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DA100 (50Hz) and DA50W Chassis – Repair Notes
Page 15 of 80
Circuit Operation
Depending on the input mains circuit, there is either 320VDC (conventional bridge rectifier/capacitor
arrangement) or 360VDC (power factor circuit) fed into the top of the chopper transformer. This voltage is also
used to charge C714 via R713 and R714, allowing the voltage on the gate of Q701 to increase. When the voltage
on the gate of Q701 reaches 4.5V, it turns on and current flows in the primary winding of T701. This will cause
pin 15 to increase and when the zener voltage of D716 is exceeded, turning Q703 on and Q701 off.
arrangement) or 360VDC (power factor circuit) fed into the top of the chopper transformer. This voltage is also
used to charge C714 via R713 and R714, allowing the voltage on the gate of Q701 to increase. When the voltage
on the gate of Q701 reaches 4.5V, it turns on and current flows in the primary winding of T701. This will cause
pin 15 to increase and when the zener voltage of D716 is exceeded, turning Q703 on and Q701 off.
Figure 5: Power Supply Schematic Diagram
When Q703 turns off, after C713 has discharged, the cycle will repeat. Obviously as the circuit operates in this
condition, there will be no voltage regulation and the supply will increase to its maximum level, which is about
220V. This is undesirable, so there has to be some kind of control based on the HT supply. Feedback is achieved
via IC705, its input (LED) is fed with a stabilised 12V supply on its anode by a shunt regulator off the +16V
supply (R765, R766 and D726). The cathode of the LED is fed with a proportion of the +150V supply via the
programmable zener diode IC706. A potential divider comprising of R743, R744 and R746 set the input voltage
to IC706.
As the brightness of the LED increases or decreases, the conduction of the transistor will change in sympathy.
The collector is taken to a crude supply line generated off pin 15, rectified by D715 and smoothed by C718. This
supply will be fed into C713 via the opto transistor; the speed of charge depends upon the luminosity of the LED.
Q703 will turn on and turn off Q701
Over current is prevented by Q702. Its base is connected across the earth return resistor (R716) in the drain
circuit of Q701. This resistor has a very low value and therefore a large amount of current has to flow before
Q702 will turn on and remove the bias on the gate of Q701. Once the voltage on the gate of the FET has been
Sharp Electronics (UK) Limited - May 2003
Revision 3
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