Guessing the voltage rating of large HV capacitor - Page 1
Forum section: electronics / projects

author: dexters_lab
author_rank: Unknown
timestamp: July 11, 2015, 02:33:17 pm
content: I have just salvaged a large capacitor from a piece of equipment, and i am trying to get an idea of it's voltage rating.Unfortunately there are no markings on it, it looks like the makers of the item it came out of deliberately removed the info sticker, the device also wont charge it (for me to measure it) without missing parts and / or circumventing a bucket load of interlocks which run through a couple of potted 'black boxes' which control the charging and interlocks. There is a transformer that is clearly the supply to charge the capacitor but even that is powered off and supplied though the potted devices. The transformer is labelled 2800V.So i am wondering if i could possibly identify the capacitor make and in turn work out the model?It's probably UK or European made dating from around 1999It measures 266mm x 116mm x 96mm, weighs about 4.7Kg and has a capacitance of about 190uFany thoughts? pic attached

author: Psi
author_rank: Super Contributor
timestamp: July 11, 2015, 02:54:09 pm
content: That's one serious capacitor, it's super dangerous to have the terminals open like that.Even a dead flat cap will naturally charge up to lethal levels from potential difference in the air, static etc.. When not in use the terminals must be shorted together for safety.At that size and voltage a cap like that is an order of magnitude more dangerous that a 230V wall socket.It's no longer just about electrical shocks when you get to that level off energy released at once.We're talking about instantaneous super-heating of the marrow inside bones causing them to crack or even explode from the pressure.</overly cautious warnings>

author: dexters_lab
author_rank: Unknown
timestamp: July 11, 2015, 03:06:30 pm
content: Quote from: Psi on July 11, 2015, 02:54:09 pmthanks for the tip PsiThankfully though i am aware of caps recovering charge. After being disconnected like that for 10 mins it recovered to about 0.5v (i actually made a point of measuring it) so no issue there which was about as long as it took for me to sort out taking the pics. It's now back where it should be with the bleed resistors in place.

author: HighVoltage
author_rank: Super Contributor
timestamp: July 11, 2015, 04:29:08 pm
content: It is very difficult to tell the voltage rating and guessing is too dangerous.You probably better off, finding out from the rest of the circuit, what voltage was applied to the capacitor, may be even measure it, if the gadget still operates.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: July 11, 2015, 04:32:40 pm
content: Hmm...?A friend owns a 10uF/5KVW oil filled capacitor, and it is roughly the same size/volume/weight as your cap, or even slightly larger IIRC. For that reason I wonder whether that is really a 190uF/2.8KVW (or higher) capacitor. Of course the thing could contain a string of series connected electrolytic caps, complete with voltage equalization resistors, but I've never come across such a contraption in a sealed case.If that is a 190uF cap and it really is charged to 2.8KV during normal operation, then the energy contained within would be around 750 Joules.For comparison of energy contained: Said friend once charged his 10uF oil cap to about 2.5KV, 31 Joules, and deliberately(!) shorted it out with one of those small jewellers screwdrivers. The part of the screwdriver between the capacitor connections instantly turned to fine metal schrapnel, accompagnied by a loud announcement that the screwdriver had handed in its resignation with immediate effect.Thankfully he was wearing glasses, yet removing the metal splinters from the skin of his face and hands was apparently not too much fun...He never repeated the experiment.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 11, 2015, 07:37:32 pm
content: Any guesses at what's inside the black boxes? Rectifiers or multipliers? Possible access to an x-ray?Seems unlikely, given the high capacitance and lack of ribbed ceramic insulator, that it would be much more than a few kV. So the 2800V (3500V DC/peak?) guess might be the thing, and the blocks should be rectifiers (or maybe some sort of control, if they have extra wires).Defib? Surge generator? Lightning tester?Tim

author: dom0
author_rank: Super Contributor
timestamp: July 11, 2015, 07:51:41 pm
content: 2.8 kV could just be the insulation rating of the transformer.

author: SeanB
author_rank: Super Contributor
timestamp: July 11, 2015, 09:28:57 pm
content: Insulators look like they can not handle more than 5kV, as above that they would be a lot taller, wider and with a smooth ceramic glazed or PTFE ribbed surface. With 200uF nominal capacitance I would guess ( short of you asking Mike to pop the potted blocks through his Xray unit and getting some holiday snaps of the inners, which probably is simply a diode bridge and some current limiting resistors) it is rated at around 4kV, so you probably would find it is a 5kV unit inside running at 4kV.Would make an almighty big flash gun driver, though with small tubes you might only have single shot per tube capability.

author: dexters_lab
author_rank: Unknown
timestamp: July 11, 2015, 11:45:41 pm
content: Quote from: T3sl4co1l on July 11, 2015, 07:37:32 pmnot looked in too much detail, i'll be doing a video teardown on it soon so have not taken too much apart yet, but yes rectification and switching seem to be what the potted devices are forit's a TMS (transcranial magnetic stimulation) deviceto all the other comments, thanks guyshopefully i will know more once i start pulling it apart properly.

author: dexters_lab
author_rank: Unknown
timestamp: July 20, 2015, 07:15:58 pm
content: quick follow up on thisi just found a document referencing the device this came out of (a magstim 200), it details charging the capacitor to 2800vi started a teardown thread in the other blog sectionhttps://www.eevblog.com/forum/other-blog-specific/teardown-of-magstim-200-transcranial-magnetic-stimulator/i'll continue the thread over there, seems more appropriate

author: calexanian
author_rank: Super Contributor
timestamp: July 21, 2015, 04:56:26 am
content: Impossible to tell the working voltage. looks like there is a label on he other side there. The working voltage is Dependant on two things in this case. First, and most importantly is the thickness of the paper used in it, and secondly the terminals used. The terminals shown here I believe i have seen used up to 5KV and that is really pushing it. I have also seen them used to caps as low as 600 WV. Just depends. The cans and lids are pretty standard. The roll of paper and foil inside is made to whatever specs the customer wants. Without a part number or values on it guessing would be a dangerous game at best.