26 Apr
2011
26 Apr
'11
12:17 p.m.
It would seem to me that if you needed a dummy load for a power
supply (and couldn't scrape up a bunch of power resistors), it would
make the most sense to use the highest voltage incandescent lamps
that you have at your disposal.
The reason is quite simple--tungsten-filament lamps (not that I
expect anyone to have any carbon-filament ones) have a very steep
resistance-temperature curve (higher "hot" resistance).
Tugsten filament lamps approximate a constant current load over quite a
range of voltages (am I the only person to rememebr the baretter?).
So your 110V 100Q bulb will draw a little under 1A at its rated voltage.
At a lower voltage it will still draw only 1A.
Let's work it out a little more accurately. At 110V, a 100W bulb draws
0.91A. If your figure for a hot-cold resistance swing of 15 times is right
(and it sounds it, we can estimate that the hot resistance is about 121
ohms. The cold resistance is 1/15 of that, or 8.07 ohms. Connect that
across a 5V PSU and it will draw 0.62A. Not really enough for a dummy load.
A 6V 6W bulb (which is easy to get) would draw 1A and be most suitable.
Or better yet, use a nichrome heating element from an
old hair dryer
or aquarium heater. Nicrhome V wire has only a 6% increase in
resistance between 68F and 1600F.
If you are really crazy, use containers of salt water with suitable
electrodes in them :-). Just don't knock them over and spill the liquid
into the machine under test.
-tony