High voltage, high capacitance, high ripple current, low cost capacitors: go! - Page 1
Forum section: electronics / projects

author: rs20
author_rank: Super Contributor
timestamp: March 01, 2014, 12:39:12 am
content: (OK, so if the title of this post had an answer, there wouldn't be many competing capacitor technologies out there...)I've got a300W xenon flash tubecoming my way from eBay, and I'm looking to drive it as a 30Hz strobe lamp*. The trouble is, the (group of) capacitors used to store the charge needs a350V working voltage,capacitance 100uF,ripple current well over 10Aor something (further details below). I've found some ceramic capacitor arrays that'll (maybe) do the job for the price of a new car; and I'm struggling to find electrolytic caps that have the necessary ripple current (digikey and element14 seem to be pretty bad at filling their parametric search with these details).For context, I present my working for these requirements, just in case I've done something wrong. Firstly, I know it's naive, but I'm going with 350V working voltage, 300W xenon tube, P = VA so the tube will have 0.85A running through it on average. Break it into 30 chunks per second and that's a charge of 0.85 A / 30 Hz = 33mC, which is the charge on a 95uF capacitor at 350V. The ripple current is the RMS current going through the capacitors, which is the sum of the charging and discharging phases of operation. Charging is, best-case scenario, a continuous inflow of current of 0.85A at all times, which is an RMS of 0.85A. The sudden discharge is considerably more worrying because the huge instantaneous current works out as a huge RMS load over time. It's hard to know how long the pulses are, but if they're one hundredth as long as the 1/30 second period, that works out to an RMS of 8.5A (thank goodness RMS only goes up proportional to the inversesquare rootof pulse length). 0.85A + 8.5A + rounding = 10A, but I think it's really near impossible to calculate this ahead of time, I'll just have to do one or two iterations an experiment/measure actual performance.Any hints and ideas for appropriate capacitor technologies or specific manufacturers/model variants would be hugely appreciated!* Yes, I know this will be incredibly irritating light; this is not intended as a pleasant day-to-day light source :-)

author: dannyf
author_rank: Super Contributor
timestamp: March 01, 2014, 01:32:23 am
content: If your design needs a part that doesn't yet exist, it is likely the wrong design.

author: Alphatronique
author_rank: Regular Contributor
timestamp: March 01, 2014, 01:43:19 am
content: http://www.ebay.com.au/itm/PHOTO-Flash-capacitor-1500uF-360v-Pulse-strobe-Low-ESR-photoflash-foto-/110963412203?pt=LH_DefaultDomain_0&hash=item19d5ef4ceb&_uhb=1from same dealer so may good start ..

author: rentner
author_rank: Regular Contributor
timestamp: March 01, 2014, 02:14:54 am
content: If you want 100uF only and High Voltage with ULTRA high ripple currents, you could just use a WIMA foil capacitor.MKP Capacitor with 100uF and 800V DC:DCP4L061008CD4KSSD (WIMA)25 Bucks in Germany. For a Capacitor with this valuse, that is good. Over 25 Amps permanent and Peak currents of 1200 AMPS!!!WIMA says, this Capacitors have absolutely no life limit in the mentioned SOAYou are welcome.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 01, 2014, 02:30:56 am
content: Ok... all well and good but...How fast do you *actually* need the capacitor to discharge? What duty cycle?Ceramic or film will discharge in about 10us, limited as much by the tube's breakdown time as by the equivalent circuit (xenon is kind of slow, about 5us to spark normally, maybe 1 or 2us at the lowest if you do fancy things like simmering).Snap-in electrolytics will discharge very fast as well. The peak currents are probably a longevity problem (nevermind the repeat rate).General purpose electrolytics have enough ESR that they take 100s of us to discharge, with much more manageable peak currents.Proper photoflash capacitors are very small, store a fair bit of energy for their size, and have lots of ESR, in fact so much that the current can be switched off by a tiny SMT transistor, thus controlling the light output.For a flash rate like you're looking for, I would guess a boxed film ('DC Link' or better), or film-in-oil (can) type would be fine. Use enough in parallel to get the energy required. Add enough series inductance and resistance to keep peak dV/dt and k_0 within ratings.So, that's a beast of a tube... is it huge, or water cooled even?Tim

