Bruce,
I am pretty sure I can figure out how to test out the draw on my heater motor...my next question is: once I know the draw (in amps), how do I translate that into what resistor should I buy? Thanks -Justin
Here are my views of the problem. If you measure the current of the 6V
motor, simple ohms law V = I X R will allow calculating an equivalent
resistor on the first try. Multiplying the V X I will give the watts of heat,
but many of us recommend running a considerable margin for trouble
free service. Note, those metal resistors with the tabs are designed to
be thermally connected to a decent heat sink to approach their rating.
Hot air in the engine compartment is reason to derate. A ceramic type
will take a lot of heat, but may operate so hot it will melt wires that
come in contact with its body.
With a resistor in place the blower on high may run fine at half battery
voltage. But when you switch in resistors for low blower, current drops
and the voltage across the resistor you added DECREASES. That
means the blower circuit will see INCREASED voltage above normal
limits. The motor speed will be higher than before, and the speed
limiting resistor (often placed in the blower air stream) will see more
current/voltage. The POWER to this resistor goes up as the square
of voltage, so the power increase can be substantial, and these resistors
may burn out sooner from the elevated temp.
In addition the above takes the heaviest circuit in these old cars, and
doubles the amount of load on the alternator.
A LINEAR voltage regulator can maintain the correct output voltage
at all times, but is just as hot and inefficient as the resistor.
You can cancel all the above problems by using a SWITCHING
style (buck) DC converter, which will produce exactly the voltage
you set with very high efficiency. These have become very cheap
barefoot lately, all these run Pos in Pos out. But they have a couple
new problems. One is the radio interference that may get into your
AM radio. By containing them in a hardware cloth box (very large
screen with cross points soldered) and use of at least a radio
suppression cap at input and output, noise can be contained.
The other issue is the sensitivity of electronic supplies to high voltage
spikes. Anything with contacts (horn, starter, blower) can generate
spikes up to dozens of times the basic voltage. These spikes are in
the microsecond length, too narrow to bother things like light bulbs.
But they can blow through transistors, as I found out in 1969. The
fix is a simple protection network at a converter port, maybe a 500
microfarad 100V cap and a 20V transorb. Radios and such already
contain the needed filters.
One more thing about buck switching converters. If the power element
shorts out, the input voltage may appear at the output, with possible
serious load damage. There are CROWBAR circuits sometimes used
to assure the load won't be damaged.
Gauges can have similar voltage regulation problems with resistors,
cured with a regulated supply. Gauges sensitive to polarity are a
bigger problem, polarity changing supplies are available at higher cost.
Bruce (building buck converters since 1967) Roe
