6.5kV 8345 Microfarad Capacitor - Page 1
Forum section: electronics / general

author: VE7FIM
author_rank: Contributor
timestamp: April 18, 2017, 02:26:23 am
content: Not sure if this is the right place for this, but wow — *this* is a capacitor...http://www.ebay.com/itm/General-Atomics-32947-6-5kV-8-1F-Energy-Discharge-Capacitor-/292085271551If my calculations are correct, when fully charged, this thing would store 1.71 x 10^8 Joules, and if completely discharged in 0.1 seconds, would release 1.71 gigawatts of power.

author: mariush
author_rank: Super Contributor
timestamp: April 18, 2017, 02:35:55 am
content: It's not 8.1 Farads, it's 8160uF +/- 10% or 8345 uF (measured) as you can see on the label.That's 0.008345 Farads, quite a lot less than 8 Farads.

author: VE7FIM
author_rank: Contributor
timestamp: April 18, 2017, 02:43:24 am
content: That makes a lot more sense...Good catch.Still, 1.71 megawatts of power is still pretty impressive...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 18, 2017, 04:28:18 am
content: FWIW, at 0.3uH ESL, the short circuit time constant would be on the order of 77us (1/4 wave time, of a ringing waveform). The resonant impedance is 6mohm, so the peak current is a bit less than 1 megaampere, so the peak power (instantaneous V*I) is on the order of 5GW.Conclusion: I don't recommend touching the terminals when it's charged.You would normally use it at much less than this, since you accumulate inductance quickly with all that bus bar (what's shown is probably about 1uH, total), and more again from whatever the load itself is.Tim

author: KE5FX
author_rank: Super Contributor
timestamp: April 18, 2017, 07:11:17 am
content: Quote from: T3sl4co1l on April 18, 2017, 04:28:18 amStill need to take ESR into account, don't you?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 18, 2017, 08:00:54 am
content: Quote from: KE5FX on April 18, 2017, 07:11:17 amAs low as Zo is, ESR should be lower still -- it's a film cap.The Q should be low, in the single digits, but not 1 or below.Low Q causes the ringing frequency (pseudofrequency) to be slightly lower (the change goes as 1/(RC)), and the ringing to decay fairly quickly, so that the first peak is noticeably lower than V(charge) / Zo (which, if you check my math, you'll notice I made a guess at).Tim

author: bitwelder
author_rank: Super Contributor
timestamp: April 18, 2017, 02:02:50 pm
content: Once fully charged, how long would you need to wait for it to self-discharge until it's down to safe levels, if nothing is plugged to its terminals?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 18, 2017, 04:54:59 pm
content: Quote from: bitwelder on April 18, 2017, 02:02:50 pmTake a regular 1uF 250V film capacitor, charge it to 250V. How long would you wait for self-discharge?Take 26 of them and put them in series. Charge the series string to 6.5kV (assume they charge evenly). How long would you wait for self-discharge?Take 26 * 8345 of the strings and put them in parallel. Charge the mass to 6.5kV (assume they charge evenly). How long would you wait for self-discharge?In each case, you have the same voltage, on the same film cap. Size doesn't matter.Tim

author: AF6LJ
author_rank: Unknown
timestamp: April 18, 2017, 05:32:41 pm
content: Quote from: T3sl4co1l on April 18, 2017, 04:28:18 amLooks like just what the engineer ordered for the aluminum can crusher I was dreaming of building. Now to get on designing that ballistic switch.

author: SeanB
author_rank: Super Contributor
timestamp: April 18, 2017, 05:48:40 pm
content: I would be a lot more worried about dielectric absorbtion that self discharge, that size will still be able to develop a pretty big punch even after a week of being stored shorted, then left open circuit for a few hours, after soaking for a day at full rated voltage.Best use i can see for one of these is to make a pulsed Bussard fuser though.Does not ship to my country though, will have to see if I can persuade the scrappie to give some of those power line ones, just to use as decoration.Looks like it also comes with a nice puck thyristor array switch as well, saves buying one if it is included.

author: james_s
author_rank: Super Contributor
timestamp: April 18, 2017, 06:10:52 pm
content: You could probably make a hell of a coin shrinker with one of those. I'm no stranger to high voltage stuff but a capacitor like that even makes me nervous. There is no second chance if you have an accident with that. It's likely to turn whatever body part contacts the terminals into a pulpy mist and you'd probably be dead before you hit the ground.

