MM0474
(2) The voltage at the base of A1Q8B determines the current output to
the POWER METER jack. This voltage is developed across a precision resistance
selected by the RANGE switch.
The RANGE switching circuit consists of 11
precision wire-wound resistors, each padded by an adjustable potentiometer
(resistors A1R27 through A1R51). Since each resistance can be set precisely by
an individual pad potentiometer, the current through any selected resistance is
known to a great degree of precision. The current at the POWER METER output is
carefully controlled to be the same as the current through the selected range
resistance. The Darlington-combination of transistors A1Q6 and A1Q7 results in
a high forward current gain. This gain minimizes any noncancellation effects
of current flowing from the collector of A1Q8B to the base of A1Q6 and the
current flowing from the emitter of A1Q7 to the base of A1Q8B.
(3) The voltage necessary at the base of A1Q8B to generate the range
currents for a 100 ohm thermistor mount is 17.750 volts.
The corresponding
voltage for a 200 ohm mount is 12.551 volts. Both voltages are positive with
respect to the -30 VDC power supply voltage. Division of voltage for the two
mount operating resistances is accomplished by A1R17 (17.750 VDC ADJUST),
A1R18, and A1R19 (12.551 VDC ADJUST).
(4) When the 431C Power Meter is used with a 100 ohm balanced thermistor
mount, 10 kHz bias signal appears at the Calibrator POWER METER connection.
Transformer A1T1 presents a high impedance to the 10 kHz signal, thus
preventing interaction between it and the calibrator circuit.
c. Regulated Power Supply.
(1) The power supply schematic is shown in Figure 6. The power supply
circuit consists of a full-wave bridge, A1CR1 through A1CR4, series regulator
Q1, emitter follower A1Q1, differential amplifier A1Q2-A1Q3, and associated
circuits. Initial filtering of the negative rectified voltage is accomplished
by C1 and the voltage is isolated from chassis ground by C2.
The negative
voltage is presented to the series regulator, Q1.
The series regulator is
referenced to a common ground by a temperature-compensated Zener diode, A1VR1.
A balanced differential amplifier, A1Q2-A1Q3, is used to compare the regulated
output voltage to the Zener reference. Current for the Zener reference diode
is taken from the regulated -30 volt side of the power supply.
(2) For explanation, assume that diode A1CR6 is removed from the circuit
and an open circuit is substituted in its place.
When the power supply is
turned on, both the reference voltage at the base of A1Q2 portion of the
differential amplifier and the power supply output voltage are zero. A zero
potential at the bases of the differential amplifier transistors A1Q2-A1Q3
prevents them and both the emitter follower, A1Q1, and the series regulator,
Q1, from conducting. In this state, the power supply is stable with an output
voltage of zero. Now
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