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4-29 Reference Supply
4-30 The reference supply is a small regulated bias
source which is similar to the main power supply. It
contains a series regulator (Q1) and a comparison am¬
plifier (U1). The three reference voltages are all derived
from raw dc obtained from a full-wave rectifier (CR1
and CR2) and filter capacitor Cl. The -F6.2V and
-7.5V outputs are developed across Zener diodes
VR1 and VR2. The reference Zener diode (VR2) is
temperature compensated. Resistor R4 limits the cur¬
rent through VR2 to establish an optimum bias level.
4-31. The regulating circuit consists of U1, Ql, and
associated components. The voltage across the reference
zener (+6.2 V with respect to +S} and the voltage at
the junction of divider R2 and R3 are compared and
any differences are detected by U1. The error voltage
is Then amplified and applied to series regulator Ql to
counteract the difference and maintain the +12.4 V
output constant. Output capacitor C3 stabilizes the
reference regulator feedback loop to prevent oscillation.
4-32 Zener diode VR1 provides an additional reference
voltage of —7.5 V. This voltage is not as tightly regu¬
lated as the other two reference voltages since it is not
controlled by the regulator circuit. The unregulated
+23 V, taken directly from the rectifier, is used only
by Option 011 or 104 crowbar circuits.
4-33 Additional Protection Features
4-34 The supply contains several "special purpose"
components which protect the supply in the event of
unusual circumstances. One of these is the output
diode (CR9 or U5; refer to applicable schematic). Con¬
nected across the output terminals, this diode prevents
internal damage from reverse voltages that might be
applied during system operation when one power sup¬
ply is turned on before another. The output diode can
withstand currents equal to the maximum current
rating of the supply for time periods of short duration
(approximately 3 minutes); or up to half the current
rating of the supply for longer time periods.
4-35 The series regulator diode (CR8 or U5) protects
the regulating transistor(s) from the effects of a reverse
current which would occur if an external voltage were
applied across an unenergized supply.
4-36 Sensing protection resistors R32 and R33 pre¬
vent the load from receiving full rectifier voltage if the
connections between the output terminals and the
sensing terminals (+S and —S) are removed inadvertently.
4-37 Thermal switch TC1 opens if the heat sink tem¬
perature exceeds a safe value. With TC1 open, the ac
input path to the supply is broken, shutting down the
supply. The switch automatically resets after the sup¬
ply has cooled to a temperature below that at which
it originally opened. The opening and closing tempera¬
tures for the switch are given in the individual parts
list for each supply.
4-4
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