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Topic: Correct Resistor for maximum light output on Red LED (Read 1 time) previous topic - next topic

Erni

You can get some guidence by reading the comments:

Quote

I used this LED in a computer case as a power indicator. I had to resort to this when the motherboard's power LED output stopped putting out the proper voltage following a BIOS update.

I recommend using a 68 Ohm 1/8 watt (or more) resistor in series when being driven directly from a 5 Volt constant voltage source, for example from a PC power supply red-black wire pair. When driven directly, as the 5 V forward voltage spec would suggest, the LED gets quite hot, and service life would be expected to suffer because of this. All LEDs need a current limiting resistor, and as far as I can tell, this LED does not have one built-in.

I experimented with 47 and 100 Ohm resistors as well. With a 47 Ohm resistor, the LED still heats up too much for my comfort. With a 100 Ohm resistor, the LED runs cool, but is dimmer than with a 68 Ohm resistor. With a 68 Ohm resistor, the LED still produces a little heat, but the brightness is good. If you can find an 82 Ohm resistor, that might be ideal, but RS does not carry that value. These results suggest that Vf (forward voltage) of this LED is roughly 3.5 Volts, which is typical for a blue LED, not 5 Volts as indicated on the package.

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