Ignoramus27088 wrote:
Got myself some capacitors for $10 apiece.

http://cgi.ebay.com/ws/eBayISAPI.dll...tem=7603382621

The specs are 22 kV, 1 uF, discharge capacitor. See

http://igor.chudov.com/tmp/cap.jpg

The seller was wrong in describing them as 22 VOLT capacitors. They
are 22 KILO volt capacitors. (the seller is Fermilab).

They are similar to this one:

http://cgi.ebay.com/ws/eBayISAPI.dll...tem=7598631638

but have twice more capacitance.

Anyway, here is my question. I have a 9 kV DC power supply. (a
Franceformer). How can I safely test these caps before selling
them. At 22 kV, they can store about as much energy as a .22 bullet,
according to my calculations.

It would be less at 9 kV, but still, obviously, very deadly.

So. What is a safe way to charge them, verify that they hold the
charge, and then DIScharge them at 9 kV.

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."

Jeff Wisnia wrote:
Ignoramus27088 wrote:
Got myself some capacitors for $10 apiece.
http://cgi.ebay.com/ws/eBayISAPI.dll...tem=7603382621
The specs are 22 kV, 1 uF, discharge capacitor. See
http://igor.chudov.com/tmp/cap.jpg
....
Anyway, here is my question. I have a 9 kV DC power supply. (a
Franceformer). How can I safely test these caps before selling
them. At 22 kV, they can store about as much energy as a .22 bullet,
according to my calculations.
....
So. What is a safe way to charge them, verify that they hold the
charge, and then DIScharge them at 9 kV.
....
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.

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, http://www.repairfaq.org/sam/hvprobe.htm 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.)

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.

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)

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.
....

I think you should read the high-voltage probe page mentioned above, and
also http://www.angelfire.com/electronic/...store_cap.html
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

Jeff Wisnia wrote:

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.
Big Van DeGraf machines produce microamps! Charging a uF to 20,000 V
at one uA will take 20,000 seconds. And, that's one of those 6 foot
tall jobs with a 6" wide belt.

Jon

On Thu, 30 Mar 2006 16:19:47 -0500, Jeff Wisnia
Gave us:

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.

If you were observant enough of the thread, you would find that even
the original poster mentioned the HV probe loading.

Roy L. Fuchs wrote:
On Thu, 30 Mar 2006 16:19:47 -0500, Jeff Wisnia
Gave us:


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.


If you were observant enough of the thread, you would find that even
the original poster mentioned the HV probe loading.


I guess my powers of observation are failing me Roy, but I always like
to check out where I missed something and try and figure out why it
escaped me.

Would you kindly point out where the OP revealed that he realized a HV
probe would serve as a safe discharging means, albeit a rather slow one?
(Before I mentioned it that is.)

Thanks,

Jeff

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

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

Jon Elson wrote:

Jeff Wisnia wrote:


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.

Big Van DeGraf machines produce microamps! Charging a uF to 20,000 V
at one uA will take 20,000 seconds. And, that's one of those 6 foot
tall jobs with a 6" wide belt.

Jon


Live and learn. I was remembering the ones I played with years ago which
IIRC put out more like 20 ua, but I do agree that even that much current
would make for an uncomfortably long wait. G

I remember having to test the radiation hardness of some satellite
equipment I designed. We went to a GE facility in Pennsylvania where
they had a "flash x-ray" generator.

It used a BMF horizontally oriented Van De Graf IIRC about 25 feet long
and two feet in diameter which took many minutes of running to build up
the requisite high voltage in a storage capacitor. The charge was
flashed through an x-ray tube to produce a humungous blast of x-ray
photons into my poor little circuits.

Thanks for the mammaries,

Jeff

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

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

On Thu, 30 Mar 2006 21:48:08 -0600, Jon Elson wrote:

Jeff Wisnia wrote:

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.
Big Van DeGraf machines produce microamps! Charging a uF to 20,000 V at
one uA will take 20,000 seconds. And, that's one of those 6 foot tall
jobs with a 6" wide belt.


How about a Wimshurst machine?

http://www.google.com/search?q=wimshurst

But wtih any kind of static generator, you'd need some kind of voltage
limiting, wouldn't you? It wouldn't do to destroy the caps while trying
to test them! Is there such a thing as a calibrated spark gap? Maybe
a shunt regulator from an old, old TV - or the whole HV section, but
again, it'd have to be calibrated somehow.

Cheers!
Rich