Philips Electromagnetic Lamp Indoor Furnishings User Manual


 
- 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 (so-called
‘top dimming’). For indoor installations, however, top dimming is of
limited practical use and at ambient temperatures below 5 ºC krypton-
filled lamps, like the Philips ‘TL’D may become unstable when dimmed.
The disadvantage of thyristor dimming where lamp circuits
incorporating glow-discharge starters are concerned,is that the dimmed
lamp will cause the starter to become conductive.At what degree of
dimming this will happen is difficult to predict, but the result is that
the starter will make repeated attempts to ignite the lamp.This is the
main reason why dimming of fluorescent lamps in a glow-switch
starter circuit is discouraged.
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.
5
141
Fig. 135. Dimming with an extra inductive
coil in series and by a thyristor.
3.16 Dimming
L
N
L
N
BB
extra 
coil
CC