Ballasts are needed to run high pressure discharge Lamps with their various gas fillings (mercury vapour, sodium vapour or metal halide).

They stabilise the lamp current and thus lamp power. The rating of the ballast is important to ensure proper lamp operation.

In the case of low-power (150 W) metal halide lamps especially, an excessive temperature rise can occur in the ballast at the end of lamp life. The ballast should therefore always be fitted with overtemperature protection.

Whereas high pressure mercury lamps strike at mains voltage, most metal halide and high pressure sodium lamps require an additional Igniter.