There are two primary types of reactive ballasts used to operate mercury vapor lamps, inductive and capacitive. With inductive ballasts, the lamp is connected directly to the output of the ballast. These ballasts have two benefits. First, the reactance of the inductor effectively limits the power available to the lamp with only minimal power losses. Secondly, the voltage spike produced by the inductor when power is first applied is often used in some circuits to strike the arc in the lamp. The disadvantage of the inductive-reactance ballast is that current is shifted out of phase with the voltage producing a lower power factor.
Capacitive reactance ballasts overcome this problem by the use of capacitors connected in series with the lamps. These capacitors maintain a constant voltage output to the lamp even when the input voltage varies and is the more efficient of the two.
Ballasts have to be correctly matched to the electrical requirements of the lamps.
