Best method/switch for dumping high voltage capacitors - Page 1
Forum section: electronics / projects

author: Lord Nicoll
author_rank: Contributor
timestamp: June 26, 2017, 04:57:53 pm
content: I love high voltage and sparks and such, and so naturally I love using rectified mains to charge capacitor banks to then discharge into things. However, I'm looking for a more controlled way to discharge said capacitors (usually only 400v 48-180 micro farad ones) however I paralleled a couple together and... based off the fact I hurt my finger a little and am still finding bits of the 10 amp switch I used, think it might be a good idea to maybe use transistors, the two caps are both 220 uF ones, 400v, so more than I usually do. Using an online calculator says that 330 (rough rectified mains) and the 440 uF of the cap, puts it at 24 joules just, so if that's accurate I guess it was too much for the switch (I was surprised it blew up but I had been used a lot on DC batteries to break high currents around 8 amps so maybe it was weakened) while the transistor I wanted to blow up just released the magic smoke normally. Anyways, would a beefy transistor do the job? Would there be any particularly good ones for the job (I'm thinking an SCR or a MOSFET would be best)I'm not massively knowledgeable about semistate switching beyond joule thief's and other light continuous loads, so that's why I thought I'd ask here,

author: Lord Nicoll
author_rank: Contributor
timestamp: June 26, 2017, 05:10:15 pm
content: Quote from: blueskull on June 26, 2017, 05:02:37 pmSCR it is, it's the only switch I've used for at all for high uses, any idea what surge rating I should go for? 100 amps peaks, maybe go really and look for a massive 1000 amp one if it exists? (I'm sure one does somewhere) or just use a few in parallel?

author: Gyro
author_rank: Super Contributor
timestamp: June 26, 2017, 05:35:54 pm
content: In terms of an actual physical switch, a really heavy duty Mercury displacement relay (contactor) is probably the only one that would stand a chance - a metal encapsulated one rather than glass. There are a few that connect Mercury reservoir to Mercury reservoir (rather than Mercury to solid contact) which would be pretty much immune to contact errosion. The Mercury to solid contact ones use a Tungsten electrode which should be the negative side if used for DC.

author: N2IXK
author_rank: Frequent Contributor
timestamp: June 26, 2017, 05:46:52 pm
content: An ignitron tube is often used as a discharge switch for high energy capacitor systems.The type 7171 is typical of this class of device, but there are higher power devices available, as well:https://frank.pocnet.net/sheets/201/7/7171.pdf

author: max_torque
author_rank: Super Contributor
timestamp: June 26, 2017, 07:34:29 pm
content: Sorry, but what was limiting the discharge current to 8A?? If oyu just put the switch in parallel across the cap, closing the contacts (mOhm resistance in the switch and in the caps!) would have lead to a very short, but very high current pulse of high power (because although the total energy released was small, it was released in a very short period of time)

author: Gyro
author_rank: Super Contributor
timestamp: June 26, 2017, 07:48:16 pm
content: Quote from: Lord Nicoll on June 26, 2017, 04:57:53 pmJust re-reading your OP. I hadn't noticed the reference to direct charging the capacitors from the mains... together with the lack of knowledge. You're likely to take some critisism from members for this, and rightly so. The only way you should contemplate this kind of experiment is by isolating the supply via a transformer, otherwise what you are doing is very unsafe. Helping you to do this is, I believe, against forum policy or at least practice.

author: Lord Nicoll
author_rank: Contributor
timestamp: June 29, 2017, 10:16:59 pm
content: Quote from: Gyro on June 26, 2017, 07:48:16 pmI should point the mains are just used to charge the capacitor, it is disconnected entirely from the mains before being dumped, otherwise the risk of forgetting to remove the mains supply and bridging it into something would have a very bad day. The system is entirely disconnected first. I don't have a lack of knowledge about mains, just high power solid state switching, I've always preferred mechanical switches so naturally I've done everything with switches and relays, but as you said, and I wanted to avoid, mercury type switches are all that can be used. It should also be noted the end goal of this is actually building a 36KJ railgun to try to break the sound barrier, doing so would definitely break the local laws regarding firearms and "muzzle" energy of traditional guns, but since a railgun uses two rails and no barrel, legally it isn't defined.

