W1BSV + Brass Rat \'57 EE)

"Truth exists; only falsehood has to be invented."

.

You cant test them properly so why bother trying. As far as safety is concerned just remember that any charge above 1kv on them is likely to kill you, above 3kv and its allmost a certainty. Capacitors like these can recharge themselves long after youve discharged them. Just get rid of them to someone who knows how to handle them.