12 May
2006
12 May
'06
5:05 p.m.
I need some help from those who are a lot better than I in hardware design...
The problem is I need to use a PC power supply (AT-class with it's own
power switch) as a 12V power source, and the only one I have access to
needs a minimum load of 3Amp on the +5Volt line.
By my rough calculations and allowing for wiggle room, I'd need a 20-watt
power resistor of 1.5 ohms to keep the critter happy. "Bare minimum" is
15W, and I could squeak by with that if I wire the fan to the 5V line
instead of the 12V... but even when I'm expected to perform a "miracle" I
like to have a little wiggle-room. ;-)
Problems I have are: 1) I need this by Monday, and 2) I don't keep 20-watt
power resistors in my back pocket. Had I a week, I'd just order the right
tool for the job - I don't have that luxury. ;-)
First thought : A car bulb. OK, they're designed for 12V, so will draw
less than the rated current at 5V (but maybe not that much less, bulbs do
approximate to constant current devices! [1]). You might get away, with
say a 21W car indicator bulb, or the 2 filaments of a 5W/21W tail and
brake light bulb in parallel.
[1] Am I the only person to remember what a Barretter is (even if I can't
remember the spelleing) ?
I was thinking I *might* be able to cobble up enough lower-end power
resistors & figure out how to stuff 'em together to draw 15w+ of power, but
then I thought:
What if I took an older 5V CPU (say... an 80486) and wired 'er up to just
Vcc & Ground - how much power would that use? [[ I've started some google
Less than you might think. Most of the power consumed by this sort of
chip goes into charging/discharging stray capacitance when signals switch
states. That's why the chips get hotter as the clock frequency rises.
WIth no clock, it'll not comsume that much power.
I could also wire-wrap all the address/data lines to
Vcc if that would add
a enough extra current draw to make 'er worth my while...
That's likely to burn the chip out if you're not careful!
-tony