Safely testing 22 kV capacitors

[Bert Hickman]

Bert, thank you for yout very interesting post. I am not interested
in coin shrinking, however, I am interested in can crushing.

I would like to know if I can do some meaningful experiments with can
crushing if I keep only two caps. (1 uF, 22 kV). I do not want to keep
more. Perhaps somehow getting a "better", faster spark would help with
getting higher instantaneous amperages?

I have a bottle of argon, perhaps I can somehow inject it into the
spark gap to trigger the spark?

As for oscillation and the implied necessity to reduce voltage, what
woul dbe the appropriate voltage to charge the caps to?

i

You could try connecting two caps in parallel, charging them to about
13.2 kV discharging them across a 3 turn work coil. However, with two
caps you'll only develop about 174 Joules. But it may be enough to dent
the can a bit to show the effect. Using four caps in parallel would be
better. You could also use two caps and a higher charging voltage, but
you'll see shorter capacitor life - increase it enough (say to 22 kV)
and you may only see one shot... :^)

The peak current is virtually independent of the spark gap. It's a
function of the energy initially stored in the cap and the inductance in
the circuit.

0.5*LI^2 = 0.5*CV^2
or
Ipeak = V*sqrt(C/L)

Assuming that work coil L is about 1 uH, bank C is 2 uF, and V is 13.2
kV, then Ipeak would be about 18.7 kA total, or about 9 kA per
capacitor. This system will oscillate at about 92 kHz, and the skin
depth at this frequency is only about 0.008", so much more of the energy
will go into shrinking the can. The peak current may or may not exceed
the capacitor's ratings - the folks at General Atomics would need to
provide you with the actual specs for these caps.

Using four caps in parallel will provide ~350 Joules at peak current of
26.4 kA at about 65 kHz, but the skin depth at this frequency is about
0.013" so shrinking efficiency will be reduced. The lower peak current
of 6.1 kA/cap may be better for longer cap life, and the higher energy
level may provide better overall performance.

Bert
--
-------------------------------------------------------
We specialize in UNIQUE items! Coins shrunk by huge
magnetic fields, our "Captured Lightning" Lichtenberg
Figure sculptures, and Out-of-Print technical Books.
Visit Stoneridge Engineering: http://www.teslamania.com
-------------------------------------------------------

 

[Zak]

Hmm ... It says "impregnant MIPB" which sounds like something
different from PCBs. You might check whether they are still in
business, and ask them.

http://www.gaep.com/frequently-asked-questions.html

IPB, IPBP, MIPB = isopropyl biphenyl

MIPB is mono-isopropyl-biphenyl

Might be current stuff. Google can turn up more...



Thomas

 

[Winfield Hill]

Bert Hickman wrote...

Yes, with a maximum bank voltage of 7.1 kV (for can crushing). BTW,
that's the MAXIMUM energy I use - but can crushing can be done with
considerably less energy. Since the work coil remains intact during
can crushing, I assume a high Q load (100% worst case voltage reversal).
Under this scenario, the capacitor dielectric system would see a peak
voltage swing of about 14.2 kV, which is an ~18% voltage reversal
based on the 12 kV faceplate rating of the caps.

Starting at +7kV and swinging by 14kV takes the caps to -7kV,
isn't that a -7/12 = 58% voltage reversal? Or does the 20%
spec refer to rapid reverse-direction voltage swings rather
than reversed-voltage polarity?

... I recently bought a Pearson Model 301 50 kA wideband
current transformer to allow for isolated current measurements,
but haven't had a chance to hook it into the system as yet.

These can probably work to beyond-spec currents at the upper
end of their frequency range. You can test this by looping
multiple turns through the sensor, simulating "extremely-high"
currents. You can also parallel the sensor loop with alternate
wire paths, etc., to extend the range and you can calibrate the
setup at lower known currents to obtain the new ratio.

It'd also be valuable to grab the voltage waveforms, which is
easy to do with capacitive dividers. For example ~1 pF on the
HV side and 1000pF on the low side for a 1/1000 divider.

Appropriate shields are also necessary, since signal strays are
competing with the 1pF main path. The long-distance output coax
can be part of the 1000pF. You can complete the circuit with HV
resistors and zener clamps to protect the probe's opamp buffer
amplifier, which helps isolate an expensive scope. A trimpot can
be used for calibration at the output amp; a 10V cal signal gives
a 10mV output signal, enough for accurate scope readings during
the cal adjustment. I have made such dividers working to 25kV,
with a 100Hz to 10MHz bandwidth, and it shouldn't be too hard to
extend any of those parameters.

