piclist 2002\10\20\021310a >
Thread: NPN LED current drive
face BY : Russell McMahon email (remove spam text)

> As a LED switcher, a small transistor should not to be seen as a current
> limiter or extra power heatsink, for this job a much lower cost resistor
> recommended, except of course if the supplied voltage is not known, so the
> transistor should be used also as a current limit device.

The place where a transistor current source IS a good idea, and the one
which I based my original comment on, is where the unregulated supply varies
for some reason. A very good example is with a battery power supply. The
processor is powered by a voltage regulator but the LED is run at constant
current directly from the battery.As the battery voltage falls the LED
remains at constant brightness. For example, a 9v "manganese alkaline"
battery will change from slightly over 9v to about 5.5v (0.9v endpoint per
cell) across its lifetime. This would lead to a factor of almost 2 change in
LED current and 4 times change in power dissipation in the LED circuit
across voltage with a 1.5v LED. Without constant current drive the LED can
be run at only about 53% of its rated current at battery endpoint.

9V batt        I = (9-1.5)/R = 7.5/R
5.5V batt     I = (5.5-1.5)/R = 4/R
I change = 4/7.5 = 53.3%

A specific example of the above is in typical IR remote controls where the
LEDs typically draw very large currents (maybe several hundred mA) for very
short periods. Here the constant current supply allows the LED to be run
near its destruction point as the battery fades from new to dead. Without
the current control capability the only choice is to spec operating current
that will be safe with a new battery and suffer performance degradation as
the battery voltage drops. Examination of most IR remote controls will show
that the current limiting resistor is indeed in the drive transistor's
emitter (whereas, if constant current was not a design aim, it would be in
the collector,)

       Russell McMahon

> As even unregulated power supply voltages can not change so much, few
> as expected, the resulting current change will be in the same order, or
> almost non perceptible by the user.  I would say that even when the
> supplied voltage changes from 14 to 17V you still can use a simple current
> limiting resistor in series with the LED and forgeting about constant
> current controlling.
> In this case, a small NPN transistor setup as a simple switcher, emitter
> ground, series resistor at base, led and resistor at collector will be
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