On 2006-10-01, Al wrote:

So, if the LED is specified to give a certain light output at 10mA at
4.5V and if the source voltage is a constant 6V, I would use two diodes
whose forward voltages are specified as 0.75V at 10mA to give me a 1.5V
drop.

Typically leds are specified with a voltage range for their limit current.
get the curren right and the voltage will be somewhere in that range, it
depends on the device and the environment.

If that particulasr led is 4.4V with 10mA flowing through it it could with
4.5V the current could be 30mA or more.

The forward voltage drops of a typical 1N914 diode are shown as:

Vf If
volt ma

0.6 3.0
0.7 10.0
0.8 30.0

And when they get warm, it drops, this can lead to thermal runaway.

ordinary resistors consistently out-perform ordinary diodes as a LED
current source.

Bye.
Jasen