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16
3.16 Dimming
Dimming
Dimming can be defined as the reduction of the luminous flux of a lamp, either continuously or in steps, by reducing the operating current.This is not always possible without adversely affecting the performance of the lamp.
Basically, dimming is achieved in one of the following ways (see Fig. 134):
-by switching a (variable) resistor or inductive coil in series with the lamp(s),
-by running the lamp(s) from a variable transformer,
-by suppressing the AC waveform of the supply current during part of the cycle by means of an electronic element (thyristor); such a device is also called a ‘chopper circuit’,
-by increasing the frequency of the supply current of an inductive coil, thereby increasing the impedance of this.
L 
N 
a)
L 
N
b)
Fig. 134. Four basic ways of dimming. a) by a variable resistor, b) by a variable transformer, c) by a thyristor circuit, d) by frequency regulation.
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c)
d) |
Resistors are now rarely used for dimming purposes, because they are inefficient and produce a lot of heat.
Variable transformers are appreciated because of their high power handling capacity, but at the same time they are heavy and expensive.
In the case of fluorescent lamps operated on electromagnetic gear dimming is mostly achieved by the extra inductive coil in series or by the thyristor circuit (Fig. 135). In both cases only inductive circuits are allowed and the parallel compensating capacitor must be placed before the dimming device. Capacitive or
-with the extra series impedance the total impedance for stabilising would become lower instead of higher,
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