"Greg Mayman" bravely wrote to "Mike Ross" (16 Sep 03  08:39:00)
 --- on the heady topic of "VEHICLE LED'S"

 -=> Mike Ross said to Jasen Betts
 -=> about "VEHICLE LED'S" on 09-13-03  23:22.....

 MR> A lamp's filament tends to behave as a constant power load. Thus as

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

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


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

Constant current?... Maybe you're under a misconception. Let's check
this and see. Here's a table which plots a 100watt lamp's behaviour:
>
 The following table shows life expectancy, candlepower, and current
 for various supply voltages. I have computed input power and
 efficiency for each of the data sets. In doing so I have assumed that
 Efficiency = Candlepower / Power Input.

 % rated V  Life      %Candlepower %Current  % Power I/P % Efficiency
 --------------------------------------------------------------------
   60       500x        10           75        45           22
   70        65x        25           81        57           44
   80        15x        40           88        70           57
   90         3x        60           94        85           71
  100         1x       100          100       100          100
  110         0.3x     140          106       117          120
  120         0.1x     190          113       136          140
  130         0.03x    250          119       155          161
>

Say we select points %V 100 and %V 130, the corresponding %current is
%100 and %119, and the power is %100 and %155. Since power is the square
of voltage we should have had a power change from %100 to %169 for a
linear resistor. The lamp power has a difference from the trend of
(169-155)/169x100 or 8.28%. The current change trend was
(130-119)/130x100 or 8.46%, so the current actually changes a tiny
bit more than the power does. Constant current or constant power? Hard
to say clearly which it is. 

 Mike
 ****

... Over a hundred billion electrons were used in crafting this tagline.
--- Blue Wave/DOS v2.30