![Fig. 135. Dimming with an extra inductive](/images/new-backgrounds/30680/3068069x1.webp)
5
3.16 Dimming
Fig. 135. Dimming with an extra inductive
extra
coil B
L
C
N
B
L |
C |
N
coil in series and by a thyristor.
-with the thyristor circuit the moment of current suppressing must be different for the inductive and the capacitive branch due to the phase shift, which is impossible to realise in one and the same device.
During dimming only the lamp current will decrease and the capacitor current will remain the same.The result is that the power factor will become capacitive and will shift to lower values.
Inductive coils, in the form of an extra ballast, are used to reduce the light output of street lanterns after a certain hour.This is done either by switching the extra ballast in series with the principal one, or by using two ballasts of half the nominal power rating in parallel, switching one off when dimming is required.
Thyristor dimmers are by far the most popular nowadays, because they are small and inexpensive.
Dimming to give half the light output is nearly always possible. By using thyristor dimmers practically any type of fluorescent lamp can be dimmed down to about 50 per cent of the nominal lamp current, which roughly corresponds to a 50 per cent reduction in light output
When dimming to below 50 per cent of the nominal current, the discharge will no longer provide sufficient heat to keep the electrodes at the proper emission temperature and continuous electrode heating becomes necessary.The heating current must be independent of the lamp current, thus a separate heating transformer will be required. Lamps operated in this mode can be dimmed to give almost zero light output (but not entirely, unless a switch is provided).They can also be started from a dimmed position.These dimming installations almost invariably operate at high frequency to prevent disturbing flicker at low lighting levels.
Frequency regulation is the most recent technology, and is employed in the Philips HF electronic light regulation ballast.With this ballast the lamp current can be regulated down to about ten per cent of the nominal value. Dimming is here achieved by increasing the frequency of the supply current.
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