High Pressure Mercury Lamps

High Pressure Mercury Lamps

• The arc tube is made of quartz and contains a small amount of mercury and argon gas.
• The arc tube houses three electrodes: D, E (main electrodes), and S (auxiliary starting electrode).
• Electrode S is connected through a high resistance R (about 50 kΩ) to the main electrode situated at the outer end of the tube.
• When the supply is switched on, the initial discharge is established in the argon gas between D and S.
• The initial discharge then spreads to the argon between D and E.
• The heat produced due to the discharge through the gas is sufficient to vaporize mercury.
• The pressure inside the arc tube increases to about one or two atmospheres in 5-7 minutes.
• During this time, a discharge is established through the mercury vapors, which emit greenish-blue light.
• A choke is used to limit the current drawn by the discharge tube to a safe limit.
• A capacitor C helps to improve the power factor of the circuit.
• Typical applications of high pressure mercury-vapour lamps include high-bay industrial lighting, flood-lighting, and street-lighting where high level illumination is required.