# LED voltage drop

Discussion in 'General Electronics Chat' started by samjesse, Aug 21, 2009.

1. ### samjesse Thread Starter Active Member

Sep 14, 2008
202
0
Hi
I am connected a 6v battery (4x1.5 AA) to a LED an took the voltage reading across it. I expected to read 2v but it read 5.65v. can some one tell me why? isn't suppose to maintain a 2v drop across it?

thx

2. ### peranders Well-Known Member

May 21, 2007
87
0
You must put a series resistor into the circuit otherwise you will distroy the poor LED.

I = desired current which is 2-10 mA for normal LED's.

Uled is 1.6-1.8 for red LED's
Yellow and green has somewhat higher values
Blue has 3-4 V

R = (Ubat-Uled)/I

Mar 24, 2008
20,765
2,536
4. ### mik3 Senior Member

Feb 4, 2008
4,846
63
You read 5.65V because the LED has burnt.

It will drop about 2V (depends on the LED colour and temperature) if it is operated at its rated current.

Thus for a LED which drops 2V and operates at 20mA you will need a (6-2)V/20mA=200 R resistor in series with the LED for a 6V supply.