Although
SCR
dimmers are still the most widely used device for
voltage
switching, they still have a few inherent problems. The
SCR
generates noise, is susceptible to short circuits and its control is limited, since the amount of switching within each half cycle is one.
Two new types of dimmers, based on semiconductors, have been developed to
switch
and control electric current. These are MOSFETS (Metal Oxide Semiconductor Field Effect Transistors) and IGBTs (Insulated Gate Bipolar Transistors), both of which implement different approaches to switching and control which seem to offer solutions to some of the problems of
SCR
dimmers. The IGBT receives its switching command along the wave movement reverse phase, as opposed to the
SCR
which receives its command along the time axis - forward phase control.
Both the IGBT and the
MOSFET
are linear components, and the opening and closing of their gate can be controlled, unlike SCRs which remain closed until they receive a command to open, and then remain open until the end of their half-cycle. These new components can open the gate at the zero point, at the beginning of the wave movement, and a special component calculates the
voltage
supplied to the lamp. Once the precise
load
has been reached, the command to close is received. In this mode of operation, i.e. switching off in order to determine the intensity, the switching time is called fall time, while the switching time is called rise time. In these dimmers, the fall time can be extended slightly so that it is not too abrupt, taking 400, 500, or even 800 microseconds, so that the system creates far less electromagnetic noise than SCRs and a
choke
is not necessary. Lamp singing, too, is eliminated, and there are no disturbances to the sound system. And without a choke, dimmers are more compact in size and much lighter in weight. The combination of reduced noise and reduced weight eliminates the need for a separate dimmer room, and these dimmers can be installed on the
stage
adjacent to the lamps themselves.
The ignition circuit for the IGBT and the
MOSFET
is more complex than that of the SCR. In an
SCR
dimmer the ignition circuit needs to calculate only the opening time of the SCR, whereas the IGBT and
MOSFET
need a continuous stream command whenever the electric power is activated by the load. The ability to control the opening as well as closing, employing either forward phase control or reverse phase control, is of major significance.
IPS dimmer strips by Rosco, Entertainment Technology.
With fixed forward phase control, the
SCR
dimmer cannot efficiently deal with fluctuations in alternating current, and any such fluctuation may influence the
voltage
across the lamp. A dimmer with an IGBT unit receives a signal determining the
voltage
required by the lamp, and calculates by itself the point along the waveform where the IGBT needs to be switched.
