How to drive an LED with an NPN transistor

124

02:38:04

1. Choose LED current 
e.g. I(led) =20mA

2. Choose collector resistor (Rc) value:
Vsupply=LED supply voltage, e.g.: 5V
Vf= LED forward voltage drop, e.g.:2V
Vr=collector resistor voltage drop

Vr=Vsupply-Vf
Rc*I(led)=Vsupply-Vf
Rc=(Vsupply-Vf)/I(led)
Rc=(5V-2V)/0.020=150Ohm

3. Choose base resistor (Rb) value (using 10x base current for saturation) Assuming that the "control voltage" is the same as the supply voltage.

Ibase*Hfe=I(led) * 10
Vsupply/Rb = I(led) * 10
Rb = (Vsupply * Hfe) / ( I(led) * 10 )

Rb = (5V*25)/(0.005*10) =  2.5K

published 9 years ago

Robert_Kidd

9 years ago

Did you change mind part way through? There is no emitter resistor in this circuit.

nemethakos

9 years ago

Thanks, it was meant to be a collector resistor!

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