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Led Problem? Answered

I hooked up 12 3mm red leds in parallel and then powered them with 2x 1.5v rechargeable batteries, and it worked, until 2 blew 3 got damaged and they all started flickering. Why, and what should I do to fix it?
Feel free to ask any questions.
Thanks Big   



Best Answer 7 years ago

Don't run leds in parallel. There is no way to control which led(s) will get slightly more current, which makes them warmer, then makes them draw more current (this is called thermal runaway) -- even identical leds will have manufacturing flaws that start small and end in catastrophic failure.

Solution: Use a current limiting resistor for EVERY string of series leds.
Use something like http://led.linear1.org/led.wiz to calculate

enter the led info, (depends on the leds)
12 total leds
3 volt supply (rechargables are usually 1.2v not 1.5v) so maybe 2.4v
led forward voltage will be between 1.5 and 4 (look it up)
led forward current -- a safe number for 3mm leds is around 15 ma

Using those numbers it outputs:
Solution 0: 1 x 12 array uses 12 LEDs exactly
+----|>|---/\/\/----+ R = 27 ohms
+----|>|---/\/\/----+ R = 27 ohms
+----|>|---/\/\/----+ R = 27 ohms
+----|>|---/\/\/----+ R = 27 ohms
+----|>|---/\/\/----+ R = 27 ohms
+----|>|---/\/\/----+ R = 27 ohms
+----|>|---/\/\/----+ R = 27 ohms
+----|>|---/\/\/----+ R = 27 ohms
+----|>|---/\/\/----+ R = 27 ohms
+----|>|---/\/\/----+ R = 27 ohms
+----|>|---/\/\/----+ R = 27 ohms
+----|>|---/\/\/----+ R = 27 ohms

The wizard says: In solution 0:
each 27 ohm resistor dissipates 6.075 mW
the wizard thinks ¼W resistors are fine for your application
together, all resistors dissipate 72.9 mW
together, the diodes dissipate 360 mW
total power dissipated by the array is 432.9 mW
the array draws current of 180 mA from the source.

LEDs in parallel is a Bad Idea.  Ideally you'll use a higher voltage and a series resistor for each one, calculated by THIS method.

What has happened in your case is that the ones with the lowest Vf (forward voltage drop) have hogged all the current from the battery, overheated and burned out.  Leave it for longer and they will all fry - one . . . by . . . one.

You need to read through this site - Your LEDs are more or less a short circuit when connected to the battery and will draw so much current they burn out.

to limit this current to a safe level you need to put a/several resistors in series with the LEDS to protect them.