This circuit takes
advantage of the voltage drop across bridge rectifier diodes to produce a
5-position variable voltage supply to a DC fan or other small DC motor.
It is not as efficient as a switch-mode circuit but it has the virtues
of simplicity and no switching hash. The four full-wave bridges are
connected so that each has two pairs of series diodes in parallel,
giving a voltage drop of about 1.4V, depending on the load current.
The rotary switch should have “make before break” contacts which
should be rated to take currents up to about an amp or so. For higher
currents, higher rated bridge rectifiers and a suitably rugged rotary
switch (or solenoids) will be required. If you want smaller voltage
steps, you could use the commoned AC inputs on the bridge rectifiers to
give intermediate steps on the speed switch.
Author: Stephen Butcher