correct voltage used in V of I x R in a simple 2 LED circuit

dcbingaman

Joined Jun 30, 2021
1,065
How much current an LED can handle depends on the design of the LED. There's no one-size-fits-all current. For some LEDs, if you run them at 20 mA you will have a very dead LED very quickly as they are intended to run at 500 µA and have max forward currents in the 5 mA range. Others are intended to be run at over an ampere and have max currents of several amps -- running these at 20 mA will likely result in no visible output, in fact some have minimum currents specified that are well above 20 mA.



Why would the resistor have to dissipate all of the power? Why would the LED also have to dissipate all of the power?

The power is going to be split between the current limiting resistor and the LED. If this is a 3 V LED operating at 300 mA, then the LED is going to be consuming just under 1 W and will need to be heat sunk accordingly. The resistor (if that's what's being used to limit the current) will have to dissipate the remaining 13.5 W.
I agree that the LED is using about 1W of power but is not part of that being dissipated as light and not heat? Thus the actual heat sink may not need to handle 1W total, though I probably would design it that way to be safe. A quick search on the internet seems to indicate 'most' LED's operate around 40-50% efficiency? That did surprise me, I was under the impression LED's operated at higher efficiencies than that.
 

WBahn

Joined Mar 31, 2012
29,976
I agree that the LED is using about 1W of power but is not part of that being dissipated as light and not heat? Thus the actual heat sink may not need to handle 1W total, though I probably would design it that way to be safe. A quick search on the internet seems to indicate 'most' LED's operate around 40-50% efficiency? That did surprise me, I was under the impression LED's operated at higher efficiencies than that.
Yes, but usually not enough to warrant taking it into account. If you design your thermal dissipation strategy to JUST handle the power delivered to the LED, you are probably in the ballpark of having 2x the capacity that it will actually have to handle and this is likely good enough.
 
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