Control Gear, Main ballast functions | Philips Electromagnetic Lamp

ELECTROMAGNETIC LAMP

		CONTROL GEAR
BALLASTS	1
5	1
1	1	Main ballast functions

In chapter 2.1 of this Guide: General aspects, section 2.1: Main ballast functions, the main functions of ballasts have been described.The term ‘ballasts’ is generally reserved for current limiting devices, including resistors, choke coils and (autoleak) transformers. Other pieces of auxiliary equipment are compensating capacitors, filter coils and starters or ignitors. Some systems use an additional series capacitor for stabilisation.

With the components summed up, all control functions which are necessary to operate standard fluorescent lamps can be carried out. Special arrangements, including sequence start, constant wattage or dimming circuits will not be described in this Guide, as such circuits are more and more being replaced by the modern high-frequency (HF)systems.

1 2 Stabilisation

In section 3.2: Stabilisation, the need for current stabilisation in fluorescent lamps has been described, resulting in the following two formulae:

	Ilamp = (Vmains - Vlamp) / Zballast
and:	Plamp = Vlamp . Ilamp . α lamp
where	Ilamp	= the current through the lamp
	Vmains	= the mains voltage
	Vlamp	= the voltage across the lamp
	Zballast	= the impedance of the ballast
	Plamp	= the power of the lamp
	α lamp	= a constant called lamp factor

From these formulae it can be concluded that the power of the lamp (and therefore the light output) is influenced by:

-the lamp voltage Vlamp , which in turn is highly dependent on

the operating temperature (see section 5.3.12:Ambient and operating temperatures) and on the lamp current, according to the negative lamp characteristic (see section 3.2: Stabilisation).

-the lamp current Ilamp , which is dependent on the mains voltage (see section 5.3.13: Effects of mains voltage fluctuations), the lamp voltage

and the linearity of the ballast impedance.

In order to avoid undesirable variations in light output as a consequence of mains voltage fluctuations, the lamp voltage must be not more than approx. half the value of the mains voltage (100 to 130 V) and the impedance should be as linear as possible.