A large drawback to this kind of dimmer
is that the portion of the current that would
otherwise produce light is instead converted
to heat. Also, no savings in energy is real-
ized: although light output is reduced, con-
nected wattage remains unchanged. In
addition, these dimmers are bulky; conse-
quently, they are no longer used.
Autotransformer Dimmers
Autotransformer dimmersavoid these prob-
lems by using an improved method of dim-
ming. Instead of converting the unused
portion of the current into heat, the
autotransformerchangesthe standard-volt-
age current into low-voltage current, with
only a 5 percent power loss.
A transformer has two coils of wire; the
ratio of the number of turns in one coil to the
other produces the ratio of the voltage
change induced by the transformer. An
autotransformer is simply a variable trans-
former: the primary coil remains fixed, while
the number of turns in the secondary coil is
varied by a rotating arm that controls suc-
cessive turns of the coil. Because electrical
power can be drawn from different points
along the secondary coil, different voltages
are achieved from the same transformer.
Because autotransformers do not con-
vert energy to heat as light intensity is
reduced, they are therefore cooler and
more compact than resistance dimmers.
Autotransformer dimmers are widely avail-
able in sizes up to many thousands of
watts.Solid-State Dimmers
Solid-state dimmersare predominant today;
they use the second of the two methods of
limiting current flow. A power control device—
such as a silicon-controlled switch (SCS)
under 6 kW, or a silicon-controlled rectifier
(SCR) over 6 kW—allows electric current to
flow at full voltage, but only for a portion of
the time. This causes the lamp to dim just as
if less voltage were being delivered (figure
11.7).INTERIOR LIGHTING FOR DESIGNERS
Figure 11.7Solid state dimming control.