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POWER SAVING CIRCUIT
The power supply circuits employed in the VB7C chassis are equipped with
the interval oscillation circuit for saving the power consumed during the stand-
by mode.
The interval oscillation circuit is composed of Q693, Q695, D693 and the
associated circuitry.
When the TV is turned Off, the voltage at point (A) is almost 0V, and Q693 and
Q695 remain Off. When the TV set is turned On, the voltage at point (A) will
increase to approximately 12V. The voltage is resistive divided by R692 and
R693 and applied the base of Q693, then Q693 and Q695 will turn On. When
Q695 turns On, the photo divide within D612 will completely turn On, and
Q605 and Q604 will turn On, then the switching transistor Q601 will turn Off.
As a result, the oscillation of the power supply circuits will stop and the output
voltages of the power supply circuits will fall down. Also the voltage at point
(A) will gradually fall down from 12V. When the voltage at the base of Q693
becomes lower than the triggering value (0.6~0.7V higher than emitter
voltage) of Q693, Q693 will turn Off, then Q695, D612, Q605 and Q604 will
turn Off.
As a result, Q601 will start oscillation and the output voltages of the power
supply circuits will be supplied.
By this means, the voltage at point (A) will increase and turn Q693 On again.
By repeating the above operation, power consumption during the stand-by
mode can be reduced.
Operating period of
Power Supply Circuits
12 V
0 V
Voltage at point A
Figure 3. Interval Oscillation
