Epson Stylus COLOR 440 Service Manual page 59

EPSON Stylus COLOR 440, 640, and 740
voltage, the power supply's cost is reduced. (Although a small
transformer is still used, it's relatively inexpensive compared with the
large, heavy transformer that would otherwise be needed.)
The RCC switching circuit works by alternately energizing and de-
energizing the transformer. Oscillation begins when the voltage at the
gate of Q1 rises through kick-start resistor R18. Current passing
through Q1 enters transformer T1, energizing its primary coils. Since
diode D51 prevents current from flowing through the transformer's
secondary coils, the transformer is forced to store the energy as a
magnetic field.
After startup, Q1 is held by voltage supplied by the transformer. This
voltage is switched on and off by IC1, a programmable unijunction
transistor (PUT). As the transformer becomes magnetically saturated,
its output voltage drops below the PUT's holding threshold, and the
PUT shuts off. This deprives Q1 of voltage, causing it to shut off and
allowing the transformer to release its stored energy. As the magnetic
field collapses, current begins flowing through the transformer's
secondary coils. (Since the current's direction is reversed during
discharge, diode D51 no longer blocks its flow.) When the transformer
has fully de-energized, the cycle repeats. The resulting pulsed output
contains less energy than the original input, since the unwanted energy
has been chopped out, and can now be used for the final stepped-down
voltage. To smooth the waveform, capacitor C51 filters the output,
which is then regulated at 42 volts by a stack of seven 6-volt zener
diodes.
As discussed above, the power supply contains a built-in delay that
enables the printer to remain on long enough to complete its power-
down operations. When you turn on the printer by pressing the power
switch on the front control panel, the power control signal (PSC) from
the main board energizes transistor Q84, which acts as a relay and
turns on the power supply. At the same time, the PSC charges
capacitor C84. As a result, even when you shut the printer off, the
capacitor continues supplying power to Q84 and keeps the printer on for
at least an additional 30 seconds.
To protect the printer, the power supply incorporates over-voltage,
under-voltage, and short-circuit protection circuitry. These circuits
contain zener diodes, which monitor the regulated output voltages. If
the 5-volt supply exceeds 9 volts, or if the 42-volt supply exceeds 48
volts, the RCC switching circuit is immediately shut off and its output
falls to zero. In addition, a voltage drop protection circuit protects the
printer in case of a short circuit in any component that draws from the
42-volt line. This also works by shutting off the switching circuit, which
occurs when output falls below 36 volts. Finally, fuse F1 protects the
power supply itself, in case of a short circuit in any primary component.
+42 VDC +5 VDC
IC51
+5V Regulator
ZD81-86,ZD51
+42VDC Line
Voltage Regulator
C51
Smoothing
Capacitor
D51
Transformer (T1)
C11
Smoothing
Capacitor
Bridge
Rectifier
DB1
Figure 2-16. PSB, PSE Board Block Diagram
Chapter 2 Operating Principles
C84,Q84
Power Drop
PSC
Delay Circuit
ZD53
+5VDC Line
Over-Voltage
Monitor
ZD52,87
+42VDC Line
Over-Voltage
Monitor
ZD90
+42VDC Line
Under-Voltage
Monitor
Feedback circuit
PC1
Photo-
Coupler
Q1 Q2,Q3,Q31
Main Switching
Feedback circuit
Transistor
Fuse
Filter Circuit
L1,R1-2
F1
C1-C4
AC Input
50