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# Help with resistor calculator for LEDs Answered

Hi everyone, I decided to try and build some LED lighting and found a few online resistor calculators but, I'm not exactly sure what to put for the voltage drop and the LED current (other than using the generic suggestions from the online calculators). So here are my questions...

Is the "forward voltage" what you enter for the "voltage drop"? If so, do you use the lower or higher number?

For the LED current, do I use the "max cont. foward current" or should I just put in the suggested 20mA? what kind of a difference would one be over the other?

These are the specs given for the LED I'm going to be using (minus the unimportant stuff).

Forward Voltage (V) : 3.2 ~ 3.8
Reverse Current (uA) : <=30
Absolute Maximum Ratings (Ta=25°C)
Max Power Dissipation : 80mw
Max Continuous Forward Current : 30mA
Max Peak Forward Current : 75mA
Reverse Voltage : 5~6V

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## 4 Replies

kelseymh (author)2010-06-10

Besides Willard2.0's excellent explanation and advice, note also the power dissipation limit of 80 mW. You want to make sure that the product of your sustained voltage and current (P = VA) is below that limit: 20 mA @ 3.6V = 72 mW, so you're fine, but you always want to do the math.

GSXR (author)2010-06-11

Thanks kelseymh, I never would have even thought about that. I've still got a lot to learn about this stuff, lol.

Willard2.0 (author)2010-06-10

The forward voltage listed is the min and max of the diode. I would enter 3.6V which is in the middle of the two. You could use 3.8V if you are trying to get every last bit of light out of them. The LED current is max 30mA, so I would enter 20mA as you never want to run the LED at absolute max specs

GSXR (author)2010-06-10

Awesome, thanks a lot! Your explanation was just what I needed. I've been looking all over the place trying to figure it out.