which type of capacitor for pulsed discharge - Page 1
Forum section: electronics / beginners

author: LaserTazerPhaser
author_rank: Regular Contributor
timestamp: April 03, 2019, 07:05:04 am
content: Are there any electrolytics 360v-500v 540uf-1000uf which can fully charge & discharge @ ~0.714Hz?Seems film will outlast electrolytic but by far are more expensive for the charge they store than electrolytics. Maybe a gang of electrolytics can do this.

author: tautech
author_rank: Super Contributor
timestamp: April 03, 2019, 07:32:26 am
content: Pulse grade caps are used in electric fence units however their values are only 20-40 uF IME.Most are 900 VDC rated and non polarized and always seem to be spade terminals.It's not unusual to see gangs of up to 6 paralleled for a couple of 100's uF.BTW, they are always marked as Pulse Grade.The run for many years @ 1 Hz charge/discharge rate.Try Tedds as they have some big grunty stuff for not too bad pricing.https://www.tedss.com/Capacitors

author: LaserTazerPhaser
author_rank: Regular Contributor
timestamp: April 03, 2019, 10:17:32 am
content: Quote from: tautech on April 03, 2019, 07:32:26 amWhat precisely depicts a "pulse grade" capacitor from non "pulse grade" capacitors?These seem to be finehttps://www.aliexpress.com/af/motor-start-capacitor.html?site=glo&g=y&d=y&origin=n&spm=2114.search0604.0.0.278f495eLHSZG8&filterCat=100006834%2C14191105%2C410405&jump=afs&groupsort=1&SearchText=motor+start+capacitor&SortType=total_tranpro_desc&isViewCP=ybut they might pack undocumented features.

author: Zero999
author_rank: Super Contributor
timestamp: April 03, 2019, 11:09:20 am
content: Define fully charge and discharge? That may seem like a silly question, but a capacitor will never fully charge/discharge, unless it's a connected to a constant current supply/load.You want low ESL and ESR for high current operation and if the duty cycle is high, then thermal design becomes a factor too.

author: LaserTazerPhaser
author_rank: Regular Contributor
timestamp: April 03, 2019, 11:40:15 am
content: Quote from: Zero999 on April 03, 2019, 11:09:20 amCapacitor is clamped to a load with a thyristor then recharged and to be discharged again at ~0.74Hz electrolytics aren't up for this.

author: Berni
author_rank: Super Contributor
timestamp: April 03, 2019, 11:53:44 am
content: In that case you are only going to be discharging it at 2.75A (540uF capacitor going from 500V to 0V over 100ms)This is not really a pulse capacitor anymore. Use something with low ESR and rated for plenty of ripple current and you will be fine. Tho such high voltage capacitors tend to not be available with really low ESR so paralleling 5 or so smaller ones is probably the way to go.Pulse capacitors would tend to be designed for discharge rates of 100A and up.

author: LaserTazerPhaser
author_rank: Regular Contributor
timestamp: April 03, 2019, 11:56:50 am
content: Quote from: Berni on April 03, 2019, 11:53:44 amThe load is very small and the current will rise well over 120amps. Only motor run or dc link capacitors seem to be up for this.

author: tautech
author_rank: Super Contributor
timestamp: April 03, 2019, 12:34:50 pm
content: Quote from: LaserTazerPhaser on April 03, 2019, 10:17:32 amContinuous pulse duty.QuoteStart Caps are not for continuous operation.Run caps will be a better choice although how they accept deep discharge might be anther matter.AFAIK pulse grade caps are constructed similar to motor grade caps.QuoteSame switching principle as used for electric fences where the charge is dumped into the primary of a transformer.

author: LaserTazerPhaser
author_rank: Regular Contributor
timestamp: April 03, 2019, 12:58:55 pm
content: Are there any motor run caps above 100uF? Only the noname brands seem to go above 100uf which can be just motor start listed as motor run.

