The
SCR
has been used as the standard power unit in theatre dimmers for most of the second half of the 20th century. The
SCR
- silicon controlled rectifier - is a unidirectional
transistor
diode which transmits current in one direction only, i.e. only during a half-cycle of the
AC
waveform. Therefore, two
SCR
units, each of which is operative during half a cycle, are necessary in order to transmit a full
AC
cycle. The two SCRs are connected to each other in an inverse parallel manner, "back to back". The
SCR
operates as a very fast electronic
switch
or gate, switching on the electric current every half cycle.
Each
SCR
switches the electric circuit 50 (or 60 in the USA) times a second, adding up to 100 current spurts per second. An ignition circuit sends a signal to the
SCR
to open the gate, enabling the current to pass through. From that point till the end of the half-wave cycle controlled by that SCR, the
SCR
will remain open and the current will pass through. At the crossover point, when the
voltage
returns to zero and the control to the gate is zero, the
SCR
closes automatically and remains closed until receiving its next control signal. From the crossover point the second
SCR
controls the current. It, likewise, will receive a signal from the ignition circuit to open the gate and enable the current to pass through .
This type of control, where the
control signal
determines the point on the time axis when the
SCR
will
switch
on, is known as forward phase control.
If the command to open the
SCR
is sent at the beginning of the cycle, the
SCR
will open and allow the current to pass during the entire cycle, causing the full
voltage
to reach the lamp, which will burn at full intensity. If the command to open is delayed slightly, and given only after the cycle has begun, the
SCR
will transmit only part of the
voltage
to the lamp. If the command to open is given close to the end of the cycle (i.e. the crossover point), the
SCR
will transmit only a small portion of the
voltage
to the lamp, causing it to burn only at a very small part of its potential intensity.
Although
SCR
based dimmers
switch
the current to the
lamp
on and off at a rate of a hundred times or more per second, the human eye cannot detect the switching, as the
lamp
filament does not cool down in the few microseconds between switching, causing the light to appear stable. In terms of power consumption 99.5% efficiency.
The electric power carried by the
SCR
reaches as much as 99.5% of the power supplied. The power losses are converted into heat and electromagnetic radiation, both of which present problems in dimmer design. An alternative to the
SCR
is the triac, which deals with both halves of the cycle and is used for certain low
wattage
dimmers, controlling domestic lighting.
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| The
SCR
is a unidirectional
transistor
diode which transmits current in one direction only |
| |
 |
| Two
SCR
units are necessary in order to transmit a full
AC
cycle. |
| |
 |
| Two
SCR
units are connected in inverse parallel. |
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 |
| After half the cycle the first SCR is switched off and the other one takes over. |
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