author: jbb
author_rank: Super Contributor
timestamp: June 30, 2017, 12:45:10 am
content: Sounds like you had a lucky escape with that first switch. Please I magine what could have happened if the switch had worked for a while, and you added more and more energy storage to the bank. The explosion when it finally failed could have caused you serious mechanical, thermal (plasma is hot) and electrical (plasma is conductive) injury.Your cap bank should always have bleed resistors (in the tens to hundreds of kOhms to make sure theyeventuallygo flat) and a dedicated voltage indicator (e.g. LED or panel meter).If you want to 'dump' the bank faster, I suggest a DC control circuit (12V, 24V etc.), control button and mains contractor. Discharge through a suitable power resistor (I.e limit current to within the switch rating). Please note that metal film or carbon film resistor types often fail - generally open circuit - after a few shots, so use heavy wire wound types.Typically a DIY rail gun is unswitched, as in the rails are directly connected (via short thick leads) to the cap bank. Switching is provided by shoving the projectile into the, err, acceleration zone. This could be done with compressed air.If you follow this method, you definitely need an isolating transformer for charging. (FYI, VARIACs generally doNOTisolate.)There are many safety concerns with a rail gun, so be careful.

author: Lord Nicoll
author_rank: Contributor
timestamp: June 30, 2017, 01:04:05 am
content: Quote from: jbb on June 30, 2017, 12:45:10 amYeah, I should point out before when the switch blew, I had foreseen it eventually and actually used a stick to push (+10 points for technology) and it was just a piece of plastic that hit me, it really wasn't that sore. I did have my multimeter as well, to see if the cap was fully empty permanently attached. I would rather not use bleeder resistors for such a bank, I'm going to need at 600v, 200,000 uF of capacitor to get the 36JK I want, which is .2F. That is an insane amount of storage. However I expected this, so I have an entire shed to build it in. I bought a 3 inch steel plate to shoot at, hopefully 3 inches is enough to stop the round (it'll probably be soft steel), however I do plan to switch it electronically because that's somewhat more controllable and I am currently working on the rails, in which the armature is titanium, as are the rails. I can't afford platinum (I mean who can for a single use) so titanium will have to do, it's really not ideal but better than steel. I might try a full Titanium round however that might go through the steel if it's successful and reaches mach 1, so idk if I want to, I already have to write out a full safety assessment for this. I also should point out I'm not doing this alone (yes there are actually two people involved, it seems that brains when used together act like resistors in parallel, halving intelligence, unlike resistors in series which would double it)Really if the expensive capacitors don't blow up I'll consider it a success. I'll probably come up with some controlled way to discharge the capacitors too, maybe an induction coil to lift a steel weight and suck power from it without a massive surge, or perhaps just a really really big spark cap that can be closed and get a super loud spark, however that is not that controlled at all. I don't think a resistive dumping method would work at 36 kilojoules even if it's just a moment. The actual amount of power might come down, depending on just how bad the cost is, I've been working part time for the better part of 3 years to afford to be able to do this (I've always wanted to build one since I was a little kid) so most of the money will go into the caps.

author: Ash
author_rank: Regular Contributor
timestamp: June 30, 2017, 04:17:07 am
content: Have you given any thought about how you will safely build, operate and work on this rail gun? Perhaps that is the first thing to get under control? Then you can 'safely' experiment with the rest. (I'm no expert, but this is where I'd start)... Also, document it, build it, and test it BEFORE you start on the really dangerous stuff.Design this system properly with interlocks and operating station that by design enforces separation from all the dangerous stuff and requires a physical lockout (key?) that you take with you when you leave the safe area, and perhaps 2 hand operation (controls are far enough apart). Above all, there must be a simple "safe" configuration - an e-stop if you like. If you kill the power everything must return to safe configuration. For me, I'd be having a contactor across the cap bank with a suitably sized resistor (bar heater?) to ensure that it can discharge the bank in a few seconds. This contactor would have to be energised to remove the shorting resistor and allow the device to be charged. Make sure that you also have similar interlocks on your charge circuit. The safety discharge resistor should be able to handle the full charge current as well. Your charge current should be limited.Then if you're ever tempted to defeat one of the safety protocols or interlocks - you know you're doing something DUMB.For firing, what about a triggered spark gap?If you only want 1 shot (or low repetition) this would allow some pretty huge current pulses.. You still need to safely contain this all behind some blast shielding (like everything else you are building), but essentially, you setup a spark gap that is larger than can be arced over by your DC voltage. You then arrange a third electrode that is closer to one of the main ones and apply a HV pulse to spark that smaller gap, this then allows the larger gap to fire and dump the charge.Make sure that this is all operated remotely with something totally non-conductive. Perhaps one of those "spar pump" air operated switches that use that clear PVC tubing? This would be used to turn on a HV generator/transformer (eBay) to spark over the small gap..?Also, I think the bleed resistors are a must, and should be across each cap. They can be high-ish value so as to not significantly affect your circuit, but the issue is that caps can "recover" their charge so you need to ensure that when you think they are discharged, they remain that way..Ash.

