Ballasts for Fluorescent Lamps

Discharge lamps are inductive loads and need high voltage initially to discharge the lamp. They also have negative resistance, meaning they are unable to regulate the amount of current that passes through them once the discharge starts. Small light sources can use passive components, such as a series resistor that limits the flow of current across its terminals. For high-powered lights, however, a resistor would waste a large amount of electricity, so a more complex regulator is required. These tasks are completed with the help of the ballast.

Electro Magnetic Ballast

Electromagnetic ballasts (EM ballast) work on the principle of electromagnetic induction to provide the starting and operating voltages of a gas discharge light. Inside each is a coil of wire and an electromagnetic field that together transform voltage. Some also include an igniter for high-power applications. Typically EM ballast works in a switch-start circuit in which the inductor is referred to as the ballast (choke) and switch works as the starter.

EM Ballast Operation 

1. Starter switch contacts are closed on application of the supply voltage.
2. Inductor current preheats the lamp cathodes by resistance heating.
3. Contacts open a few tenths of seconds later and lamp voltage rises sharply, the process repeats until the lamp strikes.
4. Inductor reactance regulates the lamp current.