dump cap>LED, damage?

[Bernhard Kuemel]

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

 

[Ian Field]

"Bernhard Kuemel" <bernhard@bksys.at> wrote in message
news:5473e$4dce88f1$557f6843$19057@news.inode.at...

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.

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]

On May 14, 9:51 am, Bernhard Kuemel <bernh...@bksys.at> wrote:

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.

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":
http://www.redcircuits.com/Page87.htm

Tim.