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dump cap>LED, damage?

Hi sec!

I want to make bright flashes by dumping a capacitor into an LED through some transistor. What capacity/voltage are safe before the LED lifetime drops significantly? Right now I have a 20 mA green LED. No specs cause I found it (several bars with lots of red, green and blue LEDs, actually).

The application would be a blinkie/throwie with solar cells charging a EDLC which drives a step up converter into a normal electrolytic and blink at intervals so it can blink through the night. So maybe 500-1000 blinks/night, about once/minute.

470 uF, 12V left no noticable damage after a few times.

Bernhard

Reply to
Bernhard Kuemel
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A possible alternative is to use a blocking iscillator with a slow charging electrolytic in the base circuit, it gives widely spaced narrow pulses and to can adjust the secondary to suit the LED.

If you want a bank of (say 24 paralell ) LEDs to make a huge flash; charge a

47u capacitor to 32V and dump it into the LEDs with a diac.
Reply to
Ian Field

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Most "real" LED's bought with a spec sheet will give a spec called "peak forward current" that is what you want. Poke around Digikey and see what typical LED spec sheets are like.

Looking at your application, it sounds like a perfect match to the LM3909. The pulse from a LM3909 in the sample circuit is about 45 mA for 6ms with a 300uF cap.

Really, look at the LM3909 examples. LM3909 "built from scratch":

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Tim.

Reply to
Tim Shoppa

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