Power Supply - Philips PM 5775 Manual

POWER SUPPLY (Fig. XIV-2)
From socket P4 the mains voltage is applied to the primary winding of mains
transformer T1, via mains switch S11 and voltage adapter $12. The voltage adapter
has two positions: 115 V and 230 V (4 15 %).
The secondary winding of T1 has a centre tapping which is connected to earth,
and full-wave rectification is obtained by means of diode bridge Z3. The voltages
on points 6 and 3 are +32 V and —30 V respectively.
The voltage on point 6 is fed to the collector of a series regulator transistor Z1 by
means of which the stabilised +20 V is obtained.
The stabiliser circuit consists of zener diode Z110, resistor R108, transistor Z109
and resistors R109, RV101 and R110.
Across the zener diode a reference voltage appears which is fed to the emitter of
Z109. The base of Z109 is connected to a voltage divider, and the voltage at the
collector will consequently change in accordance with the change of the +20 V
output voltage. The collector of Z109 is connected to the base of driver transistor
Z101. Z101 is an emitter-follower, whose emitter is connected to the base of
regulator transistor Z1. In this way the +20 V output voltage is stabilised.
C101 and C105 are filter capacitors. R101 is connected in the collector circuit of
Z101 in order to limit the current through the transistor in case the +20 V would
be short-circuited.
The stabilising circuit for the —20 V output voltage is almost equal to the circuit
described above. However, because the same type of regulator transistor (Z2) is
used — i.e. NPN-type - the zener diode is connected between +20 V and —20 V.
Apart from the two voltages +20 V and —20 V a voltage of +5 V is available.
The +5 V is obtained from the sum of the voltage across Z119 (4.3 V) and the
base-emitter voltage of Z111 (0.7 V). If the +5 V voltage changes the base-emitter
voltage of Z111 will change, and the current through Z111 will also change.
Because the collector of Z111 is connected to the base of Z112 the voltage of
+5 V will be regulated to its initial value. The regulation can only be effected with-
in a certain limited range because part of the current through the voltage divider
also flows through resistor R116 which is connected in parallel with Z112.
By this arrangement transistor Z112 is protected against overload.