question about pr bulb mod

i plan to make a 3 led pr bulb mod for my bossed flashlight, now the light is one that he got in a kit with other cordless power tools, the battery pack is 18 volts, what type of resistors do i need, and whats the best way to put in three resistors and 3 led's in the pr?

Assuming the leds have a forward voltage of 3.5volts..and current of 20mA.. You would need around 725 Ohms of resistance for each led.

For the sake of efficiency, I suggest three in series (so the same current can light all three). Three 3.6 Vf drops is 10.8 Volts. Leaving 7.2 from the original 18. Dividing 7.2 Volts by our desired current, say 30 mA (.030 Amps) gives us 240 Ohms for a series resistor. 7.2 Volts times .030 Amps means .216 Watts, best use half Watt (or even a one Watt). You can, of course, change the current by changing this resistor, twice as many ohms will be half the current (and the other way about). If you go for more current (or should that be 'when'?) be sure to scale the power rating on the resistor up as you lower it's Ohm value.

Doug Owen

Yes... Series config would be good... /ubbthreads/images/graemlins/smile.gif