author: tautech
author_rank: Super Contributor
timestamp: April 03, 2019, 01:08:32 pm
content: Quote from: LaserTazerPhaser on April 03, 2019, 12:58:55 pmNever seen one more than 60 uF.QuoteYes and if you use a Start one in a Run application they let out the magic smoke.Don't ask me how I know.For just the few $ they are I'd try some of the Chinese Run caps and see how they perform. If they get too warm you might have to use proper Pulse rated caps at more cost.I looked at getting some a while back for a sick electric fence I scored but the cost was something like $30 ea IIRC and I needed a couple.Found a crowd in NZ that manufactured them for the few electric fence manufacturers we have here that export worldwide.

author: Psi
author_rank: Super Contributor
timestamp: April 03, 2019, 01:28:52 pm
content: You could make a nice capacitor array with lots of Cornell Dubilier - 942C 2.5UF 600VDC high pulse rated caps in parallel.http://www.cde.com/resources/catalogs/942C.pdfBut that would probably be a bit overkill for your requirementsThey're 628 volts per microsecond dV/dtand a pack of 200 caps to get 500uF 600V would allow ~40 million amp discharge current.

author: LaserTazerPhaser
author_rank: Regular Contributor
timestamp: April 03, 2019, 01:35:08 pm
content: Id buy thishttps://www.ebay.com/itm/282977692008/but its way too large for the application and will destroy the load from overheat or even melting.The issue with massive ganging is the cost at which point I would rather buy a dc link cap from a reputable retailer.I only need ~5 of thesehttps://www.ebay.com/itm/6003058083but its likely loaded with undocumented features - infact its documents are likely nowhere to be found also takes up quite a lot of area.

author: Marco
author_rank: Super Contributor
timestamp: April 03, 2019, 01:52:32 pm
content: Quote from: LaserTazerPhaser on April 03, 2019, 07:05:04 amMeaningless question, the power dissipated in the capacitor is load dependent ... i^2*r and all.Lets round off well over 200Amp to mean a 0.5 Ohm load ... that's in the same region as a cheap electrolytic in that range, so you'd be burning around 50 Watt, which would make it explode.

author: StillTrying
author_rank: Super Contributor
timestamp: April 03, 2019, 02:47:52 pm
content: Photo Flash capacitors ?

author: LaserTazerPhaser
author_rank: Regular Contributor
timestamp: April 03, 2019, 02:53:58 pm
content: Quote from: Marco on April 03, 2019, 01:52:32 pmA few simulations showed massive capacitor power dissipation at 100khz of ripple (1.8a peak) with only ~800mA of rms current. There are many capacitors which can handle much more than 800mA of ripple but 90W of dissipation per charge and @ 0.74hz seems too much.Maybe electrolytics are fine for this but even photoflash capacitors aren't made to flash any more than 0.5Hz and for a given duration beyond which they will degrade.

author: madires
author_rank: Super Contributor
timestamp: April 03, 2019, 02:57:05 pm
content: Another source:https://www.wima.de/en/our-product-range/pulse-capacitors/

author: StillTrying
author_rank: Super Contributor
timestamp: April 03, 2019, 04:02:18 pm
content: Quote from: LaserTazerPhaser on April 03, 2019, 02:53:58 pmHow are you measuring the cap dissipation. If I measure the dissipation in a 0.1R serparate from the cap. the dissipation peaks are ~1.4kW, at around 1% duty (because 120A lasts only 2.5ms) that's ~14W average.

author: LaserTazerPhaser
author_rank: Regular Contributor
timestamp: April 03, 2019, 10:40:36 pm
content: Quote from: StillTrying on April 03, 2019, 04:02:18 pmThe charging happens for ~1.1 - ~1.4seconds the 100khz charge period doesen't last 2.5mS and it repeats constantly after the capacitor is discharged.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 04, 2019, 01:09:24 am
content: You've provided no real information to make a decision on. What is the actual application? Circuit? Waveforms?The PRF isn't meaningful; if it's fractional-Hz at the most gentle possible waveform, the only thing that won't do is possibly supercaps, and definitely not batteries (for full cycles, that is).Tim

