Low-Voltage Shutdown Circuit; Low-Voltage Rectifier-Filter - Tektronix 213 DMM Instruction Manual

Theory of Operation-213 Service
OFF
ON
0488
BATTERIES :
T
T- ton
1..--T-I
LOW
VOLTAGE
SHUTDOWN
AST ABLE
0470
0475
+15 VOLT
REGULATOR
0460,0465
0474
T490
CRT
RECTIFIER
FILTER
RECTIFIER
Fl L TER
-1000 V
HV
MULTIPLIER
t----+-+
75 V
.,__ ...... _+15 V
+6.5 V
-6.5 V
1481-15
Fig. 3-8. Power Supply detailed block diagram.
10 microseconds. 0484 operates when 0486 is turned off,
and it is used to remove the store charge at the base of 0488
to make it turn off quicker. CR472 and C472 prevent the
multivibrator from blocking, which would prevent it from
oscillating when initially turned on.
Low-Voltage Shutdown Circuit
The low-voltage shutdown circuit operates as a battery con-
dition indicator and battery deep-discharge protector. The
light-emitting diode (LED) labeled PWR (BTRY) is controlled
by this circuit, and indicates a low battery level when it is
not lit and the instrument is turned on. If the instrument is
not turned off when the LED extinguishes on a low battery
level, this circuit terminates instrument operation. This pro-
tects the batteries against damage that could be caused by
tleeµ tliscllarye.
When the battery is fully charged and the instrument is
turned on, pin 2 of U450 is about +2.4 volts and pin 3 is
about 2.12 volts. This causes a negative output from U450
which turns CR450 on, but does not affect SCR 0458.
When the battery voltage falls below +2.12 volts, the output
of U450 begins to rise and CR450 turns off. The battery
3-12
voltage continues to decrease until it reaches about +2.02
volts, at which time the output of U450 reaches about +4.5
volts and CR454 starts to conduct. This provides positive
feedback to U450 and causes its output to rapidly increase
toward +15 volts. The rapid increase in the output of U450
causes a positive step on the gate of 0458 that turns it on,
which effectively grounds the junction of R470 and R473,
and causes the oscillation of the astable multivibrator to
stop. When the multivibrator stops oscillating, all power
supply voltages collapse and the instrument stops operating.
0458 remains on until the POWER switch is turned OFF
and then ON to reset 0458.
LOW-VOLTAGE RECTIFIER-FILTER
~
All low-voltage power supply voltages begin as outputs from
T490 except the ±6.5 V supplies. When flyback of T490
occurs, approximately 7 volts is present at the collector of
0488 which is rectified and filtered to produce the ±6.5
volts. The +15 and +75 volt outputs of T490 are rectified
and filtered by conventional networks.
CR543, CR544, CR552, and CR554 are hot carrier diodes
which provide low power dissipation because of reduced
@