A discharge lamp is a type of lamp in which illuminance is produced by the excitation and ionization of the gas. A mercury-vapor lamp is a discharge lamp in which mercury is used for the production of visible light. This gas when ionized by application of sufficient voltage with help of electrodes, it produces electromagnetic radiations. There are two types of mercury vapor lamps depending on the vapor pressure inside the lamp. They are,
- High-pressure mercury vapor lamps, and
- Low-pressure mercury vapor lamps (fluorescent lamps).
High Pressure Mercury Vapour Lamp :
The high-pressure mercury vapor discharge lamps are again divided into three types, they are,- MA (mercury vapor lamp with auxiliary starting electrode) Type: These are manufactured from 250 to 400W ratings and operate at 200 to 250V AC supply.
- MAT (mercury vapor lamp with tungsten filament) Type: These are manufactured from 300 to 500W ratings and operate at 200 to 250V AC or DC supply.
- MB (mercury vapor lamp with auxiliary starting electrode and bayonet cap) Type: These are manufactured from 80 and 125W ratings and operate at 200 to 250V AC supply.
MA Type Mercury Vapour Lamp :
The construction of an MA type (lamp with auxiliary starting electrode) mercury vapor discharge lamp is similar to a sodium vapor lamp. It consists of two main electrodes made up of tungsten coated with barium oxide (material for an easy emission of electrons) enclosed in a hard glass tube or bulb made of borosilicate or quartz as shown below.
There is an auxiliary starting electrode near one of the main electrodes and the tube contains argon gas of low pressure and some mercury. The inner tube or bulb is enclosed in another glass bulb and the space between the two tubes or bulbs is either partially or completely evacuated to prevent heat loss.
The lamp has a screwed cap and is connected to a choke coil having different tappings in series with the lamp to give a high starting voltage for discharge and for controlling the current and voltage across the lamp after discharge. The power factor of the circuit is low due to the choke coil, hence can be improved by installing a capacitor in parallel to the supply line.
Initially, when supply is given to the lamp, the current does not flow through the main electrodes due to the high resistance of the gas. However the current starts to flow between the main electrode and auxiliary starting electrode through argon gas.
Thus the argon gas between the main and auxiliary starting electrode gets ionized producing an arc and a discharge takes place. This discharge can be controlled by varying the resistance connected in series with starting electrode.
The heat thus produced vaporizes mercury which reduces the resistance between the main electrodes. Due to the low resistance of the ionized path between two main electrodes the discharge shifts from starting electrode circuit to the main electrodes.
The heat produced by this discharge causes mercury to vaporize, increasing the pressure inside the inner tube. Thus the free electrons start emitting greenish-blue light with the help of vapor.
MAT Type Mercury Vapour Lamp :
The MAT type (mercury vapor lamp with tungsten filament) lamp is similar to the MA type, but the outer tube instead of being empty, it consists of a tungsten filament similar to that of an ordinary lamp in series with the discharge or inner tube, so that it acts as a choke or blast to limit high currents to safer value. Since the lamp does not require a blast of choke it can be used for ac as well as dc power supply.
When the lamp is switched ON, it works as a filament lamp and its full output is given by the outer tube. At the same time, the discharge tube starts warming up and when a particular temperature is reached, a thermal switch operates, and the coils of a part of the filament are cut off so that the voltage across the discharge tube increases. Now the discharge takes place in the discharge tube through mercury vapor producing visible light.
The advantage of this type of lamp is that the color of the light so obtained is soothing as it consists of a mixture of light due to the filament and blue radiations from the discharge tube. This type of lamp is manufactured in 300 to 500W ratings and there is no need for a capacitor in this lamp since the overall power factor is about 0.95.
MB Type Mercury Vapour Lamp :
MB type mercury vapor lamp operates at an extra high pressure (about 5 to 10 times greater than atmospheric pressure). The discharge tube in this type of lamp is of quartz about 5 cms long which can withstand high temperatures due to arc. It has three electrodes, two main and one auxiliary starting electrode. The discharge tube is in a plain glass bulb similar to that of MA type lamp. A high starting resistance is connected in series with the starting electrode.
The operation of the lamp is similar to that of MA type mercury-vapor lamp and it also consists of a choke coil and capacitor in order to operate. The lamp has generally 3-pin bayonet caps so it may not be in the ordinary sense.
Low Pressure Mercury Vapour Lamp (Fluorescent Lamp) :
A fluorescent lamp is a hot cathode low-pressure mercury vapor lamp. These lamps are manufactured in a variety of lengths and are available with illumination in a variety of colors. The lamp consists of a long tube coated with phosphor on the inner side. The tube also consists of mercury and argon gas at a pressure of 2.5mm of mercury. Both the ends of the tube consist of a filament in spiral form and coated with electron emission material as shown below.
The circuit consists of a choke connected in series with the filament and supply which provides voltage impulse at the starting of the lamp and acts as ballast during the lamp running period. A starter (small cathode glow lamp with a bimetal strip and a small quantity of argon gas) is also connected in series with two filaments which put the two electrodes across the supply.
Working of Low Pressure Mercury Vapour Lamp :
When the supply is given to the lamp, full voltage is applied across the starter, and argon gas inside the starter gets ionized producing a glow discharge inside the starter. This causes the bimetallic strip in the starter to bend, making the filaments circuit. Hence the two filaments get connected across the supply voltage and they start emitting electrons due to current flow which will ionize the argon gas inside the tube.
In the meanwhile, the bimetallic strip cools down sufficiently and opens the starter circuit. This causes an abrupt change of current in the choke coil resulting in an EMF of high voltage surge induced in the choke. This high voltage causes to strike an arc between two filaments through the argon gas inside the tube due to its ionization.
The heat generated vaporizes the mercury inside the tube producing ultraviolet rays. The UV rays when striking the phosphor coating inside the tube, visible light is produced i.e., the phenomenon of fluorescence. Usually, the power factor of the lamp is low and is improved by connecting a capacitor across the supply.
