Philips Electromagnetic Lamp Indoor Furnishings User Manual


 
that the no-load voltage need be no more than 25 to 30 per cent
higher than the lamp voltage.This is also the proportion of the power
dissipated by the ballast compared to the total circuit power.
2 Capacitor ballasts
A capacitor used as a ballast causes only very little losses, but cannot
be used by itself, as this would give rise to very high peaks in the lamp
current wave form at each half cycle. Only at very high frequencies
can a capacitor serve satisfactorily as a ballast.
3 Inductive ballasts or chokes
Choke coils are frequently used as current limiting devices in
gas-discharge lamp circuits (see Fig.104).They cause somewhat higher
losses than a capacitor, but produce far less distortion in the lamp
current at 50 Hz. Moreover,in combination with a switch starter, they
can be made to produce the high voltage pulse needed to ignite the
lamp.
In practice, a choke ballast consists of a large number of windings of
copper wire on a laminated iron core. It operates on the self-inductance
principle.The impedance of such a ballast must be chosen in
accordance with the mains supply voltage and frequency, the lamp type
and the voltage of the lamp, to ensure that the lamp current is at the
correct value.In other words:each type of lamp requires for each supply
voltage its own choke as a ballast with a specific impedance setting.
Heat losses, occurring through the ohmic resistance of the windings
and hysteresis in the core, much depend upon the mechanical
construction of the ballast and the diameter of the copper wire.
The right ballast for a given lamp and supply voltage should be chosen
by consulting documentation and/or ballast markings.
The Philips standard range of ballasts is for supply voltages of
220/230/240 V and for frequencies of 50/60 Hz.
5
109
Fig. 103. Schematic diagram of a
fluorescent lamp operated on a resistor
ballast in a DC circuit.
Fig. 104. Schematic diagram of a
fluorescent lamp operated on a choke
ballast in an AC starter circuit.
1.4 Types of ballasts
+
-
‘TL’ R
I
b
I
l
La
B
V
l
Vm
+
-
+-+-
0
S