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

On 12/24/2010 7:56 AM, Jim Yanik wrote:
Jeff wrote in
:

On 12/24/2010 1:54 AM, 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 haven't really been following, but I was in the borg last night and
they now have quite a few. I noticed, right on the box, of at least one
"Dimmable". There is little reason why not, unless flicker related.


YMMV


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

They're crazy-priced as is.

Getting better. I was looking at bulbs priced in the teens. I seem to
recall prices 3 times higher not long ago. But I'm no expert.

Wound up buying a CFL that looked just light a regular bulb (for $4),
with glass all the way to the screw. Got one floor lamp that a regular
CFL won't screw in.

Jeff


Not being able to dim them makes them even
less desirable as a replacement for incandescent bulbs.



LEDs are CONSTANT CURRENT-driven,not voltage driven.
they have to have some sort of current regulator inside that keeps the
current constant regardless of input voltate swings.
To dim a LED,you would have to vary the input current,and the typical triac
lamp dimmers don't do that,they vary input voltage.

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.

Yes, you are correct, except that the LEDs have to have a "threshold"[*]
voltage to light at all. If the dimmer is triggered when the AC (sine wave)
is too low the LEDs don't light. The range of adjustment will be very small. [*]as others have pointed out, this isn't an on/off thing but is highly
nonlinear


I think the key is how the current source is constructed. Limiting
the power lost there is important.

Yes, this is a very expensive thing to do, compared to a *cheap* Triac dimmer.

Can you shed any light on just what is in an LED lamp? I'm curious
now. I figured a wee chopper with a bit of output filtering. Add a
resistor or a constant current configured transistor. Just speculating.

You'd have to make a DC source, which won't be cheap because of the sine-wave
input. The source will have to store energy for the cycle.

Plus,there's a fairly narrow range of current that produces light from the
LED chip,it's not linear.Most LED dimming schemes pulse modulate the
LED,

Is that a household type dimmer like on the wall?

No, they adjust the (phase) angle of a triac firing. This controls the power
put into a resistive load just fine but doesn't work for a highly non-linear
load like a (long) string of diodes.

changing the duty cycle of the current,and the eye averages the output.
you get better efficiency that way,but it's more complex electronically.

For the money they are, they can throw a little circuitry at it!
Chopping up DC is no big deal these days. With the low current drawn and
high voltage, I wouldn't think it would need a very large cap.

Right. But you don't have DC to chop. ;-) Making the DC isn't all that
cheap (or efficient).

I have no practical knowledge of this, so I'll defer to those with
more experience.