Choosing The Resistor To Use With LEDs

Step 3 The Hard Way (Math!)

All the calculators in step 2 are just doing some simple math that you can do at home:

The formula to calculate resistance in a circuit is: R=V/I or, more relevant to what we're doing:

(Source Volts - LED Volts) / (Current / 1000) = Resistance^*

So if we have a 12v battery powering a 3.5V 25mA LED our formula becomes:

(12 - 3.5) / (25 / 1000) = 340ohms.

But wait! (you might say) When I use one of the other calculators I get 390 ohms! And indeed you do. That's because its hard to buy a 340 ohm resistor and easy to buy a 390 ohm one. Just use the nearest one you can easily find.

To learn more about this magic formula read about Ohms Law.

^* We're dividing the current by 1000 because our listing in in miliaps, or 1/1000^th of an amp.

Comments

[cekpi7]

thanks its help me :D

[sinopro]

If I use 3 LED lamps in white, each is 3.2V and 25mAH, totoal is 3.2V 75mAH, and battery is 9V, not 12V, what the resistor should be?

[Grathio]

It entirely depends how you wire the LEDs. Are they in series or parallel?

I recommend that you go to Step 1, click on one of the links there and discover the answer yourself.