So, I have these garden lights powered by solar cells. I also happen to have a power-LED over from a broken flashlight, I think its a Cree R2 or Q5 or similar. These lights charge a 1.5V 600mah Nimh battery during the day, and then light up these rectangular low-power leds.
Now I would want to experiment with simply driving a high-power LED at similar, or slightly lower power levels, meaning a very very low current. I have not measured the drive current for these, but I imagine its well below 50ma. The theory being that it will be brighter still because of the excellent efficiency of power LEDs in this power band.
I know power leds generally require current limiting or control, and that the battery voltage is not enough so I will need some sort of boost circuit if not already present.
What I am wondering is what is the most efficient way of doing this, without being overly complicated? Is it possible to do this with just a resistor, or am I still prone to current run-away at these low levels?
Best Answer
The amount of light LEDs put out has a very linear relationship with the current they are driven with, and the voltage to current graph is relatively constant as they are diodes with a characteristic junction voltage. Taken together, this means that the efficiency can rise about 25% if an LED is driven at 10% of rated brightness (in this LED, but others should be fairly similar).
In any case, regardless of the level you choose to drive the LED at, what you want is a "single cell LED driver". These are usually current-regulated, eliminating the need for any resistors which do nothing for your efficiency, and also have the ability to step-up the voltage, so you can drive a 3 V nominal LED with one or two 1.2 V NiMH cells.
Linear, Micrel, and Diodes Inc all make drivers for this purpose, and you can find many more in a search for "single cell LED driver".