with the voltage across R1. Since R2 is connected between
the -6.2V reference supply and a point that feedback action
holds near -16mV, its current remains constant. This
current flows through R1 to produce a voltage drop across
R1 proportional to its resistance setting, thus the output
voltage of the supply is proportional to the resistance
setting of R1. At the output of the voltage comparison
amplifier (U1-1), a positive voltage change corresponds to a
decrease in the conduction of Q1.
4-16 CR2 and CR3 protect the input of the amplifier
against transient overloads, C2 and R4 speed up loop
response time, and C4 and R12 stabilize the supply's
high frequency characteristics.
4-17 OR-Gate. To permit either the voltage comparison
amplifier or the current comparison amplifier to control the
series regulator transistor, the outputs of both amplifiers
are connected to the base of driver Q2 through an OR-gate
composed of CR5 and CR6. CR5 is normally reverse
biased by a negative output from the current comparison
amplifier, permitting the voltage comparison amplifier to
drive Q2 through CR6. An overload drives the output of
the current comparison amplifier positive, forward biasing
CR5 and reducing the supply output. When the overload is
removed, CR5 is reverse biased again and the voltage com-
parison amplifier resumes control of the output.
4-18 Driver and Series Regulator. The -12.4V output of
the bias supply provides the turn-on bias for series
regulator transistor Q1. Its complete current path includes
Q15, CR59, R 14, and Q1, and returns to common
through current monitoring resistor R8. (It is because this
bias current flows through R8 that the output ammeter
requires the zero offset bias circuit described in paragraph
4-43.) Through the OR-gate, either the voltage or the
current comparison amplifier controls the conduction of
driver Q2, which regulates the flow of turn-off bias
through Q1's base-emitter circuit. The algebraic sum of the
nearly constant turn-on bias through R14 and the variable
turn-off bias through Q2 controls the conduction of series
regulator transistor Q1.
4-19 Current Limit Circuit. In the +20-volt regulator,
the current comparison amplifier compares the voltage
across current monitoring resistor R8 to the fixed voltage
across part of current limit adjust potentiometer R6. The
current limit adjustment is set so that the input voltage to
the current comparison amplifier is negative in the normal
operating region, but becomes zero when the output
current increases to 0.55 amps. When the amplifier's
input voltage reaches zero, it takes control of the
regulator output voltage and reduces it as necessary to
keep the output current from exceeding 0.55 amps.
When the overload is removed, the output of the current
comparison amplifier goes negative, reverse biasing CR5
and returning control to the voltage comparison amplifier.
4-20 Turn-On/Turn-Off Control. When the power supply
is turned on or off, Q15 in the turn-on control circuit with-
holds turn-on bias from Q1 while the regulator bias voltages
are too low. This prevents an output voltage transient from
occurring before the amplifiers are properly biased. The
output of the -6.2V reference supply is also temporarily
held at a low voltage by Q14, which conducts to short that
output.
4-21 Circuit Protection Components. Diodes CR1, CR7,
and CR9 each protect the +20-volt supply from specific
hazards. Output diode CR1 protects the supply components
if a reverse voltage is applied to the output terminals. A
common way for this to occur is for an unenergized supply
to be connected in series with another that is energized. If
the output voltage is turned down quickly while a large
capacitor is connected across the output, CR7 protects
driver Q2 from excessive dissipation by shunting some of its
base current to common. The series regulator diode, CR9,
protects the series regulator transistor from reverse voltage.
Reverse series regulator voltage could occur if a deenergized
supply were connected in parallel with an energized one.
4-23 Instead of using an NPN driver and a PNP series
regulator in the negative output line as in the +20-volt
regulator, the -20-volt regulator uses a PNP driver and an
NPN series regulator in the positive output line. The -20-
volt regulator circuit is the complementary equivalent of
the +20-volt circuit in other respects, as well. Their
current limit circuits operate similarly. At the outputs of
the current and voltage comparison amplifiers in the -20-
volt circuit, a negative voltage change corresponds to a
decrease in series regulator conduction. The turn-on bias
for its series regulator transistor, Q3, is supplied from a
positive voltage source, the +7.5V bias supply, and is
switched on and off by Q13 in the turn-on control circuit.
4-24 The -20-volt supply uses the output of the +20-
volt supply as its reference voltage. As a result, both
outputs are set by a single front panel control and track
each other with in 1% in the fixed tracking ratio mode.
Two resistors in resistor network Z1 are connected in
series between the +20volt and -20-volt outputs. These
resistors are closely matched in resistance and temperature
coefficient so that the voltage across each is exactly half
of the total. The midpoint of this divider is connected to
the non-inverting input of the -20-volt supply's voltage
comparison amplifier. The amplifier's inverting input is
connected to common through R32 to hold it at zero volts.
The amplifier keeps its differential input voltage at zero by
matching the output voltage of the -20-volt supply to that
of the +20-volt supply.
4-2
4-22 0 To -20-Volt Regulator
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