Gas discharge lamps have negative resistance once lit, so a ballast supplies the high starting voltage and then limits current. Magnetic ballasts are heavy coils; electronic ballasts run lamps at high frequency, which ends visible flicker and raises efficacy. Ballast factor, the ratio of light from a lamp on that ballast to a reference, lets designers tune output. LED tube retrofits come in ballast-compatible and ballast-bypass versions.
Ballast is humming and the tubes only glow at the ends, replace it or switch the fixture to ballast-bypass LED tubes.
Do not mix it up with
- Driver: A driver regulates power for LEDs; a ballast starts and controls discharge lamps.
Related terms
HID
High-Intensity Discharge: lamps such as metal halide and high-pressure sodium that make light from an electric arc in a pressurized tube.
Driver
The electronic power supply that converts line voltage into the regulated current or voltage an LED needs.
Lamp
The trade word for a replaceable light source, what consumers call a bulb, as distinct from the fixture it goes in.
Strike
To start the arc in a discharge lamp; restrike is relighting it after it has gone out.