And, always remember to be afraid - very afraid - of the energy
stored in these caps. They will not give you any second chances.
=:^[

I'd like to ask a question about capacitor failure. Considering a
capacitor that's gradually degrading, I wonder if the final failure
can occur during charging, as opposed to discharge. This would mean
everyone should be far away behind shields whenever any paralleled
HV capacitor bank has a significant voltage on its caps.

 

[J. Clarke]

:For can crushing you need a minimum of about 400 Joules, and for coin
:crushing at least 2000 Joules. I happen to use a bank rated at 140 uF at
:12 kV. I also use Maxwell energy discharge caps, but mine are Series C
:100 kA high current type. Because mine are rated for only 20% voltage
:reversal, I only take the bank up to about 9500 volts (6300 Joules) for
:coin crushing. For can crushing, I only go to about 3500 Joules (mainly
:to reduce wear and tear on the spark gap switch).
:
:Using all of your 15 caps in parallel would give you a capacitor bank
:capable of delivering ~3 kJ, so you are in the right ballpark. However,
:can crushing (especially) and coin crushing can cause highly oscillatory
:discharges. Rapid voltage reversals are very stressful on a HV
:capacitor's dielectric system, and most of Maxwell's pulse caps are only
:rated for 10-20% voltage reversal (at faceplate voltage), so you don't
:want to run these caps anywhere near their full faceplate voltage if you
:are doing can or coin crushing - they WILL prematurely fail. And, you
:definitely don't want to be anywhere near the caps when the energy from
:the other 14 capacitors dump everything they've got into a single
:faulting cap... :^)
:
:Looking at the "Frankenstein" insulator style used on your caps, they
:are likely not rated for more than 2 - 5 kA peak (the folks at General
:Atomics can probably provide you with their actual specs):
:http://www.gaep.com/capacitors.html).
:
:Running more caps in parallel will help to share the peak current seen
:by each capacitor. If you plan to do any coin shrinking, treat the coil
:like a small bomb, with copper shrapnel being ejected at hyper
:velocities.
:
:There's more information on my site:
:http://205.243.100.155/photos/shrinker5.pdf (1 page summary)
:http://205.243.100.155/frames/shrinker.html (more gory details)
:
:And, always remember to be afraid - very afraid - of the energy stored
:in these caps. They will not give you any second chances. =:^[

Ever thought about the effect that might have on a nice chunk
of plutonium?

I'm curious, what would you expect it to be?

 

[Bushy Pete]

I have a bottle of argon, perhaps I can somehow inject it into the
spark gap to trigger the spark?


Might be worth a try, on the ARL spectrometers I play with, the spark will
not ignite at certain voltages when there is air present in the system. The
argon flush to the spark gap is typically about 5 seconds before starting
the spark.

Most of these have a small Teflon, Perspex or going back to the dark ages, a
bakelite insulating bowl that surrounds the gap and a tygon or similar
plastic type insulating gas supply line. These would typically be about 50mm
diameter and 30mm length internal dimension that would surround the
electrodes and keep the gas inside. An argon flow rate of about 4 litres
per minute will give a clean cavity within a couple of seconds, and spark
will normally occur as the gas mixes and gets rid of most of the oxygen in
the camber.

Most of the spark gaps are in the order of 3 to 5 mm and have a secondary
gap in air around the same distance. The secondary gap is in series with the
primary gap. This gives a nice spark that triggers in the 8KV range, but
this can vary quite a bit. The spark is initiated by a circuit similar to a
car ignition system, and as the voltage rises, it triggers at a voltage that
will vary with different gases or spark gaps.

However, be aware that relying on the difference between the argon and air
might not be reliable and you should treat it as if it was going to go off
with "Murphy's Law".

Hope this helps,
Peter

 

[Bert Hickman]

Bert Hickman wrote...


Starting at +7kV and swinging by 14kV takes the caps to -7kV,
isn't that a -7/12 = 58% voltage reversal? Or does the 20%
spec refer to rapid reverse-direction voltage swings rather
than reversed-voltage polarity?

