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( lll ) Other Operation
• D611 and R616:
When the AC input voltage increases, the feedback voltage produced from the
feedback coil pins 2 to 3 also increases in direct proportion to the AC input
voltage. When the feedback voltage becomes higher than the zener voltage
(5.6V) of D611, D611 will be conductive and the determinant of the time
constant for integrating the feedback voltage will change from R622 and C613
to R622, R616 and C613. Accordingly the time constant of the integrator will
become smaller, the base voltage of Q604 will increase quickly, and the On
period of Q601 will become shorter and Q601 will go Off quickly. In other
words, when the power is turned On during the higher AC input voltage, D611
and R616 will consequentially suppress a rush current coming into this power
supply circuitry.
• D614 and R617:
When Q601 is On, the positive feedback voltage is charged into C613. To
discharge the voltage, a discharge path (D614, R617) is provided. When
Q601 is Off, the negative feedback voltage is produced from the feedback coil
3 to 2 and the voltage charged in C613 will be discharged, going through
terminal 3, C613/R619, D614, R617 and terminal 2. (See T1 period in Figure
2-(G).)
• D610:
D610 is provided to obtain a sufficient current for operating the photo
transistor D612 and the transistor Q605, because the resistance of the
starting resistor R603 is too high to obtain a sufficient current from the DC
input voltage (C609). Therefore, D610 is provided to rectify the feedback
voltage produced from the feedback coil 3-2 and supply sufficient current for
operating D612 and Q605.
• R604//R606 and C608:
When Q601 goes Off, a surge voltage will be impressed between the collector
and the emitter. To suppress the surge voltage, R604//R606 and C608 are
provided.
• R614, C612 and D609:
During the On operation of Q601, D609 cannot be conductive if the positive
feedback voltage is lower than 0.6V. However, during the lower positive
feedback voltage, the current goes through R614 and C612, and turns Q601
On. After Q601 is On, the base current will be supplied through D609.
OVERLOAD PROTECTION CIRCUIT
The power supply circuits employed in the VB7C chassis is equipped with an
overload protection circuit to automatically reduce the power to almost 0 if a
failure occurs in the +16V, +12V or the 5V supplies to help prevent
secondary damage.
The overload protection circuit is composed of Q635, D627, D694 and the
associated circuitry.
In circuit operation, if all +16V, +12V and the 5V supplies are their
normal, the diodes D627 and D694 are reversed biased. If, while the power
is On, a failure is caused in any of +16V, +12V or the 5V supplies,
either of the diodes D627 and D694 will switch On, grounding the emitter of
Q635. When the emitter of Q635 is grounded, Q635 will turn On. When Q635
turns On, the photo diode within D612 will completely turn On, and Q605 and
Q604 will turn On, then Q601 will go Off quickly. As the result, the oscillation
of the power supply circuits will stop and the +B output voltages will
decrease. Also the voltage at the base of Q635 will decrease very slowly
since the voltage changed in C628 will be discharged.
When the voltage at the base of Q635 becomes lower than the triggering
value (0.6~0.7V higher than the emitter voltage) of Q635, Q635 will turn Off,
then D612, Q605 and Q604 will turn Off.
With Q604 Off, Q601 will turn On and the +B output voltages will increase.
When the voltage at the base of Q635 reaches the triggering value (0.6 ~ 0.7V
higher than the emitter voltage) of Q635, Q635 will turn On.
By this means, the +B output voltages will be suppressed.
