Resistor question

[Darth Cruor]

I was wondering, how do you calculate the resistance and power supply needed to run (any) LED?

[xwingband]

The voltage drop across the LED divided by the desired amps.

So, you'd take the battery voltage minus the forward voltage of the LED to get the drop. The best way to get the LED's voltage is to use a power supply, but if not you can use the average that the spec sheet will give you. Your target amps are also on the spec sheet.

Power supply depends on what you can fit mostly... in a resistored setup though you want to be as close to the LED's voltage as possible. Why? A resistor simply converts the extra to heat. So by being as close to it as possible you're minimizing that loss.

[Novastar]

Darth Cruor... to add to that... there really is no "set" way to measure the perfect resistor for ALL LEDs... even if we're talking about the same type of LED, color... and even the same BIN sometimes!

So... what X here said about having a dedicated "tester's" power supply (it's a special unit, you wouldn't normally have this kind of thing), there isn't really a great way to do it.

What you CAN do is try resistors that in the same "range" of the average calculation that you make. This is hard to explain, but... try a few different resistors that maybe resist (as example) 1 ohm here... 0.75 ohms here, and maybe 0.5 ohms here. A range.

These values are just made up, but hopefully you get the idea. Your calculations may yield any type of ohm resistance... maybe even 10 ohms... depends on your battery cell setup.

But stick to what X-wing said: if you're going to just light the LED with no special driver board... try to MATCH the battery voltage solution VERY closely with the LED's fwd v.