author: KE5FX
author_rank: Super Contributor
timestamp: April 18, 2017, 08:09:40 pm
content: Quote from: SeanB on April 18, 2017, 05:48:40 pmIt would make sense to keep a few megs of bleeder resistance permanently connected across one of these, I think. I can't see much upside to leaving 150+ kJ worth of energy sitting around for months or even years at a time. It'd be like keeping a live grenade in your desk drawer at work.

author: PartialDischarge
author_rank: Super Contributor
timestamp: April 18, 2017, 08:23:27 pm
content: If I lived in the US I'd definitively buy this, not only the capacitor is included but clamped ABB thyristors+firing circuitry+oscillator board, so nice.

author: AF6LJ
author_rank: Unknown
timestamp: April 18, 2017, 08:25:36 pm
content: Quote from: james_s on April 18, 2017, 06:10:52 pmYou could but the secret to that is the same as the secret to making a good aluminum can crusher. You need a switch that will go from on to off in a fraction of a microsecond.People have tried to build can crushers with SCRs but they take for ever to turn on. (milliseconds) during that time they operate in a linear mode and much of the useful energy is converted to heat.In the 1980s the Russians were trying to build a a capacitive discharge EMP generator.They tried all manner of switch mechanisms They settled on a ballistic switch, the moving contact was driven home with explosives, a compressed gas discharge system might have worked as well.. Both systems eliminate the rise time issues that exist with SCRs and other thyristers.I don't know if it is still up there...There was a YouTube video of a guy who built a can crusher with a big cap and a spring loaded switch... His system had other problems like the wire size used for the coil was too small and on one occasion his coil blew up.This stuff has fascinated the hell out of me, I use to work next door to one of Maxwell Industries plants. They built an EMP generator that took a city block offline one day due to ineffective shielding. They also built a gun that shot a .4" aluminum covered Mylar bullet that could penetrate 3/4" steel decking. There was an article in the local paper about that gun, then nothing heard.... Makes me wonder if the third generation of that gun isn't orbiting up in space above us...Some day I am going to build my can crusher.

author: james_s
author_rank: Super Contributor
timestamp: April 18, 2017, 08:29:23 pm
content: I saw a coin shrinker in person at a Maker Faire years ago, it used 3 suitcase sized capacitors that were charged by a large neon sign transformer. The switch was a spring loaded thing with large tungsten contacts, it seemed to work well but the coils were single use, they vaporized in the process. What surprised me was the noise, the discharge sounded like a large caliber handgun.

author: PartialDischarge
author_rank: Super Contributor
timestamp: April 18, 2017, 08:38:43 pm
content: Quote from: AF6LJ on April 18, 2017, 08:25:36 pmThis guy did it that way because he didn't know any better, the best way to build a can crusher, or anything that requires a fast total discharge of a kV capacitor with kAs of current flow is a thyristor: extremely fast on times, in 10us you can have thousands of amperes flowing provided the correct gate firing, and the voltage across will easily drop from a few kVs to a few volts (provided appropriate clamp force of press-pack devices and good metal to metal contact)

author: Kilrah
author_rank: Unknown
timestamp: April 18, 2017, 08:45:54 pm
content: Quote from: james_s on April 18, 2017, 08:29:23 pmYou don't need that much, in the past I've used a direct short of a simple large photo flash cap (1000uF 350V) charged to rectified 230V mains precisely to simulate a gunshot in a theatre play. Worked a treat, louder than those pyro caps... and reusablePretty sure I still have the cap in a drawer. That was about 20 years ago and the cap was taken out of a broken flash that must have been at least 10 years old back then...

author: AF6LJ
author_rank: Unknown
timestamp: April 18, 2017, 08:50:51 pm
content: Quote from: MasterTech on April 18, 2017, 08:38:43 pmThe ones I have seen in the 10,000A and up take milliseconds to turn on.In this day and age you may be able to do it with FETs, but that is a lot of current you are dumping into a one or two turn loop, make from copper buss bar.

author: PartialDischarge
author_rank: Super Contributor
timestamp: April 18, 2017, 09:07:14 pm
content: Quote from: AF6LJ on April 18, 2017, 08:50:51 pmWithout knowing the exact dimensions of the thyristors in the cap bank its difficult to know the model, but it is an ABB one and could be this one:https://library.e.abb.com/public/f15d6377c6c2ffb183257c63004dc804/5STP%2026N6500_5SYA1001-07%20Mar%2014.pdfHave a look at the specs and get marveled, that thing can pass 100kA in less than 1ms.

author: RGB255_0_0
author_rank: Frequent Contributor
timestamp: April 18, 2017, 09:15:02 pm
content: How capable is this one compared to Photonicinduction's one he used to obliterate stuff? It was the quickest way of making a smoothie I've seen.

author: KE5FX
author_rank: Super Contributor
timestamp: April 18, 2017, 09:43:09 pm
content: A hydrogen thyratron could be a possibility, but they seem to top out at 10K - 100K amps. Sure can't complain about the rise (fall) time, though.When you start looking at data sheets from EG&G, it's probably time to step away from the workbench for a while and get some fresh air...

author: AF6LJ
author_rank: Unknown
timestamp: April 18, 2017, 09:51:51 pm
content: Quote from: MasterTech on April 18, 2017, 09:07:14 pmThat's a good part but it takes 3ms to turn on...That is going to get really hot in those three milliseconds, and that means that energy isn't getting to the load.But it is faster than many I have seen that are in the 10-16 millisecond range.

author: AF6LJ
author_rank: Unknown
timestamp: April 18, 2017, 09:54:09 pm
content: Quote from: blueskull on April 18, 2017, 09:34:12 pmI am thinking it would take many in parallel.The turn-on time would be as fast as the slowest one, and the drop across many in parallel would be manageable, but it wouldn't be my first choice.

author: AF6LJ
author_rank: Unknown
timestamp: April 18, 2017, 09:57:19 pm
content: Quote from: KE5FX on April 18, 2017, 09:43:09 pmThe more you look into solid state devices, the more a ballistic switch looks like the right choice.Unless you want to get into a electrically activated pneumatically actuated, vacuum switch.And at that point; I'll stand back and watch the manufacturing and testing of the switch.This could be a mechanical engineer's nightmare.

author: james_s
author_rank: Super Contributor
timestamp: April 18, 2017, 10:17:43 pm
content: Quote from: Kilrah on April 18, 2017, 08:45:54 pmThe coin shrinker was probably orders of magnitude louder than that. It was outdoors in the open, sounded like a .44 Magnum or maybe something bigger, it was *loud*!

author: AF6LJ
author_rank: Unknown
timestamp: April 19, 2017, 03:03:01 am
content: Quote from: blueskull on April 18, 2017, 10:03:26 pmYou are right I misread the datasheet.It might do the job, I would feel more comfortable with two of them in parallel.

author: AF6LJ
author_rank: Unknown
timestamp: April 19, 2017, 12:11:49 pm
content: Quote from: evb149 on April 19, 2017, 07:00:04 amVery Cool.Thanks for passing that along, the technology has evolved to quite a high level...

author: amyk
author_rank: Super Contributor
timestamp: April 20, 2017, 02:15:22 am
content: Quote from: james_s on April 18, 2017, 10:17:43 pmThe sound is the shock wave caused by the rapid expansion of air, heated by the arc which forms when the coil explodes and vaporises.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: April 20, 2017, 03:22:37 am
content: There is all sorts of fun to be had discharging high voltage capacitor banks. In my formative electronics years I assembled a largish bank at about 600 VDC using every salvaged electrolytic I could lay my hands on. I don't remember, but it was probably a couple of thousand microfarads. The discharge "switch" was the contacts on the end of two wands. Inductance? ESR? Anything like that was far into my electronics future. Touch them to the object of interest and watch things happen. What happened was widely varied depending on the materials and other things that were uncontrolled in my crude experiments. One of the more interesting was discharge into a tungsten light bulb filament. Left a beautiful star of filaments hanging in the air. Each filament maybe half a meter long, finer than spider silk, but possible to catch and pull proving that they weren't just smoke trails. Never could duplicate that. Using fairly clean water and doing the discharge underwater had interesting effects as the mass of the water provided confinement, while the steam produced provided pressure.Of course all of this was terribly dangerous and I couldn't possibly have survived. As the Mythbuster's used to say - "Don't try this at home. I was an expert."