Over Voltage Protection Circuit - Epson LQ-1050 Technical Manual

2.2.4.4 Over Voltage Protection Circuit

As described in 2.2.4.3, the power supply circuit of this unit not only uses the output voltage stabilizing
circuit to stabilize the output voltage (Vo) and provide feedback to the switching circuit, but it also
contains an over voltage protection circuit to protect against malfunctions of the constant voltage
generation circuit.
Figure 2-16 shows the over voltage protection circuit.
If the output voltage stabilizing circuit malfunctions and the output voltage (Vo) goes over the limit:
4 0
=
then the photodiode in PC2 receives current. Consequently, the gate of the thyristor (SCR: TY 1 ) on the
receiver side conducts to pass the base current of Q 1
at reduces to O V and no voltage is induced in the secondary side winding Tt=.
The power supply must be switched off to reset the protection circuit.
—
I
o
[ V ] ,
R 15
R5
I
c 14
c
Fig. 2-16. Over Voltage Protection Circuit
to ground. Q 1 is cut off so that the potential
, D 2 0
—
c
22
2 - 1 7
c
+
I I
REV.-A
V o