-=> Mike Ross said to Greg Mayman
 -=> about "VEHICLE LED'S" on 09-17-03  10:00.....

 GM> No that isn't right, otherwise dimmers wouldn't work as well as
 GM> they do. Constant power gives constant brightness.

 MR> True but it's because dimmers act to modify the waveform shape
 MR> (power angle) and not the actual voltage amplitude.

Constant power _ALWAYS_ gives constant brightness, regardless of
the waveshape.

 GM> More accurate to say the usual incandescent lamp acts rather as a
 GM> constant CURRENT load, due to its PTC characteristics.

 MR> Constant current?... Maybe you're under a misconception. Let's check
 MR> this and see. Here's a table which plots a 100watt lamp's behaviour:


 MR> Since power is the square of voltage we should have had a

Nope, power is ALWAYS voltage times current.

It is only the square of voltage when the load resistance is
fixed at the one value. Not applicable in this case.

 MR> Constant current or constant power? Hard
 MR> to say clearly which it is. 

It's not hard at all to say which one is closer to a constant
value.

Taking the extremes of your table:

 MR> % rated V  Life      %Candlepower %Current  % Power I/P % Efficiency
 MR> --------------------------------------------------------------------
 MR>    60       500x        10           75        45           22
 MR>   130         0.03x    250          119       155          161

Note that for the same voltage increase from 60% to 130%, the
power increases by a factor of 3.44, and the current by a factor
of 1.58.

Which of these is closer to constant, that is closer to the same
value at the higher voltage as at the lower? The CURRENT!

Plot all your figures on a graph if you still don't believe me,
then you'll see which is closer to a flat, constant value curve.

From Greg Mayman, in beautiful Adelaide, South Australia
   "Queen City of The South"    34:55 S  138:36 E

... America's best buy for a nickel is a telephone call to the right man.
___ Blue Wave/386 v2.30

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