Power Supply Circuit Operation - Epson FX 1170 - B/W Dot-matrix Printer Service Manual

2.2.2 Power Supply Circuit Operation

Figure 2-22 shows the power supply circuitry in block diagram form. When AC power enters the printer from
an external power source, the filter circuit removes the noise. The AC voltage then undergoes full wave
rectification and is smoothed to produce direct current. The voltage is then fed through the switching circuit
and the secondary smoothing circuit to produce a stepped down +35 VDC voltage. A +35 VDC detector circuit
is connected to the switching circuit. This feedback control arrangement ensures a stable +35 VDC voltage
supply.
A+5VDCvoltage is generated by feeding the +35 VDCvoltagethrough the +5VDC power supply circuit, where
the +35 VDC is stepped down to a stable +5 VDC voltage. The switching regulator IC containing the
The +5 VDC voltage overload protection circuit cuts the +5 VDC line output if the voltage exceeds +7 VDC. It
stops the operation of the switching circuit and then the output of +35 VDC voltage.
voltage overload protection circuit cuts the +35 VDC line output if the voltage exceeds +36 VDC. It stops the ""
operation of the switching circuit and then the output of the +35 VDC voltage. The voltage drop protection
circuit protects the printer against a sudden voltage drop that maybe caused by a short circuit in the seconda~
circuitry of the +35 VDC line. If a voltage drop is detected, it stops the operation of the switching circuit and
then the output of the +35 VDC voltage.
An external switch is used to turn the printer power on or off. When the power switch is turned off, the
switching circuit is de-energized and the output of the +35 VDC voltage is stopped. However, since the switch
is in the secondary circuitry, the current continues to flow in the primary circuitry as long as the printer remains
plugged into an external AC power source. For this reason, you must disconnect the printer from the external
AC power source. Unplug the power cable from the AC power outlet before you perform any maintenance
work.
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