Block D (Output Circuit) - Citizen CLP-621 Technical Manual

2-5. Power Supply

2-5-5. Block D (Output circuit)

DS2, C15 and C16 perform half-wave rectification to produce +24V.
DS3 and C19 perform half-wave rectification to produce DC voltage to supply to the +5V regulator
IC (IC31). IC31 produces a stabilized +5V voltage.
2-5-6. Block E (Control circuit)
The shunt regulator (IC32) and photo coupler (PI1) detect the output voltages (+24V and +5V) and
feed the signal back to the primary side. R33, R34, R36 and VR1 are used to detect +24V and +5V
outputs. The voltage produced by these resistors is compared with the reference voltage of IC32
(2.5V). If there is a difference between both voltages, the current flowing into the photo coupler PI1
is changed. So, the signal fed back to pin 2 (FB) of IC1 in the primary side is also changed. Then,
the conduction-angle for Q1 is changed to stabilize +24V and +5V constant.
<Over current protection circuit>
The over current protection circuit detects the drain current of Q1 to check if power supply
circuit is overloaded or its output is short-circuited. If over current is detected, IC1 cuts off the
output signal from pin 5 (OUT) for protecting power supply circuit.
The drain current (Id) of Q1 is detected by R23, R22, R21 and C11, and input to pin 3 (IS) of
IC1.
If the voltage at pin 3 (IS) exceeds the internal reference voltage (0.24V) of IC1, the switching
pulse sent from pin 5 (OUT) is cut off, resulting in lower output voltage.
If over current condition due to overloading or output short-circuit continuous, the voltage at pin
8 (CS) of IC1 increases, and after a certain period of time, the voltage will exceed the internal
reference voltage (8.5V) of IC1. In this case, IC1 is latched to off mode and the output at pin 5
(OUT) is set to "Low" level (off condition). Then, the output voltages (+24V and +5V) is shut off.
To recover the latched state, turn OFF the power and, after a while, turn it ON again.
<Protection circuits in case of abnormality>
In case of abnormality, the power supply circuit automatically stops its operation to avoid
smoking or fire.
When temperature of the power supply raises abnormally, the thermal fuse TF1 opens and the
drive signal to Q1 is cut off. When the output voltage rises, the voltage across the primary
winding N2 also proportionally raises. In this case, the Zener diode ZD1 is turned ON and the
voltage higher than 8.5V is applied to pin 8 (CS) of IC1. Then, IC1 enters latch mode and stops
its operation.
To recover the latched state, turn OFF the power and, after a while, turn it ON again.
If large current flows, the fuse (F1) in the noise filter circuit will be blown out.
CLP-621 & CLP-631
2-60