Safely testing 22 kV capacitors

After the deadly explosion, as you float above your body, you think to yourself, "Yup, it failed the test, now what...?"

I doubt that the shock from a 1u cap at 20KV is going to kill unless still hooked to the power supply. However, the reaction (falling backwards and hitting your head) sure can do damage.

I saw a fellow knocked across a room by 30KV on a large string of capacitors in a radar pulse forming network.

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I agree with Jeff that using the HV probe is a safe way discharge the caps. Also, you probably should buy one on ebay, since that will give you a probe with a proper high-voltage-safe housing and high-voltage- insulated wire. If you are feeling adventurous and have some high voltage wire on hand,

has directions for making a probe, using a 200-megohm "bleeder resistor from a defunct video terminal". (Although in newer monitors the bleeder may be built into the flyback assembly and harder to use.)

How long "a while" is depends directly on the probe's resistance. For example, if it is fairly low, like 200 megohms, the time constant RC is 2*10^8*10^-6, or 200 seconds. Voltage goes as V0 * e^(-t/RC). At time t+RC, you will have 37% as much voltage as at time t. If you start at V0 = 10000V, it will take 4.6 RC, or about 15 minutes, for the voltage to drop below 100V. If the probe's resistance is reasonably high, ie 1000 to 5000 megohms, the same drop would take 75 to 375 minutes - assuming the capacitor has no leakage.

Finding out whether (or how badly) each capacitor leaks is probably the most important thing to test. If you measure the voltage, then detach the meter, reattach exactly a minute later and remeasure, you can figure out the capacitor's leakage resistance via the equation above: R = t / (C*(log V0 - log V1)). (base e logs)

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I think you should read the high-voltage probe page mentioned above, and also

which has a number of comments about testing capacitors. Also, I don't think you should start testing at 9kV all that quickly; ie, test at low voltages first to get your test setup in order, and to weed out any of the caps that are too bad to test at HV.

AIUI, a Franceformer is a neon-sign transformer that delivers AC rather than DC; if so you will need a rectifier. Correct?

If you use line voltage or some other kind of transformer, such as an oil-furnace igniter, you'll need to use a high-voltage resistor in series with it and your rectifier to limit the inflow current when charging a cap. A neon sign transformer probably doesn't need such a resistor.

To avoid damaging terminals with sparks, you might want to attach some

6" wires to each cap, with hooks or loops formed on the wire ends to let you hang a test-leads carrier (fiberglass strip at end of leads) or a shorting bar from a distance.

-jiw

For the discharge resistor, leave your self some margin. If 5KV rated, put three identical ones in series if you are testing at 9 to 10 KV.

What procedure are you proposing to non violently check the caps are actually discharged? Personally I'd want to know they were fully discharged (well past 99%) before handling them. Also does your plan include checking the HV probe after use before relying on it to indicate they are safe?

Insulation can be a problem, beware of surface leakage, all surfaces need to be extremely clean and absolutely dry. I've used glass up to about 35KV with a grounded guard ring (in case there is a flaw I haven't spotted) with no trouble so if you have a large enough quartz glass tube that should be suitable to hold the resistors. Some uncolored plastics are also suitable for high voltage work. There is no way you'd get me using wood unless it was freshly kiln dried then vacuum impregnated with paraffin wax. Corona discharge can also be a real problem. It shouldn't be TOO bad at 10KV but you still want to be really carefull to get nice smooth connections with no sharp edges or points.

High voltage har a really annoying way of finding the slightest weakness or pinhole in any insulation and many TVs have had a fine fireworks display from the side of the LOPT on my bench. Its a bit inconvenient if one is just trying to get the TV powered up successfully to decide if its condition is good enough to justify LOPT replacement but with your setup it's potentially lethal. Its also important to confirm the integrity of the grounding wires in your test circuit. I left off the ground lead to the aquadag on a large screen TV by mistake ONCE, (in 12 years) now I'm rather cautious. You goof up your grouding, yoy'd better allready have got a good deal on a pine box . . .

Wild wacky and way out there idea!! Does anyone have any idea if a suitable length of carbon core vehicle ignition cable would make a satisfactory bleeder resistor? Iggy would be dumping 100 joules into it, its got distributed resistance and sufficient insulation so the main worry is can a maneagable length take the peak power?

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Ian Malcolm.   London, ENGLAND.  (NEWSGROUP REPLY PREFERRED)
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AARGH 1/2*CV^2 so 50 joules for 10KV (up too late!). Hmm, carbon core cable is even more appealing to me.

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Ian Malcolm.   London, ENGLAND.  (NEWSGROUP REPLY PREFERRED)
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For some reason I'm not seeing the original post, so I'll tack this on here--if their condition is unknown it would be a good idea to have a solid barrier between you and the capactor when charging and discharging the first time, and do one at a time..

Also, have good ventilation in the test area--if one blows you probably don't want to breathe what comes out of it.

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--John
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Thanks, I definitely will sell all of them, maybe except one.

i

Thanks Ben, I appreciate the advice.

i

Good idea. I will definitely buy a HV probe.

i

Just for reference, the energy stored in that cap at 22 kV is approximately 242 joules. About as much as small caliber rifle bullet.

i

Ian, would you or anyone have any idea if what I have is HV resistors?

They are blue, about 3 inches long, and are marked VICTOREEN MOX-3 12 MEG.

I would use a HV probe. And then a real short for extra safety, and then I would short their terminals to cases.

Thanks for the tips.

i

Come on - slowly ramp up the voltage to 22kv - hold steady - have cap in a cage box - in case it blows - and then ramp the voltage down. The supply will draw the charge off the cap. Just remember, if they blow, the case blows...

Now if you want to measure the capacitance at some high voltage - t=RC time (seconds) = resistance times capacitance. 1t = 66.6% voltage.

Odds are they are just surplus.

Martin

Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member

Ignoramus27088 wrote:

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Discharging caps that way can ruin the cap. Just turn off the supply or apply a resistor load. The internal plates violently move - ripping themselves up.

You get away with it - smile - sell it - it blows up when used. Not a happy day.

Martin

Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member

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Ignoramus, I strongly protest your post. Here is an interesting conversation that you started, with 32 posts so far, according to the Google usenet archive, of which you have made 14, or 44% of the posts. Yet you marked all your posts "do not archive" - which means they will be removed from Google Groups in 6 days, thereby severely damaging the stored thread. With this kind of callous action on your part, one wonders why others should even take part in the already-broken, soon-to-be-bogus conversation. What the h____ are you doing here?

--
 Thanks,
    - Win

A FranceFormer is a neon sign transformer, and definitely AC.

Hmmm, better be sure that stick is dry, clean and free of cracks in the wood.

The boom might be quite impressive, as these are probably Marx generator pulse discharge caps, and may have a current capacity of tens of thousands of amps.

Jon

Winfield, I am sorry, I changed my settings to remove the XNA header.

i

Yes, they say "DISCHARGE CAPACITOR" on the dataplate. They probably were using them to power high power lasers. One of my acquaintainces worked there on lasers at some point.

i

Well, let's see. P = 22 kV squared / 1E6 = 484 Watts. Well, that isn't so amazing, but I don't know where you will find a 1 Meg Ohm resistor with a 500 W rating and a 22 KV voltage rating.

Jon