# Different colour LEDs series & parallel resistance amount?

Discussion in 'General Electronics Chat' started by jackofalltrades, Oct 21, 2015.

Oct 21, 2015
2
0
Hi, I'm just getting into electronics and I'm attempting to make a small LED lamp powered by a 300mA 9v DC power plug - and I've run into a little problem.
I've been following some instructions, and they basically stated that their LEDs are all the same colour/voltage, just with a different colour coating (i.e. white LED with red, blue plastic bit.)

However, I'm using actual different colour LEDs with a clear plastic. They have different forward voltages, I'm not sure how to do the resistance because of the red - blue - red LEDs in series?

Forward Voltages max
Red 2.0
Blue 3.2
UV 3.2

I've attached a picture of how the schematic looks in the tutorial:

Mar 24, 2008
20,772
2,540
3. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,589
389
hi,
You add up the LED colour voltages in the series network, then subtract that value from 9V.
This gives the voltage that you must drop across the series resistor.
Divide that voltage by the LED current required by the LEDs.

Typically this LED current is approx 15mA to 20mA, for regular LED's, choose 15mA.

So for your top chain, Rd,Bl, Rd thats 2.0 + 3.2 +2.0 = 7.2v
Subtract 9v - 7.2v = 1.8v
Resistor value 1.8v/0.015A =120 Ohms.

OK.

BTW: you MUST have a series resistor with an LED in order to limit the LED current, else it will fail.!

E

4. ### ScottWang Moderator

Aug 23, 2012
4,925
777
I am assuming that the LED are all rating for 20mA, and just using 80% of 20mA, that is 16mA.
You could try to calculate the R values.

mcgyvr likes this.