dump capLED, damage?

Question

Hi sec!I want to make bright flashes by dumping a capacitor into an LED throughsome transistor. What capacity/voltage are safe before the LED lifetimedrops significantly? Right now I have a 20 mA green LED. No specs causeI found it (several bars with lots of red, green and blue LEDs, actually).The application would be a blinkie/throwie with solar cells charging aEDLC which drives a step up converter into a normal electrolytic andblink at intervals so it can blink through the night. So maybe 500-1000blinks/night, about once/minute.470 uF, 12V left no noticable damage after a few times.Bernhard

Ian Field · Accepted Answer

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.

Tim Shoppa · Answer

.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 andsee what typical LED spec sheets are like.Looking at your application, it sounds like a perfect match to theLM3909. The pulse from a LM3909 in the sample circuit is about 45 mAfor 6ms with a 300uF cap.Really, look at the LM3909 examples. LM3909 "built from scratch":Tim.

dump cap>LED, damage?

Join the Discussion

Didn't find your answer?