LED refresher

Doing a little voltage measurements, I determined the greens are operating on 120uA.  2.65V dropped across a 22K (red,red, orange + measured directly) resistor equaled 120uA. Must be super bright, super efficient, very low current green LEDs.   (Years ago a company component engineer postulated that LED technology would never be bright enough to be usable outdoors or even for a flashlight!)

Other LEDs operate in 5 to 10mA ranges so the greens were a surprise.

If you wire a blue, a green, a red and a white leds in parallel you will find they light up at slightly different voltages.  And as you dial up the voltage you will see them come on at different voltages. 

The same pn# leds from a given manufacturer will usually light up at the same voltage and exhibit very close emission vs current characteristics .

aussteve posted:

If you wire a blue, a green, a red and a white leds in parallel you will find they light up at slightly different voltages.  And as you dial up the voltage you will see them come on at different voltages. 

 

This can be true if they are "fully" in parallel but not if, say, they share a single current path (say, through a shared current-limiting resistor) .  In that case, the LED with the smallest Vf will, when it starts conducting, work to shunt as much current as it can to keep the voltage constant to both LEDs.  And, if the smallest Vf (say, on a nominal yellow LED) is appreciably different from the largest Vf (say, on a nominal white LED) the voltage will never rise to the point where the white LED turns on.  One good reason for LEDs in parallel not to share their current-limiting resistor.  This post on SE demonstrates this nicely:

https://electronics.stackexcha...r-many-parallel-leds