4.5v to 3.8v led help

[piratepaul]

Hi, take no notice of the radiation bit, you can get stuff from the skip round the back of the hospital that is dangerous.
Watch your eyes though.
The voltage of the LED is 3.5 ...that means it will drop 3.5v regardless of current through it.
If you put a resistor in series with it a current of 30mA must flow through the resistor and diode, there is only one resistor value which will limit current to 30mA.

The power supply is 4.5v, the voltage across the LED is 3.5v... Thus the voltage across the resistor must be 4.5 - 3.5 = 1v.

The resistor needed = 1/0.03 = ( don't know where my calculator is just now) Ohms.

If the resistance of the diode is 1/0.03 it will work... If not it will not.

Tar&stuff.

[oram]

cool.

thanks david.

[oram]

cheers paul. that makes sense.