In rec.radio.amateur.boatanchors 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.

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)

Yes, the RMS voltage, really.

If you remove part of a cycle from a sine wave, the RMS voltage goes
down. The RMS voltage depends on the waveform.

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?

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.


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.

People like dimmers and the huge installed base of legacy dimmers
ensures they will be around for decades.

The various standards groups, such as NEMA, are actively working on
standards for the new lighting technologies and as the issue is global
and involves billions of dollars of product, will likely eventually
result in a set of standards for bulbs for legacy dimmers and standards
for new technologies.


--
Jim Pennino