Philips Electromagnetic Lamp manual 141, Coil in series and by a thyristor

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 (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.

141