author: jbb
author_rank: Super Contributor
timestamp: July 01, 2017, 12:15:23 am
content: Quote from: Lord Nicoll on June 30, 2017, 01:04:05 amThat's going to be amonsterswitch. Note that if you try to control the current with the switch, it will dissipate alotof energy. That would mean a large silicon volume and therefore a lot of money. Therefore your switches should be used as on/off devices, and control of the current pulse achieved by the other components in the circuit (e.g. cap bank value, rail inductance, extra pulse shaping Ls, Cs and Rs).FYI, to achieve robust performance with a 600V rail, you'll need to use >900V devices. In fact, I would recommend nothing less than 1200V rated devices.Quote from: Lord Nicoll on June 30, 2017, 01:04:05 amGood! It's much better to think about safety before you start. It's much cheaper to design safety properties in than retrofit them later.Quote from: Lord Nicoll on June 30, 2017, 01:04:05 amInduction coil: not a good idea. Once the capacitor voltage reaches 0, the current in the inductor will be maximum. The current won't just stop; if will then push energy back into the capacitor bank, charging them up withnegativevoltage, and wreck them. This is called 'back EMF' in the DIY railgun field.Really big spark gap: 36kJ is a lot. This will be loud and dangerous. It will subject the capacitors to large currents, which will age them.Quote from: Lord Nicoll on June 30, 2017, 01:04:05 amAll the more reason to use a properly controlled discharge scheme.Let us consider a discharge scheme using the TE1500B220RJ (220R 1500W) resistor. It's a big chunky one for mounting in free air.At start of discharge:I = 600 V / 220 R = 2.72 A. Very reasonable current to switch.P = 600 V * 2.72 A =1636 W. A little over rating. Probably OK, but keep an eye on it.To work out the discharge time, we look at the 'final' (i.e. not too dangerous) and initial voltages:V_initial = 600 VV_final = 50 V (this is the 'safety' limit, although there will still be a lot of energy in those caps!)... and use the exponential delay equation.V_f = V_i * e^(-t / \tau)Rearrange for t:V_f / V_i = e^(-t / \tau)ln (V_f / V_i) = -t / \taut = \tau -ln(V_f / V_i)Substitute in V_f and V_i:t = 2.485 \tauThis just tells us how many time constants (\tau) we have to wait after starting the discharge.\tau = RC = 220 R * 0.2 F = 44 secondsHence t = 109 seconds.Now add some margin:20% tolerance on capacitors (when new, ageing makes them lose capacitance)5% initial tolerance on resistors10% resistance change with temperature (note temperature rise is in the order of 300 deg C)Total 25%.Hence need to allow for 136 seconds (2 min 16 seconds).If that's too slow, you can use 2 resistors in parallel. This halves R, which halves \tau, which halves the discharge time to 68 seconds (1 min 8 sec)Or 3 resistors, giving 46 seconds.

