The history of
lighting control
systems begins with variable resistors, progressing thru salt-pot dimmers, mechanical clutch-operated auto-transformer and finally to electronically controlled dimmers, starting with thyratrons (1950’s), then moving to thyristors (1960’s) triacs ,SCR and very recently IGBT. (Insulated Gate Bipolar Transistor) .
The electro-mechanical systems were very large and very cumbersome to operate, often requiring a number of electricians to move rheostats simultaneously.
The advent of electronically controlled dimmers allowed dramatic reduction in required space and heat dissipation.
It became possible to control remotely the power devices and have such function like timed crossfade, flashes, effects etc….
It is interesting to read the manual of the first
electronic dimmer
and see what was emphasized:
“Any incandescent
stage
lighting
load
between rated maximum and zero may be smoothly regulated by its circuit dimmer or may be left in an intermediate position indefinitely without heating or deterioration of parts.
The operation of
electronic dimmer
is based on the principle of controlling the
voltage
(therefore the power) applied to the lamp. This method of control suffers from an inherent problem.
The lower the
voltage
the lower the color temperature, this results in light output turn yellow and move toward red end of the color spectrum. If color filters is used in the front of the lumunaire the end result may be major change in the color and intensity of the light output.
Dimmers are not the only means of controlling light output, devices, such as barndoors, shutters and irises placed in front of luminaries are all used to control the light output. (See mechanical dimmers).
A conventional (analog or digital) dimmer regulates by a technique known as forward phase control (FPS).
Circuitry in the dimmer detects the zero-voltage crossing of the
AC
power time waveform, generates a delay ranging from zero to one half-cycle and triggers the SCRs
Into conductivity. ( This delay is proportional to the intensity of the dimmer output.
Both SCRs are triggered, but only the device which is forward-based for the polarity of the half-cycle will turn on and conduct. The conducting
SCR
will remain on until the next zero crossing when it will commutate (turn off) by becoming reverse biased.
At this point the timing cycle begins again for the opposite polarity half-cycle.
In spite of popularity of this method there are some fundamental problems in using this method.
Device protection
Because the
SCR
can be turned off only at the half-cycle, the device must be capable of withstanding a short-circuit condition until the external fast-response fuse or
circuit breaker
will
cut
off the current.
Zero - Crossing
The zero crossing mechanism can be susceptible to false triggering from noise on the power time. This can cause completely unpredictable behavior auch as flashing, pulsing or half-waving.
Salt-pot dimmers:
The salt-water dimmer employed liquid instead of a resistance wire to control the
voltage
and thus the light intensity.
It consisted of a container filled with water and an electrolyte, such as salt or diluted sulfuric acid. One
electrode
was fixed at the bottom of the container and the other
electrode
was attached to the piston that was operated by
tracker wire
and pulleys.
Variable resistors:
Early dimmer switches to adjust light levels -- a variable resistor is a piece of material that doesn't conduct electrical current well -- it offers a lot of resistance to moving electrical charge. A variable resistor consists of a piece of resistive material, a stationary contact arm and a moving contact arm.
Mechanical clutch-operated
1929 early version of the Mansell magnetic clutch.
Autotransformer Dimmer
An autotransformer is an electrical transformer with two-winding transformers. The winding is equipped with many taps and automatic switchgear to allow them to act as automatic voltage regulators, to maintain a steady
voltage
at the customers' service during a wide range of
load
conditions. This auto-transformer developed about 1933 by General Radio Company -USA was named "Variac" which become a generic name
Thyristors or SCR: (1960's)
The
thyristor
is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as a switch, conducting when their gate receives a current pulse, and continue to conduct for as long as they are forward biased. Some sources define silicon controlled rectifiers and thyristors as synonymous[1]; others define SCRs as a subset of
Triacs
A TRIAC, or TRIode for Alternating Current is an electronic component approximately equivalent to two silicon-controlled rectifiers (SCRs/thyristors) joined in inverse parallel (paralleled but with the polarity reversed) and with their gates connected together.thyristors.
