Re: Long Live the Incandescent!

[Marc_F_Hult <MFHult@xxxxxxxxxxxxxxxxxxxxx>]

On Thu, 9 Jul 2009 07:33:28 -0500, "B Fuhrmann"
<b-fuhrmann-usenet@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
<e-udnXWPEa0a98bXnZ2dnUVZ_v-dnZ2d@xxxxxxxxxxxxxxxxx>:


> The [LED] lights are not dimmable (assuming that they were bright enough
>that dimming would be useful) even though they say that they are.  When used
>on a dimmer, they stay at full brightness until the dimmer is almost full
>off and  they go completely dark.
>I like to have enough light available in a room for all tasks but keep the
>lights dimmed for more typical use.  This may be able to be fixed in future
>designs by having groups of the LEDs go dark at different dimmer levels.

To (hopefully) clarify, LEDs are in fact intrinsically "dimmable". Their
light output is conventionally dimmed by holding voltage more-or-less
constant and reducing current.

But incandescent lamps are typically dimmed by reducing average voltage, not
current. So in practice, standard, conventional TRIAC-based dimmers are not
well suited for dimming LEDs because they dim by reducing average voltage and
current is held more-or-less constant (varies with filament temperature)
during conduction.

Another issue is the 'dimmer curve' of particular wall-mounted dimmer. One
description of a dimmer curve is the relationship between the rotation (in
degrees) of a physical dimmer knob to the phase angle (in degrees)of the
TRIAC conduction (output).

A typical household dimmer curve that is appropriate for incandescent lamps
may not be appropriate for an LED-based lamp. This issue is well addressed by
emerging electronic solutions as well as by some existing conventional
'professional'  dimmer consoles and panels such as those used in
entertainment venues using DMX (DMX512)

There is at least one IC  on the market that addresses both the
current-control and dimmer-curve issues by interpreting the complex "chopped"
waveform created by a TRIAC-based dimmer into the information needed by an
LED for continuous dimming with a useful dimmer curve and then providing the
appropriate current to the LEDs.

National's LM3445 detects the 0-180 degree phase angle of a conventional
TRIAC dimmer output and translates that information into a current output
with a 1:100 dimming range (about 6-1/2 f-stops of light).

http://www.national.com/analog/led/triac_dimming

Incorporation of this sort of smarter electronics into LED lamps will reduce
or eliminate the problem that Bill describes. I have a pair of dimmable LEDs
intended for ceiling cans in the kitchen which I can place on the same
circuit as halogen incandescents and CFLs in which they perform admirably.
Like CFLs, and unlike incandescents, they are not 'infinitely' dimmable, i.e,
they have an abrupt shut-off at about four to five f-stops below full
brightness.

HTH ... Marc
Marc_F_Hult
www.ECOntrol.org