author: Lord Nicoll
author_rank: Contributor
timestamp: July 02, 2017, 04:58:43 pm
content: This is my first attempt at doing a fragmented quote reply, so I will probably fuck it up.Quote from: jbb on July 01, 2017, 12:15:23 amThat is now the plan, calling it one switch as you noticed, is wrong, it'll be more like 10 heavy duty SCRs on a TEC cooled plate, I have some of those cheapy peltier cooler junctions (3 doing nothing) so I'll be using them to cool the SCRs, or at least the metal plate they're attached too. Since this is such a dangerous device the thing will already be uniformally coated, so condensation isn't a huge concern. I also do extreme overclocking so if I really wanted to I could use LN2 to make the whole thing super cold, but that isn't ideal. This won't have a high fire rate, I expect the rails to be single use so waiting 10 minutes for the SCRs to cool is not an issue.Quote from: jbb on July 01, 2017, 12:15:23 amIn terms of the bank, rail aperture and control console, they'll be built into a shed as mentioned, one with a lock. This isn't designed to be man portable, so I'm not concerned about the size. The target will be the most robust part though, thankfully I'm quite a good welder, so I plan to weld 1/2" steel plates to a 2" thick base, and then pour a couple inches of lead in to act like an anti-spaulding later and absorb some impact. I might use some RC car parts to make it a remove firing device but that also has some issues, I'll consider that in said report.Quote from: jbb on July 01, 2017, 12:15:23 amHmm, I had planned to use diodes and a steel core (induction heating not so much the coil) but yeah, the diodes would take a pounding and the steel core might melt, creating a massive problem. I could use Xenon short arc lamps as a controlled spark gap, but those are only rated at 50 volts, so would be blowup, that would be worse. I guess LN2 cooled wire resistors maybe, maybe just normal water cooled, I wouldn't want to get an almost super conducting wire from LN2. I'll reply to the resistor value in this one too. I guess a giant wire round resistor would be best, I prefer liquid cooled (I'm an overclocker after all hehe, water and LN2 are my favourites) though I don't fancy spending the money on LN2, it might not be that expensive I can imagine it'd use a lot. There is also the option of using an inverter to feed the power back into the grid although that would need more wiring than I'd like to have done, that would be a creative way to use the power, not destroy it. Ah, resistor it is, the question is do I build my own, or just buy one, I kinda want it to be water cooled because I know it won't be too hot that way, and if it blows the water would then act as a wire and slowly discharge it, and not cause an arc if it did blow.My friend, who is in college, just finished 1st year of engineering, is doing this project with me. I put him in charge of finding suitable SCR's and the triggering circuit, so I'm waiting to get an idea of what he picked. I might touch over his design if I don't like the protection and safety side, but I'll be the one building it so I can easily do that. His lab partner wanted and tried to build a railgun but failed so we're going to beat him since my friend doesn't like him an awful lot, and I too had a run in with him, me and him both being egotistical assholes with messiah complexes, however I also have anger issues and almost boxed him, so I am by no means the best person to hold the key, thank god this thing won't be able to be aimed.

author: jbb
author_rank: Super Contributor
timestamp: July 03, 2017, 08:04:18 pm
content: Before you go any further, have you checked out the excellenthttp://powerlabs.org? It hasn't been updated in ages but contains much useful information.Quote from: Lord Nicoll on July 02, 2017, 04:58:43 pmLooks fine so far ...Quote from: Lord Nicoll on July 02, 2017, 04:58:43 pmSCRs are often the weapon of choice in pulse power applications; they are robust and can handle large surge currents.However, SCRs are what we call a half-controlled switch. That means you can turn them on, and then theystayon until the current falls to (near) zero. In that respect they are quite like a spark gap. They don't offer the ability to limit their current, so you have to do that (e.g. through controlling the parameters (L, C, R) of the connected circuitry.Additionally, LN2 may be too cold. It will change the silicon properties quite a lot, and may cause weird things to happen (ornothappen). I've also heard that it may cause the bond wires to shear off the silicon due to contraction. Any active cooling should only go down to the rated operating temperature of the device (often -30 deg C or similar). For pulse discharges, a large aluminium block would probably be sufficient.Quote from: Lord Nicoll on July 02, 2017, 04:58:43 pmI appreciate that you're thinking about what happensafteryou accelerate the test mass. I believe en earthen berm (or small hill) is also a favourite. Maybe you could tilt the accelerator stage downwards a little?Quote from: Lord Nicoll on July 02, 2017, 04:58:43 pmI respectfully suggest that RC parts are not your friend in this case. They are designed to be 'good enough' and may trigger by accident due to interference. If you want a good isolating break, might I suggestPlastic Optical Fibre (POF) optics? They are comparatively cheap and (similar devices) are commonly used in large power electronics (e.g. multi megawatt motor drives).Quote from: Lord Nicoll on July 02, 2017, 04:58:43 pmYes, this is technically possible. However, a standard inverter won't do it, as they are not intended to suck an energy source from 600V down to 50V. Additionally, your railgun will kick off all kinds of nasty EMI when triggered which may outright destroy the inverter. Third, your power company may have words (such as 'please don't') about injecting power back into their grid.

