Chapter: Television and Video Engineering : Essentials of Color Television
Automatic Degaussing (ADG) Circuit
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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