The 20% spec actually refers to the maximum voltage swing during a rapid
discharge versus the rated DC voltage. In the above case we have
14kV/12kV = 1.16, or a 16% voltage reversal. See the following for more
info:
http://www.gaep.com/tech-bulletins/voltage-reversal.pdf

These can probably work to beyond-spec currents at the upper
end of their frequency range. You can test this by looping
multiple turns through the sensor, simulating "extremely-high"
currents. You can also parallel the sensor loop with alternate
wire paths, etc., to extend the range and you can calibrate the
setup at lower known currents to obtain the new ratio.

The Pearson 301X CT is rated at 50,000 amps (0.01 volts/amp, 50 ohm
output resistance, with a 3db cutoff at ~2 MHz). The 50 kA limit appears
to stem from the 500 volt maximum output voltage spec. However, another
Pearson App Note says that, if I terminate their CT's with a 50 ohm
load, I'll get half the output voltage/amp. I think this really means
that I can push the CT up to 100 kA without exceeding the 500 volt
output limit.

It'd also be valuable to grab the voltage waveforms, which is
easy to do with capacitive dividers. For example ~1 pF on the
HV side and 1000pF on the low side for a 1/1000 divider.

I do have a 60 kV Ross capacitive voltage divider. However, I'm
concerned about hooking this up to the storage scope because of the
possibility of getting substantial ground bounce when firing the system.
Measuring the current was is more attractive because of the fully
isolated measurement. Unfortunately, I don't have a battery powered
scope that I can float...

Appropriate shields are also necessary, since signal strays are
competing with the 1pF main path. The long-distance output coax
can be part of the 1000pF. You can complete the circuit with HV
resistors and zener clamps to protect the probe's opamp buffer
amplifier, which helps isolate an expensive scope. A trimpot can
be used for calibration at the output amp; a 10V cal signal gives
a 10mV output signal, enough for accurate scope readings during
the cal adjustment. I have made such dividers working to 25kV,
with a 100Hz to 10MHz bandwidth, and it shouldn't be too hard to
extend any of those parameters.

And, always remember to be afraid - very afraid - of the energy
stored in these caps. They will not give you any second chances.
=:^[

I'd like to ask a question about capacitor failure. Considering a
capacitor that's gradually degrading, I wonder if the final failure
can occur during charging, as opposed to discharge. This would mean
everyone should be far away behind shields whenever any paralleled
HV capacitor bank has a significant voltage on its caps.

It really depends on the type of pulse capacitor. The style C series I
have use extended foil construction with a paper or paper-film
dielectric with castor oil as the dielectric fluid. They are rated at
300,000 shots at rated current (100 kA) and voltage. Degradation in
these caps is usually from partial discharges and localized dielectric
damage (particularly at the edges of the foil plates) due to rapid
voltage reversals. This particular style cap is NOT self healing, and it
can indeed short out during the charging cycle. However, the thick steel
case is designed to easily contain a self-faulting cap. They are
designed to "contain" a catastrophic failure without rupturing even when
backfed by three other identical caps connected in parallel. But the
case will definitely be bulged from pressurized gas from the internal
electrical explosion.

Newer high density energy discharge caps use metallized film-foil
construction that is "self healing". This allows the manufacturer to
further push the limits of dielectric stress without risking sudden
failure of the entire capacitor. If a dielectric fault occurs, the short
will blow a small metallized bridge to that section of the capacitor
without causing any other damage. As faults progressively occur and are
cleared, the overall capacitance of the unit steadily decreases. Once
the capacitance has declined by ~5%, the capacitor has reached its end
of life.

BTW, irrespective of whether the cap's case contains the innards, high
energy capacitor failures are always exciting... :^)

Also, a wealth of technical information on high energy capacitor
construction and usage can be found at the General Atomics site:
http://www.gaep.com/technical-bulletins.html

Best wishes,

Bert
--
-------------------------------------------------------
We specialize in UNIQUE items! Coins shrunk by huge
magnetic fields, our "Captured Lightning" Lichtenberg
Figure sculptures, and Out-of-Print technical Books.
Visit Stoneridge Engineering: http://www.teslamania.com
-------------------------------------------------------

 

[Ignoramus24006]

Bert, I have been trying to find contact info on Maxwell labs to get
more in depth info on these capacitors, such as how many discharges
they are rated for, max current etc. I noticed on your page that you
mention Maxwell caps, do you happen to have any better info on them or
how to find out?

thanks

i

 

[DoN. Nichols]

Bert, I have been trying to find contact info on Maxwell labs to get
more in depth info on these capacitors, such as how many discharges
they are rated for, max current etc. I noticed on your page that you
mention Maxwell caps, do you happen to have any better info on them or
how to find out?

