On Thu, 30 Mar 2006 19:19:12 GMT, 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.

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.

On Thu, 30 Mar 2006 19:47:06 GMT, Ignoramus27088 wrote:
On 30 Mar 2006 19:43:34 GMT, Dave Hinz wrote:

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.

It'd look like these:
http://images.google.com/images?q=hi...00&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.

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.

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

According to Ignoramus27088 :
On 30 Mar 2006 19:43:34 GMT, Dave Hinz wrote:

[ ... ]

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)

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

Without one, you are SOL -- unless you get other things made for
the purpose.

Take a look at eBay auction 7603726273. This is an
electrostatic voltmeter. It draws *zero* current. The range is set by
pulling out or pushing in the small knob on the end of that long
insulator sticking out the back. I can't get this particular auction to
show me the other images, so I can't see what the scales really are.
Some recent eBay auctions lock up Opera with weird JavaScript.

O.K. Using Mozilla, it looks as though it will go up to 30KV
full scale, if I am reading that label on the back properly.

Here is another -- 7605561926

What is it about the sellers of these which keeps them from
posting the voltage scales? But these are both dirt cheap for what they
are.

You can perform an interesting experiment with one of these and
a variable capacitor of the sort used for tuning old AM radios. Set it
up with the sections fully meshed (maximum capacitance), and connect it
to the meter. Momentarily touch a source of voltage enough to get some
deflection on the scale. Now rotate the shaft of the capacitor (which
is grounded to the frame, and you will see the voltage swing *way* up,
as the number of electrons used to charge it interact with the
capacitance changing.

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

Especially ones which you can *trust*. Generally, the skin oils
from handling will produce a leakage path significantly lower than the
highest value resistors. I have seen some as high as 2.5 terra Ohms. A
spiral carbon film deposited on a ceramic body, and covered with a
transparent shrink-on plastic sleeve.

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

Well ... the discharge route is best done with a large value
resistor, attached to the end of about an 18" clear PlexiGlass rod.
From the end attached to the rod (drill and tap the rod for a screw),
run a wire off to ground. Pick about five 22 MegOhm 2W resistors in
series -- solder with short leads so it is fairly stiff, and touch this
to the capacitor's terminal. This will put about 4W in each resistor at
the full 21 KV. Too much for the resistor long term, but probably
enough for the purposes. Make sure that the lead from the rod is *well*
grounded. And observe the voltage on a meter as it discharges.

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.

Yes.

Would testing them ALSO at HV be worthwhile?

Well ... it is possible for them to have failure modes which
will only appear at high voltage, and your purchasers will probably
expect them to work at their rated voltage.

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

That is wise.

I've been bitten by as much as 60 KV, but not at high current.

My worst one was actually at 2KV -- but *lots* of current --
from a 100 uF bank of capacitors.

Enjoy,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

On Fri, 31 Mar 2006 05:03:54 +0000, DoN. Nichols wrote:
According to Ignoramus27088 :
On 30 Mar 2006 19:43:34 GMT, Dave Hinz wrote:

[ ... ]

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)

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

Without one, you are SOL -- unless you get other things made for
the purpose.

Take a look at eBay auction 7603726273. This is an
electrostatic voltmeter. It draws *zero* current. The range is set by
pulling out or pushing in the small knob on the end of that long
insulator sticking out the back. I can't get this particular auction to
show me the other images, so I can't see what the scales really are.
Some recent eBay auctions lock up Opera with weird JavaScript.

O.K. Using Mozilla, it looks as though it will go up to 30KV
full scale, if I am reading that label on the back properly.

Here is another -- 7605561926

What is it about the sellers of these which keeps them from
posting the voltage scales? But these are both dirt cheap for what they
are.

You can perform an interesting experiment with one of these and
a variable capacitor of the sort used for tuning old AM radios. Set it
up with the sections fully meshed (maximum capacitance), and connect it
to the meter. Momentarily touch a source of voltage enough to get some
deflection on the scale. Now rotate the shaft of the capacitor (which
is grounded to the frame, and you will see the voltage swing *way* up,
as the number of electrons used to charge it interact with the
capacitance changing.

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

Especially ones which you can *trust*. Generally, the skin oils
from handling will produce a leakage path significantly lower than the
highest value resistors. I have seen some as high as 2.5 terra Ohms. A
spiral carbon film deposited on a ceramic body, and covered with a
transparent shrink-on plastic sleeve.