author: rentner
author_rank: Regular Contributor
timestamp: March 01, 2014, 02:58:51 am
content: Tim, what is wrong with the Wima Cap? It is capable of destroying many things. Over 1kA is enought of a peak. The ESR is just about 2.3mOhms. It has at least 12V/uS rise time. It would take about 65uS to go to 800V. That means, this is very very fast.Well, to be honest, I would add 10uF of a super low ESR and ESL capacitor in parallel to get much higher peaks, if you even want that! This will work fine for a Strobe Lamp. This cap is a DC-Link Capacitor. No idea, what that means, but I have 4 of the High Current Wima MKPs. Giant and sexy, if I may be honest. Absolutely perfect for 99% of all situations - if you want to pay that much money.But the WIMA ones are quite cheap compared to others that cost about 10 times that much!!! They are not much worse - if they are even worse.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 01, 2014, 03:38:43 am
content: Let's see... that WIMA is... 1200A peak. With ESR that low, you certainly wouldn't want to put it directly across the tube, but handled appropriately, it should be just fine.You wouldn't want less than 800V/1200A = 0.67 ohms series resistance between the cap and the tube. That'll give a time constant of around 67us, or a discharge duration on the order of 100-200us (depending on when the tube decides to stop conducting). Reasonable enough I suppose. It could be 10 times slower still, if it's just for effect (the OP didn't state a purpose or a use..).The series resistor will have to be rated for a few hundred watts; yes, the tube will dissipate a fair amount (ideally, all 300W!), but the tube drops a fairly stable voltage, maybe 100-200V (depends on length and pressure; I have no idea what's typical for a tube that size), and the rest is dropped by the resistor. For this reason, you'll probably also need a larger capacitor (more energy) and more charging current, assuming you require the optical output corresponding to an actual 300W dissipated in the tube itself. (If that's already assumed by the tube rating, well... I have no idea.)A typical inductance would be no more than the critically damped value, L <= R^2 * C, or under 45uH. This inductance slows down the rate-of-rise, stretching the pulse further and reducing peak current; a somewhat smaller resistor value could therefore be tolerated (perhaps >= 0.4 ohms and 30uH).Note the inductor must be air cored, because a small ferrite core isn't going to do squat (it will saturate immediately), and the amount of core required to handle the whole pulse would be humongous. I mean, sure you could, but... On the upside, it'll be easy to put all the DCR into the coil (still keeping in mind it has to dissipate a lot of power!), so you don't need an extra resistor.I wouldn't recommend more inductance, because it will start resonating with the cap. That's not bad, in and of itself, but if you've got other stuff connected -- charging circuits, voltage monitor, simmer, early shutoff, etc. -- it might not be so friendly. (Really, those circuits should be designed to tolerate reverse bias, anyway -- a kiloampere isn't particularly friendly and should be handled and observed cautiously.)Note also that a kiloampere will physically push on the wires in the circuit. Not by too much, but not something you want to handle with floppy spaghetti wire. Anything 18AWG or so should be plenty, even without wire ties. Complementary wires should be tied together to keep loops small (also reducing EMI -- and yes, every shot fired *will* induce more than a few volts and amps in everything near the wiring, it's not something that can be ignored!). However, the inductor experiences N turns of force, so it should be clamped, or varnished, or potted, or something like that, lest it pop open after one or a few shots. And keep metallic or magnetic things at least one diameter away from the inductor.Tim

author: SeanB
author_rank: Super Contributor
timestamp: March 01, 2014, 04:36:39 am
content: 300W tube, will have a short life anyway. Looking at the tube it will only have a life of 1000 cycles or so at full power before failing, and at 30FPS this will be rather spectacular. Remember that the one electrode will have to handle the 300W of power dissipation, and this level is typically done with a tube that either has a massive air cooled or water cooled anode block, not a sintered tungsten block in glass with a thin lead. This will be glowing white hot within a second, and will melt through the glass in about 3 seconds. The 300W rating is for photoflash, where you have a typical 3-4 minute cooling down period between full power flashes, or a rapid fire burst of low power pulses along with a cooling down period.You want high power continuous flashes look for a used short arc Xenon arc lamp of around 500-1kW and use that as the flash tube. it will have the required cooling and will handle the high internal pressure that will be generated. Something likehttp://www.ebay.com/itm/ORC-optical-radiation-corp-350-Watt-short-arc-xenon-lamp-model-XM350HS-PFR-/251433256135?pt=LH_DefaultDomain_0&hash=item3a8a9704c7and add a starting electrode to the outside would be a thought.

author: Rerouter
author_rank: Super Contributor
timestamp: March 01, 2014, 04:49:13 am
content: what is stopping you from say using 10x 10uF caps in parallel? this is an easy way to increase the ripple current rating, and should be around the same price,

author: rs20
author_rank: Super Contributor
timestamp: March 01, 2014, 06:08:12 am
content: Thanks everyone, there's heaps of super helpful info in this thread that I'm sifting through!Firstly, SeanB, when you say 300W rating is for photoflash, are you interpreting the rating as 300 Watt-seconds (i.e., Joules)? Just making sure that's your underlying assumption there. If so, the eBay page is basically lying by using the wrong units (judging from your visual assessment of the tube, this is precisely what's going on.) I'll complain if that's the case. Thanks for the link, I'm definitely going to get started with the cheaper crappy tube I've got on the way (and not put 300W through the "300W" tube), and work my way up to the real stuff. Is the tube you linked water-cooled? I see what look like little hose connectors on it.T3sl4co1l, the actual use is a strobe light for freezing objects moving at 30Hz (or revs per second, etc). I can figure out what sorts of pulse times are the best compromise, but 1 to 2 ms seems fine, or maybe just a touch long (2ms --> 6% duty cycle will cause rather bad motion blur?). I need to figure out how much light I need; I may be able to compromise heavily on capacitance, which will make all the currents and spikes a lot more gentle. I was actually considering the idea of using an inductor as a better alternative to a resistor, seems like a nice way to keep the I2R under control without burning off any (theoretical) volts, but it's great to hear that this is a thing that's actually done in real life and not just me being silly.And finally, rentner, thanks for the hint on film capacitors. I just had absolutely no idea that 400V / 100uF could come in reasonably priced, "reasonably sized" film capacitors. They look like little beasts to behold!You will all be reassured to know that I do, in fact, know what a bleeder resistor is.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 01, 2014, 07:05:02 am
content: I don't remember if flash tubes are normally rated in joules as watts, though the unit bastardization wouldn't surprise me. Consider how many air conditioners, stoves or furnaces you might've seen which are rated in BTU (energy), when they mean BTU/hr (power). Alas...Are you looking to do this strobe stuff in broad daylight? 300W will probably be enough to do that -- even from a substantial distance. I have an ex-milstrobe unitwith RPM dial -- it's quite dim (the fastest range uses 0.1uF or something like that), but more than enough to see a frozen (or rather, aliased) spinning object under normal indoor conditions.A modern equivalent would probably deliver identical -- or greater -- optical output by pulsing a regular high power white LED at a reasonable duty cycle, without needing a flash tube. Something suitable for better viewing even under bright conditions, might need a horseshoe style tube.Tim