First item in a Google search for "Maxwell Labs" comes out as:

http://www.maxwell.com/

They are now "Maxwell Technologies", but that is a minor change.

Enjoy,
DoN.

 

[Winfield Hill]

DoN. Nichols wrote...

First item in a Google search for "Maxwell Labs" comes out as:

http://www.maxwell.com/

They are now "Maxwell Technologies", but that is a minor change.

Right name, wrong company. Through the miracle of modern corporate
buyouts, asset transfers, and whatnot, the Maxwell capacitor guys
and their products are now at General Atomics Energy Products in
San Diego. http://www.gaep.com/contact-us.html The fellows there
are very friendly and kindly sent me a FAX of the detailed datasheet
for my Maxwell #33504 100uF 10kV capacitors, a few years ago, even
though they're no longer manufactured. They apparently have file
cabinets filled with design info and datasheets, and brains filled
with experience and corporate history.

General Atomics Energy Products
General Atomics Electronic Systems, Inc.
4949 Greencraig Lane, San Diego, CA 92123
Phone: 858-522-8400 Fax: 858-522-8401

They also have a pile of useful application notes as online pdfs,
http://www.gaep.com/technical-bulletins.html One of these notes,
http://www.gaep.com/tech-bulletins/high-energy-capacitors.pdf
has a list of HV pulsed capacitors they made dating back to 1980.

One issue that hasn't been brought up is operating degradation and
pulsed-capacitor lifetime. These fellows do wear out, and the GAE,
formerly-Maxwell, folks provide detailed information so users can
estimate the remaining life in a HV pulse capacitor, and replace
it before there's trouble, e.g., Capacitor Engineering Bulletin
96-004, "The Effect of Reversal on Capacitor Life"
http://www.gaep.com/tech-bulletins/voltage-reversal.pdf

What happens in a government lab when a capacitor is removed from
service? I've seen large caps sitting over in a corner, considered
not good enough to place back into routine service, but not bad
enough to throw away either. "Could I have one of those?" I asked,
when on an open-house tour. "Probably," was the answer. So, when
we buy these big fellows on eBay, it's likely they aren't new and
unused, with a full life ahead of them. Another reason for caution.

 

[Ignoramus29226]

DoN. Nichols wrote...

Right name, wrong company. Through the miracle of modern corporate
buyouts, asset transfers, and whatnot, the Maxwell capacitor guys
and their products are now at General Atomics Energy Products in
San Diego. http://www.gaep.com/contact-us.html The fellows there
are very friendly and kindly sent me a FAX of the detailed datasheet
for my Maxwell #33504 100uF 10kV capacitors, a few years ago, even
though they're no longer manufactured. They apparently have file
cabinets filled with design info and datasheets, and brains filled
with experience and corporate history.

General Atomics Energy Products
General Atomics Electronic Systems, Inc.
4949 Greencraig Lane, San Diego, CA 92123
Phone: 858-522-8400 Fax: 858-522-8401

They also have a pile of useful application notes as online pdfs,
http://www.gaep.com/technical-bulletins.html One of these notes,
http://www.gaep.com/tech-bulletins/high-energy-capacitors.pdf
has a list of HV pulsed capacitors they made dating back to 1980.

One issue that hasn't been brought up is operating degradation and
pulsed-capacitor lifetime. These fellows do wear out, and the GAE,
formerly-Maxwell, folks provide detailed information so users can
estimate the remaining life in a HV pulse capacitor, and replace
it before there's trouble, e.g., Capacitor Engineering Bulletin
96-004, "The Effect of Reversal on Capacitor Life"
http://www.gaep.com/tech-bulletins/voltage-reversal.pdf

What happens in a government lab when a capacitor is removed from
service? I've seen large caps sitting over in a corner, considered
not good enough to place back into routine service, but not bad
enough to throw away either. "Could I have one of those?" I asked,
when on an open-house tour. "Probably," was the answer. So, when
we buy these big fellows on eBay, it's likely they aren't new and
unused, with a full life ahead of them. Another reason for caution.

Thanks Win and DoN. I appreciate you clearing up the corporate
issue. I will print out these PDFs tonight and will check out that
procedure (gotta leave for work now).

i

 

[Gunner]

spark gap to trigger the spark?


