values, paragraphs 4-15 through 4-21, which describe the
voltage comparison amplifier, OR-gate, driver, series regu-
lator, current limit circuit, turn-on control, and circuit
protection components of the +20-volt regulator circuit,
also apply to the +18-volt regulator. In the +18-volt
regulator, as in the +20-volt regulator, the turn-on bias
current for the series regulator transistor is switched on and
off by Q15 in the turn-on control circuit to prevent output
voltage transients.
4-35 Reference and Bias Supply
4-36 The reference and bias supply powers the
operational amplifiers and provides the bias and reference
voltages used throughout the supply. A shunt zener
regulates its +7.5V output. A series transistor regulates its
-12.4 V output, using 6.2-volt zener VR1 as its voltage
reference. The -12.4V output provides a constant current to
VR1, which is the primary voltage reference for the entire
supply.
4-37 Two equal resistors are connected in series
across the -12.4V output. To regulate this output,
voltage comparison amplifier U4 compares the voltage
across one of these resistors to the -6.2V reference and
controls the conduction of series regulator Q11 through
driver Q12. The voltage drop across Q11 is controlled
by feedback so that the voltages at the two inputs of U4
remain equal. Driver Q12 controls Q11 by shunting part
of the base bias supplied by R68.
4-38 During turn-on, the -6.2V reference supply is
temporarily shorted by Q14 in the turn-on control circuit.
By trying to match this low reference, Q11 is initially turned
off. While Q11 is turned off, R69 bypasses current to the
-12.4 V output until the output reaches -9 volts and the
turn-on control circuit removes the short from the reference
and enables the -12.4-volt regulator to operate normally.
4-39 Turn-On/Turn-Off Control Circuit
4-40 Immediately after the supply is energized and
until the output of the -12.4-volt regulator reaches about
-9 volts, the turn-on control circuit withholds turn-on bias
from series regulator transistors Q1, Q3, and Q7 and holds
the -6.2V reference at a low value. This prevents an output
voltage transient by ensuring that the operational amplifiers
are energized and other essential bias voltages are present
before the series regulator transistors are turned on. The
circuit also prevents an output transient when the supply is
turned off by removing the turn-on bias from the series
regulators and shorting the -6.2V reference supply as the
voltage of the -12.4 V supply falls below -9 volts.
4-41 Q13 switches the bias to the -20-volt regulator
on and off, Q14 switches the short across the -6.2-volt
reference supply, and Q15 switches the bias to the +20-
volt and +6-volt or +18-volt regulators. Q15 remains
turned off until VR2 conducts at 9 volts to switch it on.
While Q15 is off, it holds Q13 biased off and Q14 on;
when Q15 conducts, it turns Q13 and Q14 off.
4-42 Meter Circuits
4-43 Voltmeter. Two of the resistors in resistor
network Z1 are range resistors for the voltmeter. The
accurate ratio of these resistors permits a single calibration
potentiometer, R58 to adjust both ranges simultaneously.
4-44 Ammeter. The range switch connects the
ammeter across the current monitoring resistor of a supply:
R48 in the +6-volt or +18-volt supply, R8 in the +20-volt
supply, or R28 in the -20-volt supply. Each of these
resistors conducts a constant bias current for its series
regulator transistor in addition to the supply's output
current. If no compensation were used, this additional
current would raise the indicated output by up to 8% of full
scale. The resistor networks connected to each range of the
ammeter selector switch apply a bias to the meter to offset
this error. R59 calibrates all ammeter ranges.
4-4
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