author: Lord Nicoll
author_rank: Contributor
timestamp: July 05, 2017, 10:46:17 pm
content: Quote from: jbb on July 03, 2017, 08:04:18 pmThat is pretty interesting, he seems to be using much higher voltages, which is terrifying because he has so many amps, I'm working off the principle of less volts and more amps, I was looking around the 5000 amp peak range, so idk how that'll work out. His rails seem to survive ok and they're copper, so titanium should limit the current more and be a lot more resistant to degradation I hope, they might be perfectly reusable if I'm lucky.Quote from: jbb on July 03, 2017, 08:04:18 pmYeah, even on CPU's you can cold bug or break them (I tried the LN2 once and broke the CPU, I never even got to make a single submission to HWBotand I just fucked around with the rest of the LN2 making fog and ice cream) so I'll stick to an assisted peltier junction type cooler and a billet of aluminium. I think you over estimated how much control I want, I want those capacitors to be dumped as fast and heavy as humanly possible, without any arcing. I don't want anything fancy for power control, that is how shit breaks, if I want lower power I simply will charge the bank less, KISS. I'm not so much worried about that as you can tell, once it doesn't tax the switches that much (obviously it will but 10 high power SRCs should last a couple hundred firings, probably more than the test of the system will take)Quote from: jbb on July 03, 2017, 08:04:18 pmBehind the plate will be a solid concrete wall, so nothing I can fire in my yard will go through, just to be sure though I might warn my neighbours, they're used to be doing archery and occasionally arrows bounce in, so I tell them when I'm plinking away. I also weld so they're used to avoiding the windows if I do that outside. If their 100JK gun only went through plywood though then I doubt this will do much more but if for some reason mine is many orders of magnitude more efficient I'll be sure to let some physicists and the US army know, I'm sure many large checks will roll in (I can dream Harold) I do plan to make a portable one that piggybacks on this project (get it, piggyback, because it will need a backpack lol)Quote from: jbb on July 03, 2017, 08:04:18 pmI'll take that into consideration, but not many people where I live use RC cards, interference is never an issue, but a tiny car vs a massive assortment of high voltage, high amperage and plasma is probably best to have 110% control over. I guess a 9V battery and a relay with a long lead will be suitable ghetto for me. I have a shit load of laser modules and light optic crap, I probably just need a long enough optic (my audio uses it but I only have a few feet of that)Quote from: jbb on July 03, 2017, 08:04:18 pmThere has been a minor change of plans on the 600V also, I recently won in an auction 12 4700?F 400V Kemet capacitors, so if they turn out to be legit, holy hell 12 Kemet giant capacitors for £82 is an unreal deal. That puts this railgun's power at only about 1/10th of origianlly planned using 330V. I know the National Grid in Ireland does allow power to be fed back into the grid, but you need a special meter and such, but some people have solar panels (Ireland doesn't get a lot of sun shine so they aren't popular, and while we do have windy days we also have calm ones, so wind turbines are also not so popular) but people do feed excess power into the grid, the power companies are all hippy about support this tech since Ireland is a shit place for it. If they could harness rain then we'd be laughing (I know there are like 10 ways to do that, but they're all pretty low efficiency). 12 4700?F 400v capacitors is still 56400?F, which is a frightening amount to think I'll have delivered next week, but the price was less than a single brand new capacitor of this size, these are supposedly new in box from a company that overstocked these, and since they seem discontinued, might be just that, a company hoping to at least pay a guys wages for a few days from the sale.