LML & John:

>> So something like a Pulse Width Modulator for a motor would work very well
>> here?

Same concept but, with one exception, I wouldn't reccomend using the motor controller for LEDs.

While I have limited experience with duty cycle controlled DC (only works on DC motors BTW) there's one thing I seem to recall: DC motors only start turning once the duty cycle goes over a certain value and I think it's in the 50+% range. Naturally, the controls go all the way up to 100% (full on) for max speed on the motor. This is the opposite of what's typically required for LEDs where they start to light up immediately and really don't need much more than 1:7 on to achieve their max brightness (given CURRENT through the led 8X in excess of its MAX steady state current/power capability).

The exception: If the 100% on duty cycle was current limited to the LED's MAX steady state current/power capability. Then any thing less than 100% would naturally dim the LED from that point.

Note too: The series resistor is a current limiting device. In their normal operating range, LED's want to drop the same voltage all the time due to semiconductor physics. So, while the resistor will be dropping voltage, it's actual function is to limit the current through the LED. Since the LED has a preordained voltage drop, this current sets it's power dissipation. Naturally, the remaining voltage is dropped by the resistor which is carrying the same current and tada, you have the power dissipated by the resistor. Because the semiconductor/fixed voltage drop of the LED, you have to back into ohms law somewhat as opposed to dealing with two series resistors.

BTW, I'm 100% with Jeff, simplicity is the goal wherever possible and is doesn't get much more simple than a series resistor!

There is ANOTHER way of achieving the goal and that is to put LEDs in series! That is, if each LED want's to see, say 1.4v, dropping several in a series would be a way to reduce the power dissipated by the series resistor since the resistor would see less of a voltage drop (same current) it winds up dissipating less power / generating less heat...

Although this morning.... the idea of snuggling up to a nice toasty resistor seems... mmmm... strangely comforting!

Keep the blue side going round-and-round.

Go Navy! Beat Army!

Ed McCauley
Bottom Line Technologies Inc.
(908) 479-1200
http://www.bltinc.com
ed.mccauley@bltinc.com