Maximum discharge current of capacitors.

How do I find out the maximum discharge current of a capacitor before I order it?

Is it somehow related to the maximum ripple amps by some formula?

Reply to
Bill J.
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What kind? Pulse-rated caps usually have the ratings in the datasheet (e.g. Digikey).

Perhaps.

Tim

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Reply to
Tim Williams

Searches for the phrase "pulse rated" at mouser and digikey yielded no results relating to capacitors. I am building a power supply which delivers high power short duration pulses with a long charge time between pulses. Hence, I need to know the maximum amperage each capacitor can deliver so I can determine the number of capacitors needed in parallel to deliver the required power.

Reply to
Bill J.

Without knowing the voltages, and currents involved, it is hard to give a general answer. However from your description, a search for photo flash capacitors, may point in the right direction. These are designed to be low loss, and survive very high instantaneous discharge currents, with a relatively lower charge rate.

Best Wishes

Reply to
Roger Hamlett

Certain types of caps have pulse ratings. Polypropylene for example usually have max dV/dt specified and you can get 'pulse' versions of electrolytics for things like flashguns too.

Graham

Reply to
Pooh Bear

(e.g.

have

things

I just want to be able to know how fast all of the ordinary electrolytics in the Mouser, etc catalogs can be discharged so I can choose among them.

Reply to
Bill J.

(e.g.

have

things

It may be inadvisable to use an 'ordinary' electrolytic for pulse use. You're unlikely to find this info, it's not how they're intended to be used.

What's the application anyway ?

Graham

Reply to
Pooh Bear

before I

(e.g.

usually have

things

unlikely

How can a designer determine what current a given capacitor in a catalog can deliver, then? There must be a way. It is not as though max discharge current isn't an important figure, after all.

IS there a connection with max ripple amps?

Reply to
Bill J.

"Bill J." wrote in news: snipped-for-privacy@u72g2000cwu.googlegroups.com:

I think so. Low ESR (Equivalent Series Resistance) caps have lower ripple, usually. That lower resistance means they respond faster to changes on the input. I guess what matters is how deeply you want to discharge them. If you can afford to risk needing to replace them just look for low ESR standard electrolytics, and base your calculations on the given resistance value for them. If you also pick high temperature types, you'll probably improve the chances that they'll last a while. Some people suggest not expecting electrolytics to go beyond 2 to 5 years anyway, but I bet they will.

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Reply to
Lostgallifreyan

before I

datasheet (e.g.

usually have

for things

You're unlikely

It isn't for most ppl. In fact I've never seen it specced. There is in fact no probable

*maximum* . It's just a trade-off with component life.

Probably inasmuch that esr is involved.

Graham

Reply to
Pooh Bear

You mean higher ripple current. Ripple *voltage* in a circuit is determined primarily by C alone..

No it doesn't. It means they heat up less from I^2*ESR.

Or flashgun tyes.

Electrolytic life depends entirely on how abused they are ( notably temperature and delta T ). You can kill one in under a year or it can last for decades.

Graham

Reply to
Pooh Bear

before I

datasheet (e.g.

no

usually have

for things

You're unlikely

probable

The power supply I'll be building will be used to power a coil gun or another magnetic pulse generator. So I should just look for capacitors with a low enough internal resistance to fully discharge fast enough for my application?

Reply to
Bill J.

before I

datasheet (e.g.

formula?

yielded no

usually have

electrolytics for things

You're unlikely

no probable

The esr is unlikely to be a problem discharging the cap ! If it is you have a far larger problem. You might care to consider *adding* some R in the circuit to limit peak current though, it might make the device more reliable. OTOH a coil will have inductance so that's likely to be the determining factor in both peak current and time to discharge.

Graham

Reply to
Pooh Bear

You can't. It seems that you want the maximum (ie short circuit current). This is determine by the voltage that the capacitor is charged to and the ESR. Max I = Voltage/ ESR

So order those that have the lowest ESR > How do I find out the maximum discharge current of a capacitor before I

Reply to
Wdino

But not all capacitors are suited to extreme high currents without rupture or capacitor degradation. So it is important to also know whether more than one or just a few such full pulse can be achieved.

Peter Dettmann

Reply to
Peter Dettmann

before I

no probable

I'm pretty sure there IS a maximum pulse current you can do without damaging the cap, and it's less than you would be doing in this application (less than the max voltage divided by ESR). The ripple current rating is for limiting heat (full-time ripple current across ESR causes the cap to heat up), and so it stands to reason that a pulse rating (which is very intermittent) would be substantially higher. But since "ordinary" electrolytics are rarely used in such high-pulse-current applications, manufacturers don't take the time to test and characterize them for such a rating. Xenon flash is such an app which uses special caps designed for it, but they may not be available in the capacitance/voltage rating you want for your app. From my reading (ISTR at the site below, and maybe the quarter shrinker site), even "pulse rated" capacitors are rated for a fixed number of pulses and don't work as well after having given many high-current discharges.

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- they've used "regular" caps for these applications. ISTR reading that electrolytics' life is reduced, but are still usable for this. That's a great site for this stuff (coil guns and similar things), it appears to have a high concentration of people with a lot of knowledge in this area. Be sure to ask the right questions in the right forum. Even though your app for such a cap is a coil gun (which is discussed in The "Electromagnetic Projectile Accelerators" forum), you're asking about an electronic component (capacitor) so this goes in the "General Science and Electronics" forum. I actually didn't make this mistake myself, I learned it by lurking and seeing someone else get corrected on it. I think the site makes this a little less clear than it should be, perhaps I should mention something about it over there.

Reply to
Ben Bradley

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