# Which Resistor

Discussion in 'General Electronics Discussion' started by Nelix, Mar 16, 2016.

1. ### Nelix

2
0
Mar 16, 2016
Hi All

I'm sure this will be an amazingly simple question for someone to answer, however as a hobbyist in electronics I am looking for some clarification.

I have a 2 pin bi-colour LED (Red/Green) and will be running it from a PP3 battery (9v). I simply want to be able to put the led on the battery one way for one colour and the other way round for the other colour. My question is this "What size resistor sahould I use, the LED I will be using is this one:

http://uk.rs-online.com/web/p/visible-leds/2285720/

Kind Regards

Nelix

2. ### Gryd3

4,098
875
Jun 25, 2014
You can do the math yourself
You have a 9V battery, and a forward voltage drop from 2.0-2.2V .. this leaves you with 6.8-7V left over.
Let's assume the LED requires 20mA... because the specs online don't say.
What value resistor do you need that drops 7V while 20mA passes through?

KeithM likes this.
3. ### dorke

2,342
665
Jun 20, 2015
Yes it is simple.

For the LEDs you have IF=20mA.
Vf max= 2.5V
Vf Typ(red)=2V
Vf Typ(green)=2.2V

Since the resistor is common for both LEDs,
lets "aim" at 2V:

R=[9-2]/20mA=350 ohm
R=360 ohm (standard value).

Last edited: Mar 16, 2016
Arouse1973 likes this.
4. ### Nelix

2
0
Mar 16, 2016
Thanks for the help.

I had worked it out at around 340 but as mentioned I just wanted some clarification.

Thanks again, it is very much appreciated.