HP 740B Operating And Service Manual page 33

Section IV
CURRENT LIMIT Control is set to MIN (fully ccw),
R12 is set at maximum resistance, causing a larger
voltage drop across the R121AlORllAlOR8 combina-
tion for a given output current. The Current Limit
Adjustment (AlOR8) sets the range of the CURRENT
LIMIT Control.
4 -77. Output voltage also affects the Current Limit
Detector. At low voltage outputs, the sensitivity of
the circuit depends almost entirely on the current
through R12, A10R1 and A10R8 and the circuit is
calibrated to limit output current to approximately
50 mA when the CURRENT LIMIT Control is set to
MAX. At high output voltages, positive prebias volt-
age from AllR4 (Figure 7 -3) starts to forward bias
the emitter-base junction of A5Q6. This increases
the sensitivity of the Current Limit Detector causing
the maximum output current to decrease to approxi-
mately 20 mA when the output voltage reaches 1000 V.
4-78. MAIN LOOP FEEDBACK DIVIDER (Figure 7 -3).
4 -79. The closed loop voltage gain accuracy of the Main
Loop is almost entirely determined by the accuracy of
the Main Loop Feedback Divider. The divider controls
the Main Loop Gain on all ranges and functions as an
inputattenuatorfor the Meter Loop on all ranges above
1 V.
4-80. The Main Loop Feedback Divider consists of
A9R4 through A9R12 which comprise a 3 Mn decade
divider. A9R4 throughA9R9 are precision wirewound
resistors that are factory matched to have similar
temperature coefficient characteristics. A9RlO,
A9Rll and A9R12 calibrate the divider for precision
decade division ratios of 10:1, 100:1 and 1000:1.
4-81. ZERO CONTROL CIRCUIT (Figure 7-3).
4-82. The Zero Control circuit functions to set the
Main Loop zero reference in all modes of operation.
The Zero Control circuit is comprised of AlOBT1, the
front panel ZERO Control (R13) and several other re-
sistors. 1. 4 V from A10BTl is dropped across
A10R6, AlOR7 and A10R13. Part of this series
string is paralleled by SlR9, SlR10, SlR8 and R13
(ZERO Control). Varying R13 causes the voltage
across SlR10 to vary approximately ±15 JJ.V, intro-
ducing a small offset voltage in the Main Loop feed-
back path. AlOBTl discharges constantly through the
resistors that comprise the Zero Control Circuit. Dis-
charge current is extremely low - - approximately
0.2 JJ.A -- and the service life of the cell approaches
normal shelf -life. The variable offset can be used
to compensate for small external offset voltages
existing in a measurement setup or to set the Main
Loop input or output to exactly zero.
4-83. REFERENCE LOOP.
4-84. The Reference Loop supplies a stable, accurate
voltage to the Main Loop input in Standard mode. In
Differential Voltmeter mode, the Main Loop feedback
voltage (or a portion of it) is compared to the Re-
ference Loop output voltage. The Reference Loop is
comprised of the 1 V Reference Supply and the Re-
ference Divider which is operated by the front panel
VOLTAGE SET and STANDARD VERNIER Controls.
4-8
Model 740B
4-85. 1 V REFERENCE SUPPLY (Figure 7 -13).
4-86. The Reference Oven Assembly (A13) contains
the Control Differential Amplifier (A13Q3, A13Q4) and
the Oven Heater and Regulator Circuits.
4-87. A13CRl is a pre-aged Zener diode selected to
have excellent stability characteristics. A13CRl
establishes the reference voltage for the Control Dif-
ferential Amplifier (A13Q3 and A13Q4). Also, the
voltage across A13CR1 is used to develop the + 1 V
output of the 1 V Reference Supply. The operating
voltages for the Control Differential Amplifier are
generated by the +34 V Supply
(p/o A6)
and pre-
regulated by A2Q14 and A2Q15. The output from
A13CR1 is dropped across the Reference Adjustment
resistors
(p/o
AI), S3Rl and S3R2. The REF COARSE
and FINE adjustments (AlR40 and AlR4l) set the
voltage at the junction of S3R1 and S3R2 to +1 V.
4-88. The Oven Heater and Regulator Circuits func-
tion to keep the interior temperature of the Reference
Oven Assembly at a constant 80
0
C. This allows the
reference diode, A13CR1, to develop a very stable
output voltage, regardless of ambient temperature.
4-89. The Temperature Sensing Bridge is balanced
only when the oven temperature reaches 80
0
C. A
termistor, A13R7, functions ,a:s the heat-sensing
element. When the bridge is not balanced, the voