How to charge a 46F capacitor to a p.d of 15kV with out destroying power supply. - Page 1
Forum section: electronics / beginners

author: axawire
author_rank: Contributor
timestamp: January 17, 2021, 12:25:24 am
content: Hi,I need to charge a 46F capacitor with a voltage across it of 15kV with out blowing the power supply. The power supply can only supply 0.2mA max. I believe I am able to ramp up to the 15kV at about 5V increments. Would an acceptable approach be to just add a resistor of say 25000 Ohms or maybe even 50000 Ohms in series with the capacitor to make sure my increments of 5V do not exceed the 0.2mA? Or am I not thinking about it correctly or is there a better way?Thanks.

author: The Soulman
author_rank: Super Contributor
timestamp: January 17, 2021, 12:35:16 am
content: 46F or 46uF?What model power supply is it?It may have current limiting and be short circuit tolerant.Beginner? Stay save.

author: IanB
author_rank: Super Contributor
timestamp: January 17, 2021, 01:04:04 am
content: I think you have your calculations messed up a bit. Using the formula E = ½CV2, a 46 F capacitor charged to 15 kV would hold about 5 GJ of energy, which is equivalent to a little over a ton of TNT. Quite apart from the fact that you wouldn't want to blow up your house and much of the surrounding neighborhood, I seriously doubt you have a 46 F capacitor that can be charged up to 15 kV.Even if the capacitor is 46 µF, any capacitor charged to 15 kV is astonishingly dangerous. Instant death dangerous. It's a one way trip to the morgue without passing through ER.

author: drussell
author_rank: Super Contributor
timestamp: January 17, 2021, 01:04:19 am
content: Yeah, I think there must be a unit error of some sort there....As an aside, assuming zero losses it would take what, 11 years at 0.2ma constant current to put the 690000 coulombs of electrons in there?!LOL, not to mention the capacitor would probably be the size of a building.

author: Brumby
author_rank: Unknown
timestamp: January 17, 2021, 01:11:41 am
content: 46F at 15kV is a touch over 5gigajoules!IF your values are correct, I wouldn't want to be anywhere near the thing for fear of something going wrong - because if anything DID go wrong, we aren't going to hear from you except on the nightly news.Quote from: The Soulman on January 17, 2021, 12:35:16 amThis is a critical question. If you missed out the micro prefix and the capacitor is only 46uF, then this just becomes extremely dangerous, rather than potentially catastrophic.<snap>Such a question in the beginners section makes us rather concerned.What were you planning on doing with this capacitor, once charged?

author: srb1954
author_rank: Super Contributor
timestamp: January 17, 2021, 01:32:25 am
content: Quote from: IanB on January 17, 2021, 01:04:04 amThere wouldn't be anything left to take to the morgue. Unless you swept up the soot stain off the floor.

author: KE5FX
author_rank: Super Contributor
timestamp: January 17, 2021, 01:38:46 am
content: Maybe tokamak constructionwasn'tsuch a good topic for a "For Dummies" book...

author: drussell
author_rank: Super Contributor
timestamp: January 17, 2021, 01:43:31 am
content: Quote from: KE5FX on January 17, 2021, 01:38:46 amI figured the OP was modifying his lathe to do time travel like This Old Tony's Colchester....

author: amyk
author_rank: Super Contributor
timestamp: January 17, 2021, 03:45:42 am
content: 46F@15kVA barrel of oil is approximately 6GJ. The difference is that it can't release all that energy in a fraction of a second.The capacity and 5V increments mentioned by the OP makes me wonder if he has connected a huge amount of supercaps in series...Asides the safety warnings already noted above, I'd really like to see some pictures of this capacitor bank...

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 17, 2021, 04:44:21 am
content: The OP undoubtedly has a units error, and has a much smaller micro, nano or even picofarad capacitor. And while these much smaller capacitors would still be dangerous I don't think they are terribly so. A favorite prank trick of my youth was to leave such capacitors charged and laying around for the unwary. While there were many who played this game I am not aware of any serious injuries that resulted. The most common minor injuries were bruises and sprains, some when those encountering the capacitors hit something because of the muscle contractions and some when those same people found the perpetrators of the prank.

author: Brumby
author_rank: Unknown
timestamp: January 17, 2021, 04:58:14 am
content: Quote from: CatalinaWOW on January 17, 2021, 04:44:21 am@15kV?

author: m3vuv
author_rank: Super Contributor
timestamp: January 17, 2021, 05:13:46 am
content: is it a flux capacitor for a delorean??

author: rt
author_rank: Regular Contributor
timestamp: January 17, 2021, 05:16:32 am
content: The datasheet you posted in the other thread indicates this is a Matsusada S3-15P.https://www.eevblog.com/forum/repair/identify-component-connected-to-high-voltage-power-supply/msg3408212/#msg3408212These are designed for generating large electric fields at near-zero current for e.g. electron beams/etcThe inline protection resistor (white cylinder in your image) is hopefully >10Mohm and would be mainly to protect the device from short circuit. The data sheet indicates that the S3 series is not protected against continuous short circuit.In terms of voltage variation the data sheet indicates ~1% setting accuracy so you would only be able to ramp it rather coarsely to 15kV.Definitely read further if you are unsure about anything in the datasheet and ensure you understand what you need to do to protect yourself.Perhaps some of the posters here may be able to suggest some fun (and hopefully safe-ish) experiments with this equipment but it appears unsuited for charging 46F stored energy weapons.rt

author: rs20
author_rank: Super Contributor
timestamp: January 17, 2021, 05:34:03 am
content: I know this is getting old, but by my rough calculations, a 46F 15kV capacitor bank would weigh at least 140 metric tons.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 17, 2021, 06:04:11 am
content: Quote from: Brumby on January 17, 2021, 04:58:14 amWho knows? They were oil filled caps. I remember various rated voltages, some as low as 3kV some in the 20-30kV range. As far as I remember they were for noise suppression on the neon sign transformers of the day. The charging method was pretty random. An automobile ignition coil placed to arc into the cap and an interrupting switch. We had no way of measuring the voltage, but presumably it was random, sometimes only a few hundred volts, sometimes much higher. I do know that the shock was much more painful than the one from similarly sized caps charged to the 500-600 volts which was the maximum my homebrew plate supply could deliver.It was all the kind of stuff that a bunch of early teen boys get into.

author: james_s
author_rank: Super Contributor
timestamp: January 17, 2021, 06:09:04 am
content: Even assuming it's "only" 46uF, this is one of those things where if you have to ask, you shouldn't be messing with it.

author: bdunham7
author_rank: Super Contributor
timestamp: January 17, 2021, 06:28:59 am
content: Quote from: axawire on January 17, 2021, 12:25:24 amWhat do you mean by 'increments'? The way to do it would be to connect the power supply to the capacitor with a series resistor that limits the current to the 200uA. In this case that is 15000/.0002, or 75 megohms. And what size is the capacitor, actually? If it is 42picofarads(pF), OK. If it is 42 of anything else, you need to figure out what you are doing before you have an incident.

author: axawire
author_rank: Contributor
timestamp: January 17, 2021, 07:36:31 am
content: Some of these comments make me chuckle, I did not actually check to see how long charging time would take... anyway I am estimating the 46F capacitor mainly based off the formula for a radial capacitor per unit length and adding a little extra wiggle room... Capacitance per unit length C=2PiEpsilon/ln(Rb/Ra) where Ra is inside radius and Rb is outside Radius, Epsilon is 2.1, Ra=2mm,Rb=3mm and as its capacitance per unit length I then need to multiply it by the length 1.33 meters.You can imagine it as a two copper tubes one with in the other with a separating material of PTFE. This is then wound up into a coil shape to save space. This is then also dropped in a container of vegetable oil.I have also considered charging it with a van de graaff generator instead of a traditional power supply - is this maybe the better approach?

author: Slh
author_rank: Regular Contributor
timestamp: January 17, 2021, 07:56:28 am
content: Always good to put your workings. Looks like the error is that you've used the relative epsilon we where you should have used the absolute. Replace epsilon = 2.1 with er*e0 = 2.1 * 8.854*10^-12 and you'll get something closer to the right answer (in the pF).

author: bingo600
author_rank: Super Contributor
timestamp: January 17, 2021, 08:00:09 am
content: Quote from: axawire on January 17, 2021, 07:36:31 amAt these energy levels. Wouldn't that be an excellent propellant , if the temp is high enough ?Is this a "mind excersise" or ?

author: axawire
author_rank: Contributor
timestamp: January 17, 2021, 08:03:01 am
content: Oops, thanks good catch. So revising then its about 3.018e-10F, so 0.3018nF. Being charged with 15kV power supply.

author: Brumby
author_rank: Unknown
timestamp: January 17, 2021, 08:09:39 am
content: Quote from: axawire on January 17, 2021, 08:03:01 amThat's much more sane!Watch out for the 15kV, though!!!!!

author: George Edmonds
author_rank: Frequent Contributor
timestamp: January 17, 2021, 09:48:55 am
content: Many years ago now I was involved in the design and commissioning of a capacitor discharge welder that involved charging a one Farad capacitor to 2.5KV.George

author: Zero999
author_rank: Super Contributor
timestamp: January 17, 2021, 11:38:07 am
content: Quote from: axawire on January 17, 2021, 08:03:01 amIt's vitally important to be able to estimate the magnitude of a measurement/calculation. E =1/2CV2. A 10 000µF (0.01F) 50V electrolytic capacitor is a physically large capacitor, so now think about how big a 1F 50V capacitor would be and that because there's a square in the formula, a 100V 1F capacitor would be four times as big. Now imagine how big a 46F 15kV capacitor would be. Think about how much of a blast it would generate, when suddenly discharged, given a 0.01F capacitor charged to 50V, causes a loud bang and bright spark, when short circuited.Hopefully you can now see why people didn't take the original post seriously.

author: drussell
author_rank: Super Contributor
timestamp: January 17, 2021, 06:04:32 pm
content: Ah, yes...300pF ≠ 46F, only off by what, about 12 orders of magnitude.That soundsmuchmore plausible...(Still enough to be seriously dangerous, though... so be very careful if you play with this!!)

author: KE5FX
author_rank: Super Contributor
timestamp: January 17, 2021, 11:09:34 pm
content: Eh, it's enough to hurt, but I wouldn't call it dangerous to most reasonably-healthy subjects. 300 pF is probably about what you get from the aquadag in a CRT.I have a ~500 kV van de Graaff machine with a 50 pF top sphere. That hurts quite a bit more, noting that it's the V term that gets squared in the stored-energy equation, not the C.

author: dbctronic
author_rank: Regular Contributor
timestamp: January 18, 2021, 02:25:00 am
content: I used to play with old TV sets, in the 60s. The aquadag of a good sized color picture tube, at 20 kV, can kill, at least send you flying into a wall. Make sure your device has a safe (think long plastic handle) shorting bar to discharge the cap before poking busy hands around it. Regular switches won't hold off 15 kV. Also, a neon bulb with leads on well insulated handles will help verify discharge.How had you planned to measure cap voltage?

author: KE5FX
author_rank: Super Contributor
timestamp: January 18, 2021, 05:08:57 am
content: If you die from a CRT discharge, you were probably going to kick the bucket pretty soon anyway, given that the voltage and energy available isn't that much higher than a really bad carpet shock, in the 100-500 mJ range.The real risk is that you'll drop the CRT, or otherwise damage yourself with reflex action.

author: Berni
author_rank: Super Contributor
timestamp: January 18, 2021, 06:32:58 am
content: Awww now im disappointed, half way down the thread i kinda wanted to see what a 5GJ capacitor bank looks like.Seeing the OP is so unfamiliar with electronics to not notice that 46F is a humorously massive capacitance at such a voltage id say i would recommend to stay away from actual high voltage power supplies, one wrong move and you can genuinely win a one way ticket to the cemetery. Big HV power supplies can actually be more dangerous than working with explosives.So yeah i would recommend using an electrostatic machine (Like a van de graaff) to charge it up. For a capacitor of that size they can still provide enough current to charge it up reasonably fast. The energy storage in that capacitor is also low enough that you can't really get hurt from it.But think twice before scaling up such a capacitor by a factor of 10x or more, at that point it can start getting pretty dangerous.

author: Zero999
author_rank: Super Contributor
timestamp: January 18, 2021, 09:16:06 am
content: I did a very quick calculation to work out how big a 5GJ capacitor would be. Assuming 1J/cm3, based on my rough measurements of a 150µF 400V electrolytic capacitor, the volume would be 5Gcm3, so if it's a square block, it would be3√(5×109) = 1710cm, 17.1m or 52' 1 1⁄2", so about the size of a large house. I think a person wouldn't survive if they were standing right next to it and the terminals were shorted. There would be a huge explosion, with intense UV and probably even X-ray radiation.

author: amyk
author_rank: Super Contributor
timestamp: January 18, 2021, 02:33:02 pm
content: I was expecting supercapacitors, a common size of those is 3000F@2.7V or around 10kJ each, and they are closer to 10J/cm^3. That's still enormous.

author: Berni
author_rank: Super Contributor
timestamp: January 18, 2021, 03:36:36 pm
content: 17m cube of capacitor does sound rather terrifying indeed.Yes supercapacitors can store an impressively large amount of energy, but the low voltage makes them a lot less intimidating, more to the level of large high discharge rate rated batteries. The low voltage doesn't allow you to let loose all of that energy in a milisecond.However, when you have 10kJ at thousands of volts this is the result:Your hand would do something similar if you happened to be unfortunate to touch it. Now if you scaled this up to 5 GJ.... well that is 500000x more violent... yikes.

author: drussell
author_rank: Super Contributor
timestamp: January 18, 2021, 05:09:46 pm
content: Quote from: KE5FX on January 17, 2021, 11:09:34 pmThat depends on the size of the CRT, etc. obviously, but I figure once you get past the level of about 100 pF and up past the 10-12 kV range, you're crossing the line into danger territory.It's not likely that you'd kill yourself electrically with the < 1J energy unless you're really,reallyunlucky, but you quickly get into the territory of being able to injure yourself from the jolt once you get past the typical old B&W tube level (say about 100 pF at 12-15 kV max for those old tubes) and into the larger-tube color TV range with more capacitance and higher voltages. You quickly pass from the annoying zap you get from a large static discharge into the "so I woke up smoldering in the corner against the wall" zone.I built a ridiculously high power version of "The Big TC" tesla coil from the July 1964 issue of Popular Electronics back in my highscool days, usingthreeof the big window-glass capacitors instead of the recommended one, or the insanity-leveltwocapacitors as listed in the article, as well astwohigh voltage transformers in series driving the primary spark gap. I didn't die. None of my friends there playing with it died. I never even zapped myself with it as I already had a healthy respect for HV, especially something that can throw an arc 18+ inches at 300+ kV. The EM radiation from that beast is strong enough that it erased the autodial memory contents from a GE 2-line desk telephone located 30+ feet away, sent 1980s-era electronics into hissy-fit mode, confused microprocessors from the other side of the basement, etc., but I didn't ever zap myself. You really don't want to zap yourself with those levels, even though you probably won'tdie.It is going to seriously hurt, though.My worst zap ever was actually a static discharge. We were stacking up those padded gym floor mats that velcro together one day in one of the basement gyms at my high school. The entire floor of the gym had been covered with them for some wrestling thing or something. As we dragged them across the floor and stacked them, they built up crazy high static charges, which was kinda fun, people got zapped a bit, ha ha, hardy har har, etc. It started to get a bit sketchy though with one of the big stacks of mats. You could FEEL the static from farther and farther away as that particular stack grew and none of the static had been discharged from that stack somehow. All the girls' hair were sticking out at it, and we were literally just throwing the folded mats onto the top of the pile from a distance since nobody wanted to go near that stack anymore...I was standing about 20 feet away, laughing at the situation when I made the unfortunate decision to point at it and say "somebody's gonna get zapped by that pile, you canFEELthe stati........" when suddenly there is thisSNAPas a freakin' lightning bolt jumped from the stack of mats to my awaiting, pointing finger.... Deargoddid that hurt. Oh, wow did that hurt. I literally fell down.People were laughing and asking me if I was dead because at least something like 10 other people were looking directly at me pointing at the thing, taunting the pile at the time and SAW the giant spark jump. DAMN, did that ever hurt. I couldn't move my arm for several minutes, it was just seering pain... My arm even still hurt the next day. I was actually starting to think I had permanently damaged something but it did finally go away. I'm fine, but like I said, I have a healthy respect for high voltage, regardless of the actual number of joules involved.To the OP... Please, justdobe careful.

