Safely testing 22 kV capacitors

or

Thanks Roy. That's encouraging. I will try to get appropriate HV equipment and then will test the caps. I may keep one for myself.

i

I was thinking of that, actually. Iggy, what would you need for some of these?

The Wayback Machine

will have the old version of the site, of course.

Just think if those Win and those other people got fed up and had Goggle remove all of their archived posts? Then the archives would be nearly useless. All that would remain would be newbies asking incomplete questions, flames, and idiotic remarks.

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

On Sat, 01 Apr 2006 15:57:14 GMT, "Michael A. Terrell" Gave us:

There was a time when they archived regardless of the flag setting.

With all the money google has, one would think that they would have several hundreds of terabytes of archival data online and available. One would think they would have other service providers using their servers as the master archive.

That is still no excuse for a poorly planned web interface to Usenet.

I think that I would need all my caps, and some way to vent the copper gas from exploding copper coils. In other words, too violent for me.

I want to keep one or two caps at most. I think that I could build a coke can crusher (you can do a google search for "can crusher capacitor"), using materials readily available, such as solid copper wire, fiberglass, pvc pipes, and steel balls. I think that 2 uF at 18 kV could be enough to deform the cans.

i

How about a Wimshurst machine?

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

Mark, thank you VERY much for looking it up.

i

Well of course it is! That's what makes it fun... And as I recall he had huge warning signs plastered all over the site, too. And triggered the shrink blast from outside the garage while standing behind a healthy barricade like a truck.

That's why I tell all my customers the old saw that is SO true: "I'm a trained professional - If you see me running, try to keep up."

But usually minus all the pictures.

-->--

--
Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700 
5737 Kanan Rd. #359, Agoura CA 91301 (818) 889-9545
Spamtrapped address:  Remove the python and the invalid, and use a net.

Bruce,

I like the saying that there are no old, bold captains.

i

Iggy, does that transformer output alternating current or direct current?

--Winston

AC, as it turns out.

i

I used to crush quarters. "copper gas" is the least of the problem you will encounter, and not the reason the solenoids explode.

20 kV is low for quarter crushing.

If it matters, I used 14uF of 50kV energy discharge caps. 30-35kV worked the best, although inductance is of the circuit is as important as the voltage.

I didn't dare to connect a scope up to measure the waveforms.

Use pipe not wire for can crushing.

That means I do not have enough capacitors.

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

i

Nice catch!

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):

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. =:^[

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

In article , Bert Hickman wrote: : :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): :

: :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?

--
Bob Nichols         AT comcast.net I am "RNichols42"

Not much- plutonium (and manganese) are the most resistive metals, IIRC. They would induction melt great, though. Both of these facts have the same reason: the more resistive a metal is, the less bEMF it makes and the more induction power it consumes, with less reaction force (Lenz' law).

Basically, the discharge's energy would go into heating up the block of pluotonium, with little force, if any. Although I wonder if you could melt or vaporize the surface, what with conductivity, skin effect and instantaneous power being what they are.

Tim

-- Deep Fryer: a very philosophical monk. Website:

I thought you used four 70uF 12kV caps in series parallel, for a 70uF 24kV rating? The coin crushing is where you count on the wire coil disintegrating within the first half cycle, with the arc rapidly extinguishing to limit reversal the voltage? Or are you charging to a smaller fraction of the 24kV bank faceplate rating?

How much of the energy is taken up by the coin crushing and coil stretching?

That's using 140uF? That would be two paralleled 70uF 12kV caps from your bank, charged to 7kV, or about 60% of the cap's faceplate rating? Implying only 35% voltage reversal while staying under a 20% limit? Is that with a 3-turn coil, which would be about 1uH? What's the Q of the 13kHz resonance?

So, sticking to 60% of the faceplate rating, that'd be 13kV allowed on the full 15 x 1uF = 15uF cap bank, which would be only 1.3kJ available, where 3.5kJ is needed for can crushing?

For a 3-turn 1uH coil and 140uF caps at 7kV, that's 83kA peak in your case, Bert? 83kA/15 = 5.5kA. But a higher voltage would allow using more inductance and lower peak currents.

--
 Thanks,
    - Win

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

Although my capacitor bank is capable of being configured for 24 kV (+/-

12 kV with cases of all four caps grounded), I presently use only half of the bank since it provides excellent results and is completely compatible with previous charging/control/safety hardware from the earlier bank that used three 54 uF 15 kV GE caps. [The previous GE caps weren't up to the task and they began catastrophically failing. Ruptured cases, gunky arc-blackenned Geconol dielectric fluid oozing onto the floor.... it wasn't pretty.] The pair of Maxwell caps have delivered well over 6000 trouble free shots over the last few years.

It's really hard to say... but I'd be surprised if even 50% of the energy actually ends up going into shrinking the coin. Considerable energy goes into explosively ejecting coil fragments. A fellow shrinker in Texas has calculated fragment velocities of up to 5000 fps.

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.

I haven't measured the actual circuit Q, but anticipate it's at least

15-20, with most of the losses coming from the spark gap. BTW, can crushing is quite hard on spark gaps - lots more evaporated metal than with coin crushing. 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.

Also, my current coin shrinker is really not very "efficient" for crushing cans since its operating frequency is comparatively low. The system actually oscillates at about 11 kHz (including loop inductances from cabling, capacitors, and spark gap switch). The compressive force on the can is a function of skin depth, which at 11kHz is about 0.024". Since a typical aluminum beverage can only has a wall thickness of about .0035" (about 1/6th of the skin depth), most of the work coil's magnetic field passes through the can, leaving only a small portion to do crushing. Using a lower capacitance, higher voltage bank would work significantly better for can crushing.

However, the current 140 uF system is almost ideal for crushing coins (from an esthetic and practical standpoint). Lower capacitance/higher voltage systems begin encountering coil flashover problems once you go beyond ~20-25 kV. The coins also begin to develop "toroiding" (i.e., having thicker edges versus the interior). For example, here's a Silver Eagle 1 Oz coin shrunk with higher voltage lower capacitance shrinker in Texas: http://205.243.100.155/photos/HVStuff/Silver_Eagle@15.4kJ.jpg

Using 60% voltage derating should work assuming these caps are rated for

20% reversal. And, since it uses a higher operating frequency, can crushing should be considerably more "efficient". 400 Joules should be more than sufficient to demonstrate the effect.

Yes. However, increasing the inductance lowers the operating frequency, reducing can crushing "efficiency", so there's a trade-off. YMMV...

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