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Setting LED voltage/resistor value?

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drummerdimitri:
So I'd like to add a small LED to indicate that the power switch is on in a very basic circuit, however I've never known how to figure out what voltage to set them at (in this case resistor value to drop 12V DC).

Do I need to use my DMM in its diode test function? Will that give me the minimum voltage but not the max or ideal voltage?

I know different LED colors will have different required voltages. Surely I would then need to use Ohms law to figure out what resistor value I need to drop the VCC down to.

ledtester:
All LEDs have voltage drops of 2 to 3 volts and will be adequately illuminated with 10 ma, so just base your initial resistor calculations on that and adjust as necessary.

coromonadalix:
Some leds will light at 1.25v  ... and have 20 ma current consumtion ,,,

MarkF:
You do not set the voltage for LEDs.  You set their current.
Most individual LEDs have a max current of 20mA and a forward voltage drop between 1.5V to 3V.

If you want a quick and dirty calculation, just ignore the LED voltage drop and calculate a resistor based on the supply voltage and a 15mA to 20mA current.  Realizing that the current will be slightly smaller due to the voltage drop across the LED.

Otherwise, the resistor value = Rmin = (Vsupply - Vled) / Iled

drummerdimitri:
Good to know thanks!

What about the safe max current for LED's? Surely they must exhibit some strange behavior that I could capture on my graphical sampling DMM before they die  :-//

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