25 Instant Start Ballast

In an Instant Start fixture, the ballast houses an autotransformer, which steps up the supply voltage to a sufficient value to strike the arc between the two ends of the tube.

The instant start ballast uses cold-cathode starting, requiring the striking voltage to be significantly higher than the pre-heat ballast. Without filament pre-heating, this typically occurs between 400 to 500 volts, but the striking voltage for some instant-start fluorescent lamps is in the range of 700 to 800 volts. This high striking can also reduce lamp life.

This high voltage value could also represent a safety hazard to someone replacing a lamp. To overcome this, a switch is incorporated in one of the lamp holder sockets which ensures that the ballast will not be energized until the lamp is fully seated in the fixture.

Because no cathode preheating is required, the lamp lights, like an incandescent lamp, at the instant the switch is closed.

Lamps designed for use with the instant start ballast commonly only have a single-pin base. One end of the mounting has a spring, which must be depressed in order to fit the lamp in. This spring disconnects the terminals from the ballast to ensure that no dangerous levels of voltage are present to injure anyone changing the tubes.