A fluorescent lamp with a glow type starter operates through a sequence involving a gas discharge, bimetallic strip, and inductor to initiate the arc needed for the lamp to illuminate.
Here's a breakdown of the process:
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Initial Power Application: When the power switch is turned on, voltage is applied to the circuit. This circuit includes the fluorescent tube (with filaments at each end), the glow starter, and a ballast (inductor).
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Glow Starter Operation:
- The glow starter contains a small glass bulb filled with an inert gas, typically neon or argon. Inside this bulb are two electrodes, one or both of which are bimetallic strips. Importantly, these electrodes are initially open (not touching).
- The applied voltage creates a glow discharge (a visible glow) between the open electrodes in the starter bulb. This discharge heats the bimetallic strip(s).
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Contact Closure: The heat from the glow discharge causes the bimetallic strip(s) to bend due to the differential expansion of the two metals in the strip. After a second or two, the bimetallic strip(s) bend enough to close the circuit between the electrodes.
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Filament Heating and Current Flow: When the contacts in the starter close, a current flows through the filaments at each end of the fluorescent tube and through the now-closed starter. This current heats the filaments, causing them to emit electrons. This is critical for initiating the arc within the fluorescent tube.
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Starter Opening and Ballast Function: After the contacts close, the glow discharge ceases because the voltage across the starter drops. Without the glow discharge, the bimetallic strip cools and the contacts open again. This opening action is crucial.
- The rapid interruption of the current flow by the starter opening causes the ballast (inductor) to generate a high-voltage pulse. This pulse is necessary to ionize the gas inside the fluorescent tube.
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Arc Ignition: The high-voltage pulse from the ballast ionizes the gas (typically mercury vapor and argon) inside the fluorescent tube. The electrons emitted from the heated filaments are accelerated through the ionized gas, creating an electric arc.
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Sustained Illumination: Once the arc is established, it provides a continuous path for current to flow through the fluorescent tube. The arc excites the mercury atoms in the gas, causing them to emit ultraviolet (UV) light. This UV light strikes the phosphor coating on the inside of the tube, causing it to fluoresce and emit visible light.
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Ballast as Current Limiter: The ballast then acts as a current limiter, preventing excessive current flow through the fluorescent tube, which would damage it. It also maintains the correct voltage for efficient operation. The starter plays no further role until the lamp is switched off and then on again.
In summary, the glow starter initially heats a bimetallic switch to preheat the tube filaments. When the switch opens, the ballast generates a voltage spike to start the arc. Once the arc is going, the starter plays no further role.
