On 11/29/2014 4:30 AM, Rob wrote:
Jerry Stuckle wrote:
For incandescent, yes. But not for LED bulbs. If that were the case,
why would some LED bulbs be dimmable and others not?

"is it dimmable" refers to the use of a phase-cut TRIAC dimmer in combination
with a retrofit LED bulb designed to run on mains power.


No, some bulbs are not dimmable, period. Some are. None work reliably
with triac dimmer controls.

This combination does not work for all LED bulbs, because the electronics
in the bulb are trying to put a constant current through the LED independent
on the mains voltage, and now you are cutting down the mains power.
(not even the voltage, really)


True - but non-dimmable bulbs do not work with other dimmers for the
same reason.

You probably have a 13.8V stabilized DC supply in your shack.
Can you use it as a variable-voltage supply by putting a dimmer in
front of it?


Completely unrelated. The LED electronics and the power supply work
completely differently.

No, not really. When you turn down the dimmer, the supply will first
try all it can do to keep the voltage at 13.8, and at some point it can
not achieve that anymore and the voltage will drop, but it will not
be stabilized anymore.

Similarly, a LED bulb may keep constant light emission for a large part
of the dimmer setpoint range, and at some point it goes down in intensity
in an erratic way.


Pretty much, yes. But that is true for any non-dimmable bulb with any
dimmer.


We are just in the first phase of LED lighting deployment, the
"compatability" phase where the bulbs are still using existing form
factors of incandescent bulbs, and are supplied with mains voltage
that is converted to current inside the bulb. It is not the best
solution to use such bulbs in combination with existing dimmers.

Of course a next phase will be to use more reasonable form factors and
connection, where the electronics have a light level setpoint that
determines the LED current and the mains voltage is directly applied
without intermediate phase-cut dimmer.


There are already dimmers made for LEDs which work well. And there are
commercial systems which use low voltage to drive the LEDs. But these
are all proprietary; there are no standards for LED lighting yet.
Eventually, maybe standards will be developed for the commercial market,
but right now there is zero push for such standards.

It's going to be years (if ever) before such standards are created for
the residential market. There are too many reasons to stick with 117V
wiring.

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