Epson PX-8 Technical Manual page 46

REV.-A
The collector of 05 is connected to the VB line (+5V) through inductance L 1 and the emitter is
grounded. When the transistor repeats switching on and off, therefore, a voltage as shown in
Fig. 2-1 5 appears at the collector. This voltage is filtered by the electrolytic capacitor, C3,
through the diode D1 to a voltage that exceeds VB. This results in a potential difference across
the emitter, and base of the transistor 04, which causes the transistor to conduct, outputting a
DC voltage of approximately 7V at its collector.
Because this output is connected to the constant-voltage circuit, consisting of the resistors R13
and R25, and the zener diode ZD5, the actual output voltage is fixed at +5V by the 5V break-
down voltage of the zener diode. When the 04 output voltage rises above +5V, ZD5 breaks
down at +5V, putting transistor 022 in conduction, which forces the switching signal to ground
level. The output voltage is always maintained at +5V by disabling the switching of 05. Varia-
tion of load is handled by the relatively large capacitance of capacitor C8 (220 J,lF), connected
at the collector of 04.
Clock 35 kHz
Switching pulse
at 14D, pin 14
Q5 emitter
Q4 collector
(before filtered)
5V
1 - - - - - - - - - - '
I
G
1 - - - - - - - - - ' Zener breakdown
Fig. 2-15 +5V Regulator Voltages
2-22