author: drussell
author_rank: Super Contributor
timestamp: January 18, 2021, 05:32:24 pm
content: Thesereallyarefun....Mine hastwotransformers in series andthreecapacitors in parallel, andyesthe capacitors really do glow blue around the edges. The whole thing looks like it is radioactive or something when it is powered up.It is sitting in my basement, knocked down into partially un-assembled form. I don't think it's even been powered up in about 25 years. I now even have a single, even bigger, more ridiculous high voltage neon sign transformer to try with it that I acquired many years ago but have never got around to trying out. One of these days I'll re-assemble it and fire it up.Out in the middle of nowhere, away from any electronics, etc. of course! I wonder what kind of carnage it would cause to the circuitry in a modern cell phone anywhere in the vicinity?Haha, I should probably bring it out here to the lake to play with it where I have half an acre to space things out a bit, instead of potentially blowing up all the neighbors' electronics trying to fire it up in the city.Perhaps a youtube video is in order.Edit:Fix typos, add link toPopular Electronics - July 1964 issue

author: drussell
author_rank: Super Contributor
timestamp: January 18, 2021, 06:02:13 pm
content: Quote from: Berni on January 18, 2021, 06:32:58 amAt 0.2ma and with the variable voltage, the OP's HV supply is probably reasonably "safe" (in a relative sense) and my worry about them charging up capacitors with a Van de Graff generator is that if the breakdown voltage of the cap is relatively high, they might actually end up with an even more dangerous charge on the actual cap.Being careful and thinking ahead is key here, regardless.QuoteIndeed.At these levels things can go from fun to sketchy to downright scary very quickly and easily. A grounding rod on a long insulated stick is a must.Stay safe!

author: David Hess
author_rank: Super Contributor
timestamp: January 18, 2021, 09:00:32 pm
content: The common solution is to use an inverter to drive a 100s to 1000s of volts into a capacitive voltage multiplier with either primary side current limiting, or secondary side by monitoring the current through the capacitor at the ground point.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 19, 2021, 09:22:16 am
content: Post a picture of the bad boy. If your figures are right, then fully charged it will have the same stored energy as 2379 kilos of TNT. 2.4Kilotons. Really?! You may destroy a small town, sod the power supply!CV=IT so at 15Kv and a constant current of 1 amp it would take a week to charge. And at 0.2mA, 110 years, good luck. Put your Mum on, I need a word with her.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: January 19, 2021, 10:24:10 am
content: really a pity that it's not 46F at 15kV; the TO would be a fair candidate for the Darwin Award

author: BlackICE
author_rank: Regular Contributor
timestamp: January 19, 2021, 12:43:10 pm
content: Interesting question. I tried something similar although with a smaller capacitor. I learned a lot about reverse diode leakage current ahd capacitor leakage. Not easy task. I was charging something like a 50 nanofarads. I triggering ignition coil through some high voltage diodes in series to charge the capacitor. I can only get it up to about 5 KV. Whereas without charging capacitors I can easily get 15 to 20000 volts. The diodes were leaking current faster than the ignition coil could charge the capacitor. I gave up the experiment because I was trying to make this battery powered and there was no way I can do that with a reasonably sized battery. Leakage of the diodes just wasted too much power.

author: drussell
author_rank: Super Contributor
timestamp: January 20, 2021, 05:35:31 pm
content: Quote from: Terry Bites on January 19, 2021, 09:22:16 amLike the OP, I thinkyoumight also want to check for unit errors there...1000 Kg of TNT is approximately 4.2 GJ, and even if the OP's capacitorwere10 GJ and therefore approximately equivalent to 2379 Kg of TNT, that is still only approximately 2.4 tons, not 2.4kilotonsTNT equivalent, nor is it 2.4 x thousand tons mass or whatever unitslip you made...A "kiloton (of TNT)" is 4.184 terajoules (4.184×1012J).

author: drussell
author_rank: Super Contributor
timestamp: January 20, 2021, 05:53:58 pm
content: Quote from: HB9EVI on January 19, 2021, 10:24:10 amEven 46µFwould be ghastly enough, let alone a farad!While the originally stated capacitor was obviously absurdly large, It's a good thing that the OP is only talking about 300 pF or so, which is not too worrying. Even the 3 x 3000 pF capacitors on my Tesla Coil are quite terrifying enough for me!

author: David Hess
author_rank: Super Contributor
timestamp: January 22, 2021, 06:21:11 pm
content: Quote from: HB9EVI on January 19, 2021, 10:24:10 amThat is beyond the level used in nuclear simulations which rely on explosively pumped flux compression, and those capacitor banks often exploded. I have seen one inch aluminum bus bars with holes punched through them from those failures.

author: Zero999
author_rank: Super Contributor
timestamp: January 22, 2021, 06:54:06 pm
content: Quote from: drussell on January 20, 2021, 05:35:31 pmA 5GJ 15kV capacitor rapidly discharging would still produce one hell of a bang, with lots of ionising radiation. I wouldn't want to be anywhere near it going off.