Electronics > Beginners
How do I calculate the current draw of an LED if I don’t run it at full power?
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Blitzschnitzel:
There are a ton of online LED resistor calculators but they all assume I want the full brightness.
So, if I have a 3V 20mA LED and want tor run it at 5V I need a 100 Ohm resistor. How do I calculate the current draw if I use a 330 Ohm resistor? I’m not really sure how to apply Ohms law here. Or is it just something depending from LED to LED?
iMo:
Each color has got its own "forward voltage drop Uf" at the LED's pins when a current If flows through it.
For example red is 1.7V, blue is 3.5V etc. Google for the voltages.

Now, the current via the led will be:

If = (U - Uf)/R, where U is your battery or source voltage and the R is the serial resistor.

Example with 5V source, 330ohm serial resistor and a blue led:

If = (5V - 3.5V)/330ohm = 4.5mA

https://www.lumex.com/led-color-guide.html
Blitzschnitzel:
Fantastic! Thank you very much. :D
Brumby:
When you decrease the current through an LED, the forward voltage drop also decreases - but not in a linear fashion.

The formula given by imo will give you a reference figure that will be good enough for most circumstances.  If, however, you want to be precise, you will need to refer to the datasheet for the LED - but I'm inclined to say you won't really need to do that.
Blitzschnitzel:
And how is it when I do it the other way around and want to achieve a specific voltage?
I have a 3V 20 mA LED and want to run it at 2.6V. Do I then look up the current from the curve on the datasheet and use the current draw at 2.6V to calculate my resistor or do I still assume 20mA?

I have this white 7 segment LED:
https://datasheet.lcsc.com/szlcsc/SUNLIGHT-SLR0281DWC1BD_C225876.pdf
What confuses me is that the second page says 20mA at 3V but the graph on page 5 says 35mA at 3V!? Did they just mislabel that?
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