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Chapter: Television and Video Engineering - Essentials of Color Television

Automatic Degaussing (ADG) Circuit

There are many degaussing circuits in use. Figure shows details of a popular automatic degaussing circuit. It uses a thermistor and a varistor for controlling the flow of alternating current through the degaussing coil.

AUTOMATIC DEGAUSSING (ADG) CIRCUIT

 

There are many degaussing circuits in use. Figure shows details of a popular automatic degaussing circuit. It uses a thermistor and a varistor for controlling the flow of alternating current through the degaussing coil.

 

When the receiver is turned on the ac voltage drop across the thermistor is quite high (about 60 volts) and this causes a large current to flow through the degaussing coil. Because of this heavy current, the thermistor heats up, its resistance falls and voltage drop across it decreases.

 

As a result, voltage across the varistor decreases thereby increasing its resistance. This in turn reduces ac current through the coil to a very low value. The circuit components are so chosen that initial surge of current through the degaussing coil is close to 4 amperes and drops to about 25 mA in less than a second.

 

This is illustrated in Fig. Once the thermistor heats up degaussing ends and normal ac voltage is restored to the B + rectifier circuit.

 




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