Working of a Fluorescent Lamp

Design:
A fluorescent lamp consists of a sealed glass tube filled with an inert gas (usually argon) at low pressure. On each side of the tube you will find a tungsten electrode. Both these electrodes are wired to an electrical circuit, which includes a starter and ballast, is hooked up to an alternating current (AC) supply. The tube also contains phosphor powder, coated along the inside of the glass.
Working:
When the lamp is first turned on, the current cannot pass through the tube initially because the gas inside it is not ionized and hence the tube light circuit acts as an open circuit. The current travels through the path of least resistance, which is through the bypass circuit, and across the starter switch.


This current then passes through the circuit heating up the filament in each electrode, which are located at both ends of the tube (these electrodes are simple tungsten filaments, like those found in incandescent light bulbs). This boils off electrons from the metal surface, sending them into the gas tube, ionizing the gas. Once the gas is ionized, it will provide a conduction path for the current to flow. The arc which started in argon gas quickly warms up the mercury liquid stuck to the side of the tube.

The mercury boils or vaporizes into the arc stream. The arc now easily passes through vaporized mercury. This generates radiant energy, mainly in the ultraviolet range. The UV light is converted into standard visible light using a phosphor coating on the inside of the tube.
The Starter:
The starter is basically a time delay switch. Its job is to let the initial high current required to flow through to the electrodes at each end of the tube, causing the filaments to heat up and create a cloud of electrons inside the tube. The starter then opens after a second or two. The voltage across the tube allows a stream of electrons to flow across the tube and ionize the mercury vapor. Without the starter, a steady stream of electrons is never created between the two filaments, and the lamp flickers.
The Ballast:
The ballast works mainly as a regulator. Its main aim is to providing the necessary circuit conditions for starting and operating the lamps. In a fluorescent lamp, the voltage must be regulated because the current in the gas discharge causes resistance to decrease in the tube. The AC voltage will cause the current to climb on its own. If this current isn’t controlled, it can cause the blow out of various components.
Newer Designs:
Today, the most popular fluorescent lamp design is the “rapid start” lamp. This design works the same as the basic design described above, but it doesn’t have a starter switch. Instead, the lamp’s ballast constantly channels current through both electrodes. This current flow is configured so that there is a charge difference between the two electrodes, establishing a voltage across the tube. Another method used in instant-start fluorescent lamps, is to apply a very high initial voltage to the electrodes. This high voltage creates a corona discharge, which causes an excess of electrons on the electrode surface that forces some electrons into the gas. These free electrons ionize the gas, and almost instantly the voltage difference between the electrodes establishes an electrical arc.
Q. What if D.C supply is given instead of A.C supply?
A. If the supply to the lamp was DC, the cathode side would be brighter and more intense since there are more free electrons spewing off of the tungsten electrode there. Also, the electrode which is acting as the cathode would become weaker as it lost tungsten atoms and the lamp would not last as long. Since we use AC the electrons or ions break off one side, reach the other. During the next cycle are sent back. Also the lamp tube has a nice uniform brightness on both ends.