# How do you know the dischage amps of a capacitor?

Discussion in 'General Electronics Chat' started by PulseLED, Jan 24, 2014.

Jan 24, 2014
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2. ### ericgibbs AAC Fanatic!

Jan 29, 2010
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hi,
We need to know the specification of the LED or part number, datasheet etc.
E

3. ### ErnieM AAC Fanatic!

Apr 24, 2011
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The current you can get out of a cap is actually quite simple to predict mathematically, it is just:

I = V / R

Yep, ohms law. V is the capacitor voltage, R is the sum of the load resistance plus the internal resistance of the cap. Oft you can ignore the internal resistance (though I doubt this at 30A).

It may be hard to find ratings to handle this pulse: it is essentially a short circuit.

Also, since the voltage on a capacitor decays exponentially, so does your current.

If this was my project I would be looking for a different power source, even a battery.

4. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,575
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hi Ernie,
He has not told us what the forward voltage drop is of the LED.

If its a White type, it could be ~3.2V, so he will not discharge the cap below this voltage, there will be only about 1.8V available from a 5V charged cap.
Eric

5. ### AfdhalAtiffTan Active Member

Nov 20, 2010
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The current discharged by capacitor can be calculated with both V=IR and I=ie^r.

But it is still pretty hard to estimate as current drawn by LED is not linear.

The best way would be to control it by using more precisely timed digital circuit, like 555 or any mcu.

Just curious, are you trying to build a strobe light?

6. ### MrChips Moderator

Oct 2, 2009
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Unless your LED can handle 30A, pulsed or otherwise, you are going to destroy the LED.

7. ### PulseLED Thread Starter New Member

Jan 24, 2014
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Thankyou greatly for the replies guys.

So there's no real easy way to determine the discharge current as you can't readily measure the resistance of a LED? heres the LED specs:

CREE
XLamp XT-E LED

Size (mm x mm)3.45 x 3.45Maximum drive current (A)1.5Maximum power (W)5Light outputUp to 456 lm @ 5 W, 85°CTypical forward voltage @ 350 mA, 85°C (V)2.85Viewing angle (degrees)115

So the 5v 30A 1us was only round abouts figures, if it were 30A tapering off over 3us it would be fine. So I'm trying to do it as simple as possible, avoiding 555's and the like. From my research these high power LED's should be able to handle 30x the max current is pulsed for such a short time.

Yes I'm trying to make a very fast strobe, not to the intensity of a normal strobe (I think that's out of reach) but enough to generate some image.

8. ### ErnieM AAC Fanatic!

Apr 24, 2011
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I looked at Cree's data sheet but cannot find the 30A 1uS pulse you mention. Can you link to that data sheet?

From what I see is the max current is 1.5A continuous, no mention about pulse.

9. ### Austin Clark Active Member

Dec 28, 2011
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Check-and-guess is probably your best option. Trying to calculate this out, even with the datasheet (which probably lacks the information you need), will be very difficult if not impossible.

10. ### ronv AAC Fanatic!

Nov 12, 2008
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When I model it I can see 2 problems:

Vf goes to about 8 volts at 30 amps. So 5 volts won't be enough.
The other is the ESR(equivalent series resistance) of the cap. The one you show has an ohm or 2 so can't supply 30 amps. You can use ceramic caps.

It may turn into a bomb- A flash of light and a wisp of smoke.

11. ### PulseLED Thread Starter New Member

Jan 24, 2014
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What I saw wasn't specific to the Cree LED that I have, rather another 1.5A LED driven at 30A pulse or more, here is the article http://iopscience.iop.org/0957-0233/21/7/075402/

I'm ok with destructive testing until I get the results I'm after, but really don't even know where to start, what capacitor type, voltage and capacitance?

I don't claim to know much, but I just had a random thought. Could you discharge a photoflash capacitor (330v 800uf) into a transformer to drop it down to low voltage, increase the amperage then into a LED. It would be great if I could just convert a camera flash unit.

The problem I'm trying to overcome is the afterglow of a xenon flash creating the longer then wanted pulse of light. I know this is normally done with an air flash although tat presents a lot of problems in itself.

12. ### ronv AAC Fanatic!

Nov 12, 2008
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What is the fastest speed of your camera?
What are you actually trying to do?

13. ### PulseLED Thread Starter New Member

Jan 24, 2014
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Xenon flash bulbs will only go as low as 40us or there about's. The faster the flash the less blur on fast moving objects and 40us just is too slow to capture a bullet.

Anyway unless someone stops me I'll pull the little 1.5v->330v transformer from a disposable camera, flip it around, hook it up to an LED and discharge the camera capacitor into it.. and see what happens

14. ### ronv AAC Fanatic!

Nov 12, 2008
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So what your looking for is something on the order of 10 usec. so you can see it every 1/3 of an inch or so?
It's getting easier. I think I would just use say a 12 volt power supply and several LEDs in a series parallel arrangement to get the light I wanted. You can over drive them a bit if you like to keep the numbers down, but I'm betting you want like 10 Usec. on and 10 off, so I wouldn't go for more than 50%. You can just use a standard power supply and a big FET switch to turn them on and off.
I have to go now and most of the day tomorrow, but expand on the requirements and I'm sure someone will help.

15. ### MrChips Moderator

Oct 2, 2009
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Xenon flash circuits don't work like that.
The xenon flash bulb is non-conducting until a high voltage pulse is sent to the trigger.
This causes ionization and avalanche breakdown of the xenon gas. The charge from the storage capacitor is dumped through the flash bulb once this happens.

You cannot simply replace the flash bulb with an LED in such a circuit.

16. ### KL7AJ AAC Fanatic!

Nov 4, 2008
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The problem is, most capacitors don't give maximum current specs UNLESS they're designed for high current, such as R.F. vacuum caps.
Eric

17. ### PulseLED Thread Starter New Member

Jan 24, 2014
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Oh ok, I understand. Would a way around this be to use the high voltage pulse instead of directly from the cap? or alternatively use the trigger voltage to trip a IGBT/mosfet allowing the capacitor to dump?

18. ### takao21203 Distinguished Member

Apr 28, 2012
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LEDs have internal resistance. Your 30 Amps is not going to happen.

19. ### ronv AAC Fanatic!

Nov 12, 2008
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Forget about the cap. While you may put one on the output of the supply to help provide the pulse current you can simply turn the LED on and off with a FET.
While LEDs do put out more light with higher current they are less efficient past their optimum so you are better off with more LEDs after a point. How often you want to pulse it and how much light you need would take it from guessing.

20. ### PulseLED Thread Starter New Member

Jan 24, 2014
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I only want a single brief pulse, to take a single photo. I bought a couple MOSFET and IGBT on ebay to experiment with, but have never used them before.