author: LaserTazerPhaser
author_rank: Regular Contributor
timestamp: April 04, 2019, 03:43:33 am
content: Quote from: tautech on April 03, 2019, 01:08:32 pmWas it a no name brand cap? Those probably have inadequate or no safety measures implemented.Quote from: T3sl4co1l on April 04, 2019, 01:09:24 amI was considering which capacitor is ideally suited forhttps://www.analog.com/en/products/lt3751.htmlmassively powerful cap charger set to with 3.5amps peak 1.2amps rms at the secondary (shunt set to 9.8mR) it will run for hours and electrolytics might have disfavorable features for this application in which they cease to perform as a capacitor and in an ideal situation they have adequate safetey measures implemented. Its actually far more than 0.74Hz mentioned earlier with the 9.8mR shunt is nearly 1.9607Hz which puts even more of a load onto the capacitor. I just need >300v >32j per discharge cycle.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 04, 2019, 05:52:12 am
content: That's literally half the application. Can you please tell everything?Tim

author: tautech
author_rank: Super Contributor
timestamp: April 04, 2019, 07:12:11 am
content: Quote from: LaserTazerPhaser on April 04, 2019, 03:43:33 amThe one I smoked was a start cap....they have a short duty cycle and the motor centrifugal switch removes them from circuit after a second for two at the most unlike motor run caps that are connected to mains while the motor is running.In short (pun intended) a motor start rated cap is not rated for continuous connection like a run cap.They may both look the same but are for different purposes.

author: Gyro
author_rank: Super Contributor
timestamp: April 04, 2019, 09:11:24 am
content: Typically, Motor start capacitors are 'thermally under-rated' bipolar electrolytics (to get the high capacitance) whereas Motor Run are metallized film, usually Polypropylene these days.

author: LaserTazerPhaser
author_rank: Regular Contributor
timestamp: April 04, 2019, 09:41:05 am
content: Quote from: tautech on April 04, 2019, 07:12:11 amYou didnt mention the capacitor branding. Maybe epcos, kemet, avx or some other brandname makes entirely film wound start caps for long service life and low disintegration rates.Quote from: T3sl4co1l on April 04, 2019, 05:52:12 amI suppose I didn't include load impedance which varies with thermals from 0.35R - 0.45R aside from that not sure what else you were looking for. Capacitor will be discharged into this load which is nearly identical as placing a screwdriver across its terminals due to the low impedance of the load. But the load wont be spot welded as a screwdriver would be since its attached to a large barrier terminal block.

author: StillTrying
author_rank: Super Contributor
timestamp: April 04, 2019, 10:09:35 am
content: Quote from: LaserTazerPhaser on April 04, 2019, 03:43:33 amA constant 1.2A charges 1000uF to 300v in 250ms, with only ~144mW dissipated if the ESR is 0.1R. Even at 2Hz it's less than 100mW average in the cap. so I don't think you need to worry too much about the cap dissipation during charging, just the discharging.

author: LaserTazerPhaser
author_rank: Regular Contributor
timestamp: April 04, 2019, 01:36:02 pm
content: Quote from: StillTrying on April 04, 2019, 10:09:35 amWere is the 0.144w?Capacitor averaging 24.969W and this is constant charging.

author: StillTrying
author_rank: Super Contributor
timestamp: April 04, 2019, 02:14:52 pm
content: Were is the 0.144w?1.2A * 0.1RCapacitor averaging 24.969W.I think you have to take the 0.1R ESR out of the cap and measure the dissipation in the 0.1R separately, it's only 10s of mW using that LT sim.

author: Berni
author_rank: Super Contributor
timestamp: April 05, 2019, 05:17:28 am
content: Quote from: LaserTazerPhaser on April 04, 2019, 01:36:02 pmYou are including the power that is getting stored inside the capacitors capacitance. This is where most of the power goes hence why you see a significantly larger figure. To get the power dissipated inside ESR you need to add the ESR externally as a resistor and then measure the power on that, this is the power that goes to heat, the rest is stored.Also to get the matching results for that equation a current source should be used to charge it.And yes this internal heating is going to be thousands of times higher in power during discharge, but at least short lasting so it wouldn't cook a capacitor, but might damage any sensitive internal structures where hotspots possibly develop.

author: Marco
author_rank: Super Contributor
timestamp: April 05, 2019, 05:57:02 pm
content: Not only the power is higher for short discharges, also the energy ... because for shorter complete discharges the resistive component of the load is by necessity smaller, hence you dissipate more energy in the ESR. With equivalent ESR and load resistance he could be looking at say 50W of average power and that needs a lot of capacitor and surface to dissipate.