20mA LED Ballast I use this circuit for general use to power white LEDs, like a headset light I use on my desk and a keyboard light for my computer. I built it into a tiny Radio Shack project box (270-288), which they no longer sell. I put heat-shrink tubing around the female output connector (RS P/N 273-1743), so that the contacts are not exposed, but you can still plug and unplug it.

Note that if the voltage limit is set too high, the LED may blow out when the LED is first connected to the energized ballast circuit. Set R2 to minimum resistance before the first time the circuit is powered up with the intended load on it. Bring the voltage limit up (increase the resistance of R2) just past the point where the LED(s) doesn’t get any brighter.

SCR1 and R2 thru R4 can be deleted if the LED(s) will be permanently connected. Without the SCR, C2 will EXPLODE if the circuit is powered up and unloaded.