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Section IV
Paragraphs 4-5 to 4-17
4-5. Alternating current from the autotransformer
is rectified in a bridge rectifier CRI-4. The output
of the bridge is a full-wave rectified ac voltage. This
voltage is filtered by the large capacitor C1. Thus
the voltage supplied to the series regulator is almost
pure direct current.
4-6. SERIES REGULATOR.
4-7. The series regulator consists of two transistors
in series, one of which is driven by the series regu-
lator driver Q3.
The other transistor is driven by
Q11, the third stage in the amplifier. The action of
these circuits is explained in paragraphs under these
titles.
4-8. OUTPUT CIRCUIT.
4-9. Resistor R18 is connected between the output of
the series regulator and the negative output terminal.
An ammeter connected across R18 is calibrated to
read the current to the load. Voltmeter M2 is con-
nected across the output and reads the output voltage.
The output is available on the front panel terminals
and on the rear terminal strip for remote application.
4-10. VOLTAGE REGULATING CIRCUIT.
4-11. The voltage regulating circuit consists of the
FINE VOLTAGE adjustment R24 (refer to figure 4-1),
the COARSE VOLTAGE adjustment R25, the amplifier
circuit (QI3, Q12 and Q11 and associated circuitry)
and the series regulator Q2.
The diode limiters,
CR20 and CR21, limit the input to the amplifier in both
the positive and negative directions. Consider what
happens when the output voltage drops. This drop may
be a slow shift in output voltage or an ac signal on the
output.
AC signals are fed to the amplifier through
C5 and R40. This low-impedance path increases the
loop gain for ac signals. When the voltage on the base
of Q13 (see figure 4-3) becomes more positive (less
negative as when the output of the supply drops) Q13
draws more base current since the emitter-base bias
is increased (Q13 is a npn-type transistor).
This
signal is amplified and reversed in phase.
When
coupled to Q12, this reversed signal again causes
more current to flow since the forward bias is again
increased (Q12 is a pnp-type transistor).
4-12. Diodes CR18 and CR19 are zener diodes which
provide direct coupling between the stages in the
amplifier and maintain proper collector voltages on
the transistor elements. These diodes are kept in the
zener breakdown condition by the currents flowing in
resistors R35, R36, R38, and R39.
The signal is
amplified and inverted in Q12.
Capacitor CIS, in-
ductance Ll and resistor R31 in the emitter circuit
of Q12 help stabilize the loop.
The emitter of Q12
is connected through these components to a point which
is separated from the positive output terminal by re-
sistor R33. Load current through this resistor injects
current feedback into the loop which improves the load
regulation.
The inverted signal from Q12 tends to
4-2
Model 722A
reduce the base current in Q11 (a pnp-type transistor).
The signal is amplified and inverted in Q11 and fed to
the base of Q2.
This signal tends to increase the
current through Q2 (another pnp-type transistor).
Increasing the current through Q2 increases the volt-
age out of the supply.
Since the original action was
a decrease in voltage this action will tend to restore
the original conditions, regulating the supply.
4-13. The dc output voltage is set by varying the
COARSE and FINE VOLTAGE controls R24 and R25.
In explaining this circuit, resistor R51 can be thought
of as a constant-current source.
Most of this cur-
rent normally flows out through R24 and R25 to the
series regulator with only an insignificant amount
going to the base of Q13. Thus as the resistance of
R24 and R25 is increased the voltage on the negative
output lead will go further negative since the constant
current through these resistors will develop a greater
voltage drop.
4-14. CURRENT-POWER LIMITER.
4-15. Transistor Q4 plus associated circuitry forms
a protective circuit which limits the current and power
dissipation in the series regulators and load. Refer-
ring to figure 4-3, no current flows through Q4 under
normal conditions since the base is biased positive
with respect to the emitter.
The maximum current
available for the base of Q2 is fixed by the resistors
RIO and Rll and the reference supply. Normally, the
current not used by the base of Q2 to supply a certain
load current passes through the collector of Ql1.
4-16. When the instrument is in the current-limiting
condition, it is essentially a constant-current supply
rather than a voltage supply.
The voltage derived
across R18 is coupled to the base of Q4 through CR8
and R14. As the load current increases, this voltage
increases to a level where Q4 begins to conduct. Any
attempt to increase the load current further merely
turns Q4 on harder, depriving Q2 of the necessary
base current to sustain this larger load current. This
action tends to keep the load current constant. Since
the output voltage is below the desired output voltage
in the current-limiting condition, the amplifier turns
off Ql1.
With Q11 turned off the only paths for the
current through RIO and R11 are into the base of Q2
and the collector of Q4.
4-17. Referring to figure 4-3, as the load resistance
is further decreased the output voltage decreases
since the current is held constant. This tends to in-
crease the voltage across the series regulator. When
this voltage reaches a certain value, the zener diode
CR6 will start to conduct.
This current appears to
Q4 to be due to increasing load current and Q4 tends
to conduct more current away from the base of Q2 thus
decreasing the load current and limiting the maximum
dissipation in the series elements.
Diode CR8 acts
as a 0.7 volt battery since it is forward biased at all
times by the current through R12. Resistor R14 ad-
justs the lower limit of the current limit range while
R20 adjusts the upper limit.
The CURRENT LIMIT
potentiometer R5 adjusts the current limit to any value
between the upper and lower current limits.
01155-1
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