DIYbanter - View Single Post - Why aren't many / most LED light bulbs dimmable?

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On Fri, 24 Dec 2010 18:24:52 -0500, wrote:

On Fri, 24 Dec 2010 15:32:11 -0600, "
wrote:

On Fri, 24 Dec 2010 15:02:02 -0500, wrote:

On Fri, 24 Dec 2010 06:25:05 -0600, G. Morgan
wrote:

Home Guy wrote:

I'm seeing more LED lightbulbs turning up on store shelves.

I don't think I've seen one yet that is ok to use with a dimmer switch.

I can understand why CFL's can't be put on a dimmer - but why not LED
bulbs?

This is new, last 3 years. They had to develop a ballast that was
adjustable. CFL's and LEDS are not a good comparison.

They're crazy-priced as is. Not being able to dim them makes them even
less desirable as a replacement for incandescent bulbs.

L.E.D.S. Are going to difficult (impossible) to dim. Remember
they are DIODES that only need .7V to illuminate. AFAIK ---
LED's are not dim-able.
Try 3. something volts to light a white LED. The only way to "dim"
LEDs is to PWM them with variable pulse width/duty cycle. The dimming
range is quite narrow.

Not true at all. Using PWM, or a variable current, you can get a very
substantial dimming range (with less change in color than an incandescent).
It's just a PITA and a phase-control (Triac) wall dimmer ain't going to do it.

The same can be done for "overdriving" an LED. Shourt duration pulses
can significantly increase the visible light output without
overheating the junction.

No, it doesn't increase the light output at all. You may be able to see it
with less output because a flashing light catches the eye, but as long as it's
a "constant" light output (i.e. not visibly blinking) the light output of an
LED is pretty much a linear function of the *average* current through it.
Flashing of an LEDm above the eye's critical fusion frequency does not
increase efficiency, rather the opposite. The efficiency of an LED goes down,
at high currents, as it heats.
MANY high output led applications are pulsed "overdrive"
applications, and believe me, they DO put out a LOT more light.

No, they don't, for any reasonable reading of that sentence. The physics
doesn't allow it (the opposite, in fact).

Driving
them steady at those currents would blow them in a matter of minutes,
but pulsed at 15-20% duty cycle at up to 4 or 5 times rated current
they still deliver almost rated lifespan, and, if I remember
correctly,over 5 times the rated light output.

The *average* current is all that matters. The average also does the heating,
so it's a no win to pulse them, other than it's the easier way to dim them.