Tektronix 4112 Service Manual page 319

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Schmitt Trigger
Refer to Schematic A3.
The Schmitt trigger turns on power to the Pulse Width
Modulator circuit when the output of the Logical On/ Off
switch charges C379 to 21 V. Once the + 24 V supply
comes up, it ordinarily holds C379 to that level. If the
voltage falls below 11 V, however. the Schmitt trigger
snaps off and shuts off the power supply.
cs
4 +CS
5
-CS
CURRENT LIMITING
SON 10
SON
8 GNO
rh rh
THEORY OF OPERATION
Pulse Width Modulator
The Pulse Width Modulator circuit regulates the
+
5 V
supply by varying the on time of the switch ing transis-
tors. It also provides preliminary regulation for the other
supplies.
Figure 5-5 is a simplified block diagram of the Pulse
Width Modulator LC. and associated components . An
oscillator alternately enables two output transistors
while the error amplifier determines their duty cycles.
Transistor
QA
controls one switching transistor
(Q521 )
through Pin 11 and the Base Drive circuit. Similarly, Q
0
controls the other switching transistor (0515) through
Pin 14. The RC timing circuit is set for twice the
inverter frequency of
20
kHz.
COMP
9
3132· i •
Figure 5 - 5. Pulse Width Modulator l.C.
620-0295-00 POWER SUPPL Y
5-7