Safely testing 22 kV capacitors

[Ignoramus27088]

Got myself some capacitors for $10 apiece.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=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?ViewItem&item=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

 

[Dave Hinz]

Got myself some capacitors for $10 apiece.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=7603382621
The specs are 22 kV, 1 uF, discharge capacitor.
Nice!

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

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

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.

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)

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

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.

 

[Joel Kolstad]

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

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.

 

[Ignoramus27088]

Nice!

Thanks Dave. I agree.,

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

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

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)

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.

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.

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

 

[Ignoramus27088]

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

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.

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

 

[Dave Hinz]

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

It'd look like these:
http://images.google.com/images?q=h...US:official&percentage_served=100&sa=N&tab=wi

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

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

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.

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

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.

Would testing them ALSO at HV be worthwhile?

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.

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

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

 

[Tim Shoppa]

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

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.

 

[Dave Hinz]

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

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.

 

[Ignoramus27088]

It'd look like these:
http://images.google.com/images?q=h...US:official&percentage_served=100&sa=N&tab=wi

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

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

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.

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

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.

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

 

[Ignoramus27088]

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.

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

i

 

[Ignoramus27088]

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!

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

 

[Ian Malcolm]

Got myself some capacitors for $10 apiece.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=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?ViewItem&item=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

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.

 

[Lloyd E. Sponenburgh]

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.

It's really, really fun to drop them in a bucket of water while fully
charged<G>.

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

 

[Ignoramus27088]

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.

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.''

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

Great point.

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

i

 

[Ignoramus27088]

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

 

[Leo Lichtman]

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.

 

[OBones]

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.

 

[Ignoramus27088]

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

 

[Jeff Wisnia]

Got myself some capacitors for $10 apiece.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=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?ViewItem&item=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."

 

[[email protected]]

Got myself some capacitors for $10 apiece.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=7603382621

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

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.

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.