On Fri, 24 Dec 2010 18:41:14 +0000 (UTC), "A. Baum" wrote:

On Fri, 24 Dec 2010 11:58:12 -0600, zzzzzzzzzz wrote:

On Fri, 24 Dec 2010 12:43:40 -0500, wrote:

On Fri, 24 Dec 2010 12:11:20 -0500, Jeff Thies
wrote:


I hadn't thought about this, but aren't light dimmers just varying
the fraction of a half wave (pulse width)? It would seem to me that it
should work with LEDs (although not particularly well) but I don't know
what circuitry is in an LED light.

The problem is the clock rate. LEDs respond virtually instantly and at
60 or 50hz the flicker would be horrible.

As can be seen with LED Christmas lights. It's fairly cheap to double
that (full wave bridge or more LEDs), which would solve the problem.

Uh the problem is an led fires at a certain voltage.

No, as has pointed out here by others, it doesn't "fire at a certain voltage".
It's light output is highly nonlinear WRT voltage, but there is no "on" or
"off", rather a continuum.

To dim the led you
need to apply that voltage via a PWM. Chop the firing voltage up into
pulses too fast for the eye to notice.

The output of an LED is a function of the average current through it. You can
either use PWM to change the average current or vary the current directly. The
light output is a pretty linear function of the current through the LED so
either method works.

The smaller the width of the pulses the dimmer the led.

No, the less the *ratio* of "on" to "off" the dimmer the LED. The important
variable is the *average* current.