Safely testing 22 kV capacitors

IPB, IPBP, MIPB = isopropyl biphenyl

MIPB is mono-isopropyl-biphenyl

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

Thomas

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?

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.

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.

--
 Thanks,
    - Win

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

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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

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:

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.

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...

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:

Best wishes,

Bert

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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

According to Ignoramus24006 :

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

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

Enjoy, DoN.

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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.

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,

One of these notes, 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"

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

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

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

Gunner

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

- Proverbs 22:3

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'.

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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?

--
 Thanks,
    - Win

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

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.

just

it's

Messy - yes... :^)

You can see a full can of Red Bull being crushed here:

Bert

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-------------------------------------------------------
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
-------------------------------------------------------

...

All I can say is, you guys are nuts! ;-)

Cheers! Rich

Well, it's those who start with a LED-flasher, then move on to salvaging caps from disposable cameras. The next step is the microwave x-former. That's when they start celebrating Tesla's birthday. Now comes the neon supply and finally they do coin shrinking and can crushing. Only when they have ended up with the Darwin Award we do not read any more posts. BTW I did coin shrinking with 7yrs. by putting them on the train rail and can crushing I still do with my foot.

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ciao Ban
Apricale, Italy

Hey hey. I think you meant "at 7 yrs. of age", but there is a lot of sense to your methods. On the other hand, we can easily go a quarter mile by walking or on a bicycle, but some people find it entertaining to do it with nitro-fueled funny cars in a few seconds.

It's arguably stupid, but ah heck, those few of us should enjoy wasting energy for fun while it's still possible and legal.

We have IMHO the right to do foolish things, Rich is smoking his head off, there are the Audiophools, others watch Big Brother on TV. Just be prepared to get answers from engineers in this NG.

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ciao Ban
Apricale, Italy

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..."