# proper resistance??

Hello, may I have a question, I don't understand calculate in this, please help me. "For instance, given this LED with a 3V voltage drop (forward voltage), 20mA operating current, and a 9V source, we can calculate that the proper resistance is 300 ohms."

How to calculate **proper resistance** = 300 ohms??

Thank you very much.

Ohms's Law is R = V/ I

R = Resistance measured in Ohms

V= Voltage measured in Volts

I= Current measured in Amps

In this case Resistance = (Supply Voltage - Forward Voltage)/ Led Current Rating.

First, you must convert the 20 mA to Amps.

1000 mA = 1 A, Therefore, 20 mA/1000 mA = 0.02 A

(9 V - 3 V) / 0.02 A = 300 ohms

I hope this helped

An easy way to remember the relationship between Voltage, Current and Resistance using Ohm's Law is by this triangle..

To find voltage: V = I x R

To find Current: I = V / R

To find Resistance: R = V / I