Specifications

CA10 Course Notes
Sharp Electronics (UK) Limited
CE Technical Support Group Page 9
will allow Q704 to conduct more and the LED will become brighter.
As the LED becomes brighter, the transistor within IC705 will conduct more allowing C713 to charge
faster and thus turning on Q703 quicker. Q703 will discharge Q714 and turn Q701 off, making the on
time of Q701 shorter. The emf will decrease and the voltages induced in the secondary windings of
T701 will also decrease.
Supply decrease
If the 150 volt supply line were to decrease, the voltage on the base of Q705 would decrease. As the
emitter of Q705 is held steady by the zener diode D717, at 6.2V, the collector will start to rise. Q704 will
conduct less and the LED will become less bright.
As the LED becomes less bright, the transistor within IC705 will conduct less giving a lower voltage on
the base of Q703 and C713 will charge more slowly. Q703 will now turn on later in the cycle and allow
C714 to charge up for longer. This will turn on Q701 for longer and the current through the primary
winding ( 12 and 17 ) of T701 will increase. The emf will increase and the voltages induced in the
secondary windings of T701 will also increase.
Note
The voltage present at the junction of R784/R728 and R727 will be exactly 6.7464V when the supply is
150V. As the emitter of Q705 is held at 6.2V by the action of zener diode, D717, the collector will follow
the base voltage. As the h of Q705 ( 2SC2412 ) is high it will amplify the voltage on its base so that
fe
the voltage swing on its collector will be great enough to vary the base voltage of Q704 dramatically.
Q704 is effectively a current amplifier which allows the LED inside the OPTO coupler to be driven
through its range of intensity.
C720 ensures that no short time constant variables effect the operation of the regulation circuit. If the
regulation were to change too quickly, there may be a chance that the power supply will oscillate at a
high frequency - this is undesirable.
As the tolerances in this part of the circuit are high and impedances are also relatively high, there is a
risk of circuit malfunction if a voltmeter with too low an impedance is used. In worst case this could lead
to failure of the power supply. It is recommended that if fault finding has to be carried out in this part of
the circuit, then an oscilloscope is used.
Protection
Protection is provided should one of the five volt supplies, STB5V and +5V, drop to an unacceptable
level. This is achieved by monitoring the +7V supply which is used to generate the STB5V and +5V
lines.
Should the +7 volt supply drop to a point whereby diodes D726 and D727 start to conduct, the voltage
on the emitter of Q705 will drop. This will cause the collector of Q705 to drop, turning Q704 on harder,
making the LED brighter and turning on Q703 sooner, dramatically decreasing the discharge time of
C714 and thus the ON time of Q701. Therefore reducing the current flowing in the primary winding of
T701.
When the current flowing in the primary winding of T701 decreases, so will the secondary voltages. This
will cause the voltage generated by D722 ( +7V rectifier ) to also drop. This will reinforce the above
action and the power supply will attempt to shut down. During this cycle, there will come a point where
it becomes impossible to maintain the conduction of Q703 ( as the charge on C713 decays to below
+0.65V ) and the power supply will start up again. If the condition of the +7V ( low supply ) line continues
the set will continually trip.