Epson PC AX Technical Manual page 37

REV.A
PRINCIPLES OF OPERATION
A voltage stabilization circuit repeats the above functions and stabilizes the
12V line.
If a stable Vout voltage is lo\/er than +12 volts, let down a RV2 resistor
value. If let down a resistor value, the Vref voltage decreases and the
cathode current decreases. After that, the Q3 collector current decreases and
the Vout voltage rises.
In the same way, if a stable Vout voltage is higher than +12 volts, let up the
resistor value of volume RV2.
c) Fan Revolution Speed Control Circuit
FAN
When temperature increases,
~
value of resistance decreases.
TH1
12V - - - - - . . . - - - - ,
GN D- - - - -....
-------<:)------'
FIGURE 2-2-130 FAN REVOLUTION SPEED CONTROL CIRCUIT
d) Overcurrent Prevention Circuit
If a current is supplied more than a fixed current to +12V circuit,
operational amplifier outputs high level signal and a transistor is turned on.
Next, the DB terminal voltage rises, and it causes the voltage of the output
terminal of the HA16654 not to output a pulse. When the overcurrent
prevention comes to such a condition, SCR of PC1 keeps working. To return to
the former condition, turn ON the power switch, then OFF after thirty seconds.
R26
current
R22
R38
+
FIGURE 2-2-140 OVERCURRENT PREVENTION CIRCUIT
2-9