Question

With reference to fluorescent lamps.

Describe a switch type starter.

Describe a transformer quick start.

List two reasons for inclusion of capacitor to fluorescent light circuit.

The circuit in Fig. shows a typical switch-start circuit.

The starting action is initiated by a glow type starter switch which is connected between opposite ends of the tube.

When the supply voltage is applied to the circuit, the full mains voltage appears across the starter switch. A glow discharge occurs between the starter contacts which quickly heat up, bend and touch each other. This allows current to flow through the lamp cathodes which will cause the tube ends to heat up and glow before the tube actually strikes. The tube strikes when the starter switch re-opens as it cools down during its closed (non-glow) period. When the starter switch opens it interrupts an inductive coil (choke) circuit which produces a surge voltage across the tube which then strikes.

Fig. Transformer-start fluorescent lamp circuit

The lamp discharge begins as soon as the cathodes reach their operating temperature. A capacitive effect between the cathodes and the earthed metalwork of the fitting ionises the gas and the tube strikes very quickly

Capacitors are used with discharge tubes for:

Power factor correction (PFC)

Radio interference suppression (RIS).

The PFC capacitor is used to raise the supply power factor to around 0.9 lagging. Without this capacitor the power factor may be as low as 0.2 lagging due to the high choke-coil inductance to cause the supply current to be 4 to 5 times larger than normal.

Radio interference from discharge tubes is caused by the ionisation process of the discharge through the tube. This is suppressed by a capacitor fitted across the tube ends. In glow-switch circuits, the RIS capacitor is actually fitted within the starter. Typical RIS capacitor values are around 0.0005 µF.