Resistance of a LED.

Hi all, sorry if this is a stupid question.
I have googled and searched the forums here but i just cant find the answer.

When working out the voltage drop over one of two resistors I use the following calculation:

R1hm:/RTotalhm: * Vin

This is fine if im just using two resistors.

But If instead of having two resistors, I have one resistor and a LED.(Please see the attached image.)

I need to calculate the amount of voltage that will flow through the LED, but i dont know how to calculate this as i dont know the resistance of the LED, and for some reason i cant get an acurate reading from my multimeter.


thanks for your help.

An LED doesn't have a resistance, and you can't apply ohms law to it. What it does have is a voltage drop, which will be listed in it's data sheet.

All you need to do is subtract that from the supply voltage, then use that to calculate the resistor based on the current you want.

Nice one mate, thanks for the ultra speedy reply.
That one has been doing my head in.

Red LEDs are about 1.8V. Then the 360 ohm resistor has 7.2V across it and Ohm's Law calculates the current to be 20mA. If the resistor is 3.6k then the red LED will have a voltage of about 1.75V and the current will be about 2mA.

A white or blue LED is about 3.4V. Then the resistor in series with it will have less voltage drop so the current will be less than a red LED.