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Tim R[_2_] Tim R[_2_] is offline
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Default What resistor to use for a single LED

On Friday, April 13, 2018 at 8:27:27 AM UTC-4, wrote:
On Friday, 13 April 2018 13:15:14 UTC+1, Tim R wrote:
On Friday, April 13, 2018 at 4:43:18 AM UTC-4, wrote:


But I do
like the idea of putting two LEDs reversed polarity in there. Easy
enough to do...


Now I'll ask what is certain to be a dumb question. But maybe I'll learn something.

If we have two LEDs in parallel in reversed polarity, and we put 12 volts across them, aren't we exceeding both Vf and Vr? will they share the current, at some calculable ratio?

Sorry for my ignorance, that electrical circuits class was in the 80s.


They both see 12v. Manufacturers only spec them to 5v but IRL they can survive far more.


NT


Apologies, this is way too elementary for you all, but I'm a manager now, they haven't let me do anything technical for years.

Well of course they can both survive, you have a 1k current limiting resistor in the circuit. If the forward biased LED were alone, it would drop 1.6 V, the resistor would drop 10.4, the current would be 10 mA. (Assuming automotive 12 V) If the reverse biased LED were alone, it would drop 5 V, the resistor would drop 7, the current would be 7 mA. When both LEDs are in the circuit, does some current flow through each? or does the 1.6V of whichever one is forward biased limit the other to 1.6V, in which case it will never conduct, so there's no problem.

If Radio Shack still existed I wouldn't even ask. I'd just go buy two diodes and see what happened.