Theatre Design & Technology - Oct 1972 - 27

to control output voltage. Its use led to development of
large Iighting systems with efficient low-level dc control
systems having simple slider potentiometers for complete
control of as many dimmers as needed (Fig. 1). The adjustable dc control signals from the potentiometers are
converted by the dimmer's control circu itry into gating

simple systems, such as those usually found in elementary
schools. In moderate to large systems, however, they are
cumbersome and unwieldy in physical size and in control
methods. Average dimmer rating is 6 kW apiece, and an
autotransformer of that size is a 90-pound mass of iron
and copper with a large di rectly coupled handle. An average installation may have 30 dimmers. If more than a few
need to be changed at one time, motor drives must be
used. Several dimmers can be operated at once from the
same master control, but, to have different initial and
final light levels for many dimmers, considerable electromechanical control is needed.

signals for the thyristors. A maximum potentiometer setting provides the full 24-volt dc control voltage to the
dimmer's control terminals. producing maximum dimmer
output voltage an'd thus maximum light output from the
lamps.
Each. group of dimmers, called a "pre-set," is given a
totally independent set of desired initial and final light
levels, and the dimmers change proportionately between
those levels depending on the position of the master fader
control potentiometer. This proportionate changing is
called "fading"; the whole sequence is called a "fade."
(See Use of Dimmer Control at lower left.) Submaster
potentiometers can be used to control smaller groups of
dimmers by appropriate auxiliary switching.
A new advantage that came with thyristor dimmers was
the ability to adjust dimmers for an apparent linear
change in light output with a linear control change. It was
too cumbersome to mechanically provide that feature
with autotransformer dimmers because the relationship
between apparent light and dc control level is an exponential function. With thyristor dimmers, almost any control
curve can be developed electronically at low cost and in
very little space without requiring moving parts or me·
chanical linkage. The relationship between control setting,
actual light, apparent light, and lamp voltage is shown in

A new type of dimmer came into being during the
1960s with the development of power thyristors. It is
compact and easily controlled by gating a thyristor bridge

Use of Dimmer Control
I n the simple example illustrated in Fig. 1. both preset A and
preset 8 light vah'es are set initially on the individual dimmer
control potentiometers. At the beginning of Scene 1 on stage. the
master fader handle is in preset A position and the grand master
handle is at minimum. Bringing the grand master handle to maximum illuminates Scene 1 with all the preset A levels. To start into
Scene 2. the master fader handle is gradually moved to the preset
8 position. and all the light levels gradually change to the preset 8
values. Preset A is then deenergized. and the light levels for Scene
3 are set into its potentiometers. A t the next movement of the
master fader handle. the new values in preset A light up the stage.
Preset 8 is then deenergized. the light levels for Scene 4 are set up.
and so on until the first act is over and the grand master handle is
again moved down to minimum. blacking out all the lights on
stage. There are further levels of sophistication. but this example
shows how simple the control can be.
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Preset B

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1-Control system for thyristor dimmers consists of interconnected potentiometers that regulate the level of the dc control voltage going
to the dimmers. Potentiometers for individual dimmers are arranged in two or more groups that are called "presets" because. while one
group is controlling light levels. the desired light levels for the next scene can be preset in the other group. With the master fader in
position A all diodes in preset 8 are reverse biased. so preset 8 has no effect on the dimmers; with the master fader in position 8 all
diodes in preset A are reverse biased. so preset A has no effect on the dimmers.

~ THEATRE DESIGN AND TECHNOLOGY

OCTOBER, 1972

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