Currently, 12v lead acid battery is the most economical source of stored energy and so, trying to make the most out of it, I try to design - redesign on that voltage.
Here is an old redesigned white LED drive which I have replicated many times and may be of use to some of you. Current dropping resistors are not reliable in the long run and after wasting many a LEDs I have found this circuit works well. BTW the design is specific for lead acid battery -- its useful voltage of 12.7 to 10 volts
3 white LED in a string is just right for a 12v battery and linear regulation is efficient enough for such an application. Current through the LED is best kept below 20ma and is controlled by the 33 ohm resistor. Trial and error is applicable in setting this resistor but 33 ohms 10% is a good value as it is found to keep the value between 15 - 20 ma .
The circuit works well 10+ volts. I have been using multiple strings of the same circuit for 2 yrs without failure till date.
Thanks to the people who posted the PNP version that I first saw
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