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Maximum discharge current of capacitors.

B

Bill J.

Jan 1, 1970
0
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?
 
T

Tim Williams

Jan 1, 1970
0
Bill J. said:
How do I find out the maximum discharge current of a capacitor before I
order it?

What kind? Pulse-rated caps usually have the ratings in the datasheet (e.g.
Digikey).
Is it somehow related to the maximum ripple amps by some formula?

Perhaps.

Tim
 
B

Bill J.

Jan 1, 1970
0
Tim said:
What kind? Pulse-rated caps usually have the ratings in the datasheet (e.g.
Digikey).


Perhaps.

Tim

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.
 
R

Roger Hamlett

Jan 1, 1970
0
Bill J. said:
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.
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
 
P

Pooh Bear

Jan 1, 1970
0
Bill J. said:
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.

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
 
B

Bill J.

Jan 1, 1970
0
Pooh said:
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

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.
 
P

Pooh Bear

Jan 1, 1970
0
Bill J. said:
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.

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
 
B

Bill J.

Jan 1, 1970
0
Pooh said:
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

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?
 
L

Lostgallifreyan

Jan 1, 1970
0
IS there a connection with max ripple amps?

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.
 
P

Pooh Bear

Jan 1, 1970
0
Bill J. said:
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.

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.
IS there a connection with max ripple amps?

Probably inasmuch that esr is involved.

Graham
 
P

Pooh Bear

Jan 1, 1970
0
Lostgallifreyan said:
I think so. Low ESR (Equivalent Series Resistance) caps have lower ripple,
usually.

You mean higher ripple current. Ripple *voltage* in a circuit is determined
primarily by C alone..
That lower resistance means they respond faster to changes on the
input.

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

Or flashgun tyes.
, 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.

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
 
B

Bill J.

Jan 1, 1970
0
Pooh said:
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

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?
 
P

Pooh Bear

Jan 1, 1970
0
Bill J. said:
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?

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
 
W

Wdino

Jan 1, 1970
0
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 in their specs for a given
voltage rating.
 
P

Peter Dettmann

Jan 1, 1970
0
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 in their specs for a given
voltage rating.

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
 
B

Ben Bradley

Jan 1, 1970
0
Responding in another old thread:

... < snippage >

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?

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.

Sign up on http://www.4hv.org - 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.
 
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