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

Well ... the discharge route is best done with a large value
resistor, attached to the end of about an 18" clear PlexiGlass rod.
From the end attached to the rod (drill and tap the rod for a screw),
run a wire off to ground. Pick about five 22 MegOhm 2W resistors in
series -- solder with short leads so it is fairly stiff, and touch this
to the capacitor's terminal. This will put about 4W in each resistor at
the full 21 KV. Too much for the resistor long term, but probably
enough for the purposes. Make sure that the lead from the rod is *well*
grounded. And observe the voltage on a meter as it discharges.

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.

Yes.

Would testing them ALSO at HV be worthwhile?

Well ... it is possible for them to have failure modes which
will only appear at high voltage, and your purchasers will probably
expect them to work at their rated voltage.

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

That is wise.

I've been bitten by as much as 60 KV, but not at high current.

My worst one was actually at 2KV -- but *lots* of current --
from a 100 uF bank of capacitors.

DoN, thanks a lot. I will buy a HV probe, the advantage of which is
that it can also help me discharge the caps. When I try them, I will
let you know. I have a plexiglass rod of some sort (from a hospital
dumpster), I will try to test it with a megohm meter and see if it is
suitable.

i

On Thu, 30 Mar 2006 19:47:06 GMT, Ignoramus27088
wrote:

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?

Measuring the capacitance will tell you that they are still capacitors, but
not that they will hold rated voltage. You could cgarge them up to the
capacity of your HV supply, let them sit a while then discharge them with a
well insulated shorting bar to see if they still held a charge.

BTW be careful about using the HV from a color TV, they can run well over
22KV on larger sets,

["Followup-To:" header set to sci.electronics.design.]
On 30 Mar 2006 19:43:34 GMT,
Dave Hinz wrote
in Msg.

They're not PCB-era, are
they? Because if they are, you just inherited someone else's problem.

It says "Impregnant: MIPB". That's monoisopropyl biphenyl which doesn't
seem to contain halogenes (PCB is polychlorinated biphenyl).

robert

On 30 Mar 2006 19:43:34 GMT, Dave Hinz Gave us:

They're not PCB-era, are
they? Because if they are, you just inherited someone else's problem.

You obviously didn't look at the auction page very well.

On Thu, 30 Mar 2006 19:47:06 GMT, Ignoramus27088
Gave us:

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

Low voltage testing is not definitive of their capacity at the
voltages they will be expected to be working at, and will likely be
off by quite a margin.

On 30 Mar 2006 19:59:23 GMT, Dave Hinz wrote:

On Thu, 30 Mar 2006 19:47:06 GMT, Ignoramus27088 wrote:
On 30 Mar 2006 19:43:34 GMT, Dave Hinz wrote:

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.

It'd look like these:
http://images.google.com/images?q=hi...00&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.

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.

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

Sounds like the fine beginning of a do it yourself rail gun kit

Gunner


"The importance of morality is that people behave themselves even if
nobody's watching. There are not enough cops and laws to replace
personal morality as a means to produce a civilized society. Indeed,
the police and criminal justice system are the last desperate line of
defense for a civilized society. Unfortunately, too many of us see
police, laws and the criminal justice system as society's first line
of defense." --Walter Williams

On Thu, 30 Mar 2006 19:47:06 +0000, Ignoramus27088 wrote:

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

Of course you can!! Do you have a piece of plexiglass about 15"-18" long,
maybe 1"-1.5" wide? Just put 100X 1M resistors in series, and pick off
1/100 of the voltage at the bottom resistor. Silicone them down in sort
of a zig-zag, solder the leads one to the next, trim them - leave a round
solder glob at each junction to cut down on corona, and slather silicone
all over the top and around the assembly.

Use a very high-impedance, input-protected voltmeter, like a VTVM ;-)

Good Luck!
Rich

According to Rich Grise :
On Thu, 30 Mar 2006 19:47:06 +0000, Ignoramus27088 wrote:

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

Of course you can!! Do you have a piece of plexiglass about 15"-18" long,
maybe 1"-1.5" wide? Just put 100X 1M resistors in series, and pick off
1/100 of the voltage at the bottom resistor. Silicone them down in sort
of a zig-zag, solder the leads one to the next, trim them - leave a round
solder glob at each junction to cut down on corona, and slather silicone
all over the top and around the assembly.

Use a very high-impedance, input-protected voltmeter, like a VTVM ;-)

You can't get more high-impedance than the electrostatic
voltmeter which I posted a link to last night. (Probably the auction is
closed by now.) But that kind of voltmeter would allow monitoring the
voltage real-time until you decide that it is now time to discharge the
capacitor.

Enjoy,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---