On Tue, 5 Nov 2002, Rodney Woten wrote:

> Anyone on
> the list know of a simple way to control the speed on a 12 VDC motor????

There are any number of ways you could do it.  If all you're after is a
two-speed solution, a string of rectifier diodes or a power resistor in
series, with a switch to bypass for full speed, would do nicely.  With
diodes you get roughly a 0.6V drop per diode.  (This may not give you
enough of a speed drop with a reasonable number of them.)  With a power
resistor you can get whatever drop you want -- but make sure you calculate
the wattage that will be dissipated.  It can be surprisingly high.  That
resistor will get hot, so be careful where you mount it.

Your rheostat idea would work essentially the same way, with the same
power dissipation limitations.  High-wattage rheostats can be hard to come
by.

If your motor draws less than 1 amp, you could design a circuit to vary
its speed using the LM317 variable voltage regulator.  All the excess
power will be dissipated in the regulator, so you can use a low-current
rheostat instead.  Remember to use a heat sink on the regulator if the
current is more than 0.5 amp.

Or you could just get some ball-bearing computer fans and fab your own
brackets.  Some of those are *very* quiet, and can be had fairly cheaply
in a variety of sizes from surplus catalogs.  Look for ones with a fairly
low RPM rating, they tend to be the quietest.  You can also cannibalize
them from old computer power supplies, since the fan usually runs off the
12V output.

             _ _
  __ _ _   _| | |  | David M. Brodbeck (N8SRE)              Ypsilanti, MI
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