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Safely testing 22 kV capacitors

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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,

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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

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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

Reply to
James Waldby
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I took Iggy at his word since he said it was DC and he seems knowlegable about what he wants to do.

So tell us Iggy, is it truly a 9 kv DC supply?

If it isn't, maybe we can use our metalworking and mechanical skills to help him design a syncronous motor driven mechanical rectifier switch to get the DC he needs,

Jeff

Reply to
Jeff Wisnia

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.

Reply to
J. Clarke

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

i
Reply to
Ignoramus27088

Thanks Ben, I appreciate the advice.

i

Reply to
Ignoramus27088

Good idea. I will definitely buy a HV probe.

i
Reply to
Ignoramus27088

Don, now that you say that, no, I am not sure that it produces DC. I will check.

i
Reply to
Ignoramus27088

I think that Don is right, the franceformer does not say DC or AC, so I think that it is most likely AC.

i
Reply to
Ignoramus27088

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
Reply to
Ignoramus27088

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:

Reply to
Martin H. Eastburn

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

Ignoramus27088 wrote:

Reply to
Martin H. Eastburn

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?

Reply to
Winfield Hill

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

Reply to
Jon Elson

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

i
Reply to
Ignoramus6399

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
Reply to
Ignoramus6399

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

Reply to
Jon Elson

And, about the starting energy for a defibrillation sequence. Get that sucker in the wrong point in your waveform, and you're _staying_ dead.

Reply to
Dave Hinz

That'd seriously limit it's usefulness in charging a cap, unless you've got some of them thar fancy AC capacitors.

Reply to
Dave Hinz

I have a bunch of 1N4007 diodes (1000v rated). If I put, say, 20 of them in series, would that be sufficient to rectify 9,000 VAC safely? I have read some articles discussing that since leakage amps are not identical, that I need to put resistors in parallel with them. If so, these need to also be 1kV rated resistors, right?

If not, would anyone have a suggestion for a 30 mA rectifier for 9 kVAC?

thanks

i
Reply to
Ignoramus6399

Connecting differently charged caps like these together with wires will end up with no wires and much noise. And, any resistance, from just the wires themselves to high value resistors will burn off half the energy transferred (if the wires live long enough to complete the energy transfer.)

And, if you have a 0.5 uF cap charged to 18 KV, and connect it to a discharged 1 uF cap, you would expect the final voltage to be

2.25 KV, I think, due to the smaller cap value (2 1 uF caps in series) charging the 1 uF, and the loss of energy on transfer. This is a standard exam problem.

Jon

Reply to
Jon Elson

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