Low-Voltage Power Supply Circuit - Canon LBP-1760 Service Manual

CHAPTER 2
D. Low-Voltage Power Supply Circuit
1. Outline
AC power is supplied to the low-voltage power supply circuit in the engine controller when the
power switch (SW101) is turned ON. The low-voltage power supply circuit supplies +24VDC and
+3.3VDC required for the printer.
+24VDC is supplied to drive the main motor, scanner motor, and fan and to the high-voltage
power supply. +3.3VDC is used for the sensors and ICs on the engine controller PCB. +24VDC
is divided into, +24VA, which is normally supplied from the low-voltage power circuit and,
+24VB, which is interrupted when the door switch (SW101) is turned OFF by opening the upper
cover.

Low-voltage power supply circuit

Fuse
( FU1 )
Fuse
( FU2 )
Noise
filter
2. Protective functions
If a shortcircuit or other problems on the load side cause an excessive current flow from the
+24VDC or +3.3VDC power supplies or generate abnormal voltage, the excess-current protec-
tion system and excess-voltage protection system automatically shut off the output voltage to
protect the power supply circuit.
If excess-current or excess-voltage protection system is activated and the power supply cir-
cuit does not output DC voltage, it is necessary to turn the power OFF, correct the problem in
the faulty load, then power the printer ON again.
The circuit has two fuses which break and cut off the output voltage if excess-current flows
through the AC line.
2 - 16
To heater control circuit
ACH
Noise
filter
Power switch
( SW1 )
Zero-cross
circuit
Transformer
Rectification
smoothing
circuit
Switching
FET ( Q1 )
Constant-voltage
control photocoupler
Control IC
( IC1 )
Error state latch
Figure 2-2-8
ACN
Rectification
smoothing
circuit
( T1 )
Rectification
smoothing
circuit
Constant-
voltage
control
circuit
( PC2 )
Protective
photocoupler
circuit
( PC1 )
/ZEROX
To CPU
+24VA
+3.3V