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1.3 Ignition and
In the case of electromagnetic control gear, a combination of preheating and a high ignition peak is obtained by using a normal choke ballast and a preheat starter or an electronic ignitor.
Energy is supplied to the discharge in the form of electrons.The lamp current, just like the mains voltage, is sinusoidal, with a frequency of
50 or 60 Hz. If the energy flow is zero (at lamp current reversal) the lamp stops burning and in theory would have to be
This could be done by supplying additional energy to the electrodes via a higher lamp voltage, the way it is done when initially starting the lamp. But from the moment the lamp has reached its stationary condition, the lamp voltage is constant.
And yet, in practice the lamp does not extinguish at current reversal. Why not?
The phase shift introduced by the inductive element of the ballast ensures that the mains voltage is not zero at that moment. Because of the inductive properties of choke coil ballasts a phase shift ϕ occurs between the mains voltage and the lamp current (see Fig. 102). So, at the moment of current reversal the lamp voltage would be equal to the mains voltage, since the voltage over the ballast is nil.The difference (gap) between the mains voltage and the average lamp voltage as a consequence of the phase shift ensures proper
Fig. 102. Phase shift between supply voltage and lamp current (and lamp voltage) in a
V, A
gap
Vm
Il
Vl
t
discharge lamp with an inductive ballast. In the case shown, the supply voltage is sufficiently high for
ϕ
A
1 
4 
Types of ballasts
1 Resistor ballasts
Current limitation by means of resistor ballasts is a very uneconomic form of current limitation, because in the resistor electrical energy is dissipated in the form of heat. Nevertheless, until the advent of electronic circuitry, use of a series resistor was the only way of stabilising fluorescent lamps operated on DC, for example the ‘TL’R lamp (see Fig. 103). For stable operation on a resistor ballast, it is necessary that the supply voltage be at least twice the lamp voltage under operating conditions.This means that 50 per cent of the power will be dissipated by the ballast.A considerable improvement in efficiency can, however, be achieved by using a resistor with a very pronounced positive temperature characteristic (an ordinary or specially constructed incandescent lamp serves well for this purpose).
A
108
