Mecury Vapor Lamps: Operating principle and components


Mercury Vapor lamp is one of many kinds of electric lamps designed to produce light from electricity. Other types of electric lamps available in the marled include the Incandescent bulb, LED bulb, Florescent, sodium Vapor, and Metal Halide.
Electric lamps could be broadly divided into two categories.
1)    Incandescent
2)   Gaseous Discharge Lamp.
Gaseous discharge lamps could be further divided into High pressure and low-pressure gaseous discharge lamps. Mercury Vapor lamps fall under the Gaseous discharge lamp category. Other lamps in this category are Metal halide and high and low-pressure sodium lamp and fluorescent lamp.
All gaseous discharge lamps generate light using the same principle i.e. passing an electrical arc through a gas. Mercury Vapor lamps fall under the High-Pressure Gaseous Discharge lamp category. They are also commonly referred to as High Intensity Discharge (HID) lamps. Metal Halide and High-pressure Sodium (HPS) lamps are also called HID lamps. All HID lamp contain similar components and therefore have the same working principle.

Working principle of a HID lamp

An HID lamp has no filament, the light is produced by what we call the arc tube inside the bulb. The arc tube is filled with a very precise mixture of Argon and Mercury metals. It could be Sodium or Metal halides depending on the type of HID lamp we are looking at. Inside the arc tube, there are three electrodes in total; two operating electrodes at the top and bottom of the tube and a starting electrode. The starting electrode is a much smaller electrode also located at the bottom of the arc tube beside one of the operating electrodes. The starting electrode and argon gas in the tube makes the lamp self-starting. The starting electrode is located so close to the main electrode such that once the power is turned, the voltage potential between the starting and main electrode breaks down the argon gas and allows electrons to jump through the air from the starting electrode to the main electrode (arc). The heat from this arc vaporizes the mercury into the arc stream and arc strikes between the two main electrodes. The intensity (glow) of the arc increases as the lamp warms up because the heat from the arc causes the mercury to emit a bluish light.  The glow increases until the lamp gets to full brightness. 

An Orange light is emitted in the case of sodium vapor and white light in the case of metal halide.
The larger the arc grows the lower the resistance which leads to more current being pulled until the lamp shatters in your face, this is why a ballast is needed to limit current.

 Wiring Diagram.

 Below is the most common configuration of a HID lamp.


Ballast
The job of the ballast is to provide a high voltage impulse that produces the initial arc to vaporize mercury. The ballast also limits/regulates the flow of current through the arc tube and maintain a steady voltage after the lamp must have reached full glow.

Capacitor
The capacitor is connected in series with the lamp for power factor improvement. The type of ballast HID lamps use are called magnetic ballast which is basically an inductor coil. The disadvantage of an inductor is that current is shifted out of phase with the voltage. Current lags voltage producing a poor power factor. A poor power factor in an oversimplified explanation means that the circuit will draw more current than it actually needs to function, wasting power, Overloading the generator or transformer and making the conductors carry more current than then need to. 

 A poor power factor can be improved by adding another load that draws an equal but opposite reactive power from the circuit. A capacitor is used in this case because, in a purely capacitive circuit, current leads the voltage.

 Think of it this way. we want the current and voltage waveform to be in phase. Inductors push current out of phase by making it lag 90°, capacitors push current out of phase by making the current lead 90° so if an Inductive ballast has pushed current out of phase by making current lag, when we pass it through a capacitor it tries to make the lagging current to begin to lead. 
This pushes the current waveform closer to the voltage waveform. where we want it.

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