From: Richard (metal@fullwave.com)
Date: Sat Apr 26 2003 - 17:44:28 PDT
Light bulbs are nonlinear resistors. I.e., their
resistance -varies- as the voltage drop across
them varies. A typical light bulb has a 'cold'
resistance only one-TENTH its 'hot' resistance.
In other words, you will likely find that a mostly
cold 120v 100 watt bulb will be only a fraction
of the resistance of a hot 12v bulb; and it will
not drop 12v across itself.
If you want to use bulbs as dropping resistors,
the very best bulb to use would be one identical
to the actual tail-light bulb. Then you will drop
exactly half the volts across the 'dummy' bulb.
Also, if you mount them in the cab, behind some
very dark glass/plastic, you'd have a bulb-failure
indicator. If either of the bulbs in a SERIES
circuit fails, BOTH will go dark.
Regarding the brakes, it really depends on how
many amps the brakes draw at 12v. If it's only
a few amps, then that 24-12 converter might be
a nice idea. If it's 10, or 20 amps, then that's
probably a pretty expensive converter.
In regards to power-resistors, note the following:
1) they get QUITE hot in normal usage. Be sure
to mount them accordingly; i.e. space around them,
no wires or objects swinging into them, provision
for good airflow to cool them.
2) their wattage rating is 'optimistic'. If you're
smart, you'll plan on only about half the rating.
I.e., if your calcs say you'll be dissipating 50w
of power, use a 100w resistor. A 50w unit in
that situation would run hot enough to melt solder.
3) old electric-dryer elements work well as
low-ohm high-power resistors. Same for old
electric space-heater, toaster-oven, etc. elements.
The alloys are extremely difficult to solder, but
they spot-weld nicely, or can be clamped using
screws/nuts. Pop-rivets are pretty iffy, due to
the high-temps and repetitive wide temp swings.
hope this helps a bit,
Richard
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