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THEORY OF OPERATION
3-1
GENERAL CIRCUIT DESCRIPTION
The regulation is accomplished in a manner which is
similar to vacuum tube type circuits . A power type
transistor in series with the rectified output and the
load, acts like a variable resistor which maintains a
constant output voltage or current as selected by the
controls.
The power transistor is controlled by a
two stage amplifier which amplifies any changes in
the relative amplitudes of the output voltage and the
reference voltage.
The electronic short circuit current limiting switch
is a unique feature which is not normally found in the
vacuum tube counterpart of the supply. This circuit
senses any increase in current above a pre-selected
value and in turn controls the conduction of the
power transistor to limit the peak current to the
pre-selected value.
A detailed description of the
Power Supply and its operation follows. Refer to
the schematic diagram to identify the various com¬
ponents.
3-2
MAIN AND AUXILIARY SUPPLY
DESCRIPTION
Transformer T1 supplies ac voltages to the main
and auxiliary supplies.
The main supply consists
of silicon rectifiers CR1 and CR2 and capacitor C2.
This supply furnishes about 43 volts to the regulator
circuit. Silicon rectifier CR3, CR4 and capacitors
C3, C4 and C5 supply -20 volts, which is required
for operation of the control circuits.
Regulator transistor Q1 acts as a variable series
resistance to lower the voltage to the desired value,
as set by the front panel VOLTAGE ADJUST control
R19. Q1 conducts more current when the base volt¬
age goes more negative with respect to the emitter.
3-3
REFERENCE VOLTAGE
CR7 is a reversed biased diode operating in the
break-down condition. The diode maintains a con¬
stant nominal 7 volts across itself, establishing a
constant reference voltage between the negative out¬
put lead and the base of Q4.
Q4 is an emitter
follower which repeats the reference voltage at its
emitter terminal, less a constant internal base-
emitter drop of about 0.2 volt. The voltage at the
emitter has a low source impedance, making it
insensitive to normal variations in current flow.
The
output voltage from the (+) output bus is
sampled by VOLTAGE ADJUST control R19 which
causes a current flow through R20 and R21. The
regulator
action
maintains a constant
current
through R19 and R20.
3-4
REGULATION CYCLE DESCRIPTION
Assume the output level has been set with R19 and
some change has occurred which causes the output
voltage to rise.
The voltage at the base of Q3 is
that which would appear across a forward biased
diode and is essentially constant.
The electron
current flow through R20 is constant.
When the
output voltage rises, part of the normal electron
flow into the base of Q3 is diverted through R19.
The reduced base-emitter electron current of Q3
reduces the collector-emitter electron
current
flow from R17 by a factor of approximately 100.
Since fewer electrons flow into the collector of Q3
from
R17
and thte - 16 volt bus, the voltage at
R16 goes in a negative direction.
This causes
more electrons to flow through R16 into the base
of Q2. Increased Q2 base to emitter current causes
much higher collector-emitter current. Increased
Q2 collector current raises the voltage at R5 which
reduces the base to emitter current of Ql.
The
reduced base-emitter current of Ql increases its
collector-emitter resistance to electron current
flow, hence increases its collector to emitter volt¬
age drop. That voltage drop increases just enough
to compensate for the initial output voltage rise,
maintaining the output voltage at a constant level.
3-5
SHORT CIRCUIT CURRENT LIMITING
CIRCUIT
The current flow to the load is sensed by a voltage
drop across Rll ABCD.
Silicon diode CR5 is
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