Service manual
FO-IS125N
5 – 26
[3] Circuit description of power supply PWB
This power supply unit has the function to convert the AC120V (50/
60Hz) to DC24V, and provide these outputs to the control PWB. The
following explains the function of each block. (See Fig. 17)
1. Filter circuit block
This circuit reduces the outgoing noise through the input lines which is
generated in the power supply unit, and prevents the invasion of the
noise from the lines. (the excessive surge such as the thunder is pre-
vented by the varitstor (Z1).)
2. Rectification and smoothing circuit block
This circuit rectifies and smoothes the AC input, and provides the DC
voltage to the switching circuit block.
3. Switching circuit block
This circuit converts the DC voltage (provided from the Rectification
and smoothing circuit block) to the high-frequency pulse voltage by
FET (Q1)’s switching (on/off repeat), and provides the energy to the
transformer (T1). It discharges the energy (charged during the FET
ON time) to the secondary side during the FET OFF time through the
secondary windings. The output voltages on the secondary side pro-
vided by the energy depend on the ratio of the winding turns (primary:
secondary) etc.
4. Control circuit block
This circuit block controls the output voltage by transmitting the
detected 24V voltage to the primary control circuit through the photo-
coupler (PC1). In case of the over-current, this circuit reduces provid-
ing the energy to the transformer. In case of the over-voltage, this cir-
cuit reduces providing the energy to the transformer by letting the
Power-Zener (D104; connected between the 24V output voltage and
GND) into the short mode and letting the over-current protection circuit
work.
5. +24V output circuit block
This circuit block rectifies and smoothes the high-frequency pulse volt-
age provided by the transformer, and provides the DC24V output to
the control PWB. The output voltage is adjusted by the variable resis-
tor(VR101).
6. +24V MAIN output circuit block
This circuit block supplies DC24V output to control PWB through a
connector (CN2) from 24V provided by the transformer.
7. +24VS circuit block
This circuit block supplies DC24V output to +24VS through a connec-
tor (CN5) from 24V MAIN.
8. AC output circuit block
This circuit block supplies AC output from AC input to control PWB
through the optical-isolator (PC2) and the power-relay (RL1) by sig-
nals (+24VS /HEATER ON).
9. Zero cross circuit block
This circuit block rectifies the AC input, and provides the ZC signal to
control PWB through the photo-coupler (PC3).
Fig. 17
(1) FILTER
CIRCUIT
BLOCK
(2) RECTIFICATION
AND SMOOTHING
CIRCUIT BLOCK
(3) SWITCHING
CIRCUIT
BLOCK
(4) CONTROL
CIRCUIT BLOCK
(7) +24VS CIRCUIT
BLOCK
TRANSFORMER
INPUT
AC OUT
(8) AC OUTPUT
CIRCUIT BLOCK
POWER RELAY
OPTICAL ISOLATOR
CN5
+24VS
HEATER ON
+24V MAIN
+24V SUB
Ach
GND
ZC
PHOTO-COUPLER
(9) ZERO CROSS
CIRCUIT BLOCK
(5) +24V CIRCUIT
BLOCK
(6) +24V MAIN OUTPUT
CIRCUIT BLOCK
CN2