Epson LQ-2550 Technical Manual page 79

V Current Limiting and Voltage Regulator Circuits
2.2.5.1 +5
The + 5 V current limiting and voltage regulation is performed by controlling the ON/OFF time of -.
switching transistor Q 1.
Current Limiting
Figure 2-25 shows the + 5 V current limiting circuit.
The +5 V current limiting is controlled by
Reference voltage Vr.fs which is obtained by dividing the + 5 V output by R24 and R23 is supplied to
the plus side (pin 3) of IC 1. Voltage
detection resistor R20, is supplied to the minus side (pin 2) of IC 1.
As the load at R20 increases,
+ 12 V (HIGH) to O V (LOW)
of the photo transistor in photo coupler PC 1 constant. At this time, the photo diode and photo transistor
of PC 1 turns on, transistor Q2 turns on, and Q 1 turns off.
The collector current for the transistor of PC 1 is supplied by D3, C 10, and the primary side of coil T1 ~s.
a comparator (IC 1 ).
T1
10
3
I I
R3
3'
I
L -- -- -
P c l
Figure 2-25. +5 V Current Limiting Circuit
which is proportional to the current flowing through current
by Zener diode ZD 1 and R 14. ZD 1 keeps the voltage at the cathode side
L3
m -
(
1
1
2-32
T
I
+ 5
. . . .