Safely testing 22 kV capacitors

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,

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

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

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?

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.

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

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

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.

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

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

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.