Might be worth a try, on the ARL spectrometers I play with, the spark will
not ignite at certain voltages when there is air present in the system. The
argon flush to the spark gap is typically about 5 seconds before starting
the spark.

Most of these have a small Teflon, Perspex or going back to the dark ages, a
bakelite insulating bowl that surrounds the gap and a tygon or similar
plastic type insulating gas supply line. These would typically be about 50mm
diameter and 30mm length internal dimension that would surround the
electrodes and keep the gas inside. An argon flow rate of about 4 litres
per minute will give a clean cavity within a couple of seconds, and spark
will normally occur as the gas mixes and gets rid of most of the oxygen in
the camber.

Most of the spark gaps are in the order of 3 to 5 mm and have a secondary
gap in air around the same distance. The secondary gap is in series with the
primary gap. This gives a nice spark that triggers in the 8KV range, but
this can vary quite a bit. The spark is initiated by a circuit similar to a
car ignition system, and as the voltage rises, it triggers at a voltage that
will vary with different gases or spark gaps.

However, be aware that relying on the difference between the argon and air
might not be reliable and you should treat it as if it was going to go off
with "Murphy's Law".

Hope this helps,
Peter

Add a short length of wire and you can probably shut down all the
cells sites and your neibors tv and cordless phones for a shorttime
too
<G>

Gunner



"A prudent man foresees the difficulties ahead and prepares for them;
the simpleton goes blindly on and suffers the consequences."
- Proverbs 22:3

 

[Bruce L. Bergman]

Add a short length of wire and you can probably shut down all the
cells sites and your neibors tv and cordless phones for a shorttime
too
<G>

Gunner

Hell, I'm surprised that the people running these spark-gap
transmitters haven't been visited by their local cellular, commercial,
broadcast and Ham operators - en masse, and in the classic "Torches
and Pitchforks" meeting initiation method. ;-)

That thing probably splatters the radio spectrum enough without
deliberately attaching a broadcasting antenna to it. The only saving
grace would be it's a very short pulse - not long enough to get a
normal DF lock.

I'd build and operate the whole coin shrinker rig in a Faraday Cage
to catch most of the RF, with the cage inside an all steel shipping
container ala "Mythbusters" for 'ballistic containment failures'.

--<< Bruce >>--

 

[Winfield Hill]

Bruce L. Bergman wrote...

I'd build and operate the whole coin shrinker rig in a Faraday Cage
to catch most of the RF, with the cage inside an all steel shipping
container ala "Mythbusters" for 'ballistic containment failures'.

The interference generated is in a single pulse lasting
only 20-50us at most, so why bother? What's a little
nuclear-EMP between friends?

 

[Cydrome Leader]

That means I do not have enough capacitors.

I never said it wasn't enough. This is what experimenting is for.

Thanks... So, what would you say, would 1 uF at 22kV crush a can?

Probably. Can crushing is actually pretty boring though. I crushed more cans just
putting them in the coil form than from actually using it. Cans are so weak, it's
just not interesting.

I might try again with full cans of pop. That would be messy, but more pleasing
to watch, and a bit harder to pull off.

 

[Bert Hickman]

Cydrome Leader wrote:

Probably. Can crushing is actually pretty boring though. I crushed more cans just
putting them in the coil form than from actually using it. Cans are so weak, it's
just not interesting.

I might try again with full cans of pop. That would be messy, but more pleasing
to watch, and a bit harder to pull off.

Messy - yes... :^)

You can see a full can of Red Bull being crushed here:
http://members.iinet.net.au/~pterren/misc1.htm#Can crusher 2

Bert
--
-------------------------------------------------------
We specialize in UNIQUE items! Coins shrunk by huge
magnetic fields, our "Captured Lightning" Lichtenberg
Figure sculptures, and Out-of-Print technical Books.
Visit Stoneridge Engineering: http://www.teslamania.com
-------------------------------------------------------

 

[Ben Bradley]

In rec.crafts.metalworking and sci.electronics.design, On 3 Apr 2006

Bruce L. Bergman wrote...

The interference generated is in a single pulse lasting
only 20-50us at most, so why bother? What's a little
nuclear-EMP between friends?

Now you got me intestered^wcurious^wwondering at what distance such
a discharge would destroy a cellphone's RF front end...

"Excuse me while I retract my car radio antenna before I set this
thing off..."