Setting LED voltage/resistor value?

[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 = R_min = (V_supply - V_led) / I_led

[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 

[DimitriP]

Another LED Series Resistor Calculator lives here (and in a few other places):

https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-led-series-resistor

[drummerdimitri]

Another LED Series Resistor Calculator lives here (and in a few other places):

https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-led-series-resistor

Nice to see another forum member that share the same name 

[ledtester]

20 mA is what I limit continuous current to, but most LEDs can be pulsed at much higher currents (on the order of 100 mA for ordinary LEDs, much higher for specialty LEDs - check each device's datasheet.)

An LED without a series resistor doesn't last long:

[MarkF]

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 

You would need to check the datasheet for the particular LED.  I don't know of any way to measure max current.

As a rule of thumb, the max LED current is 20mA.  Going over their max current will shorten their life.  That is why I suggested using 15mA instead of 20mA.  It's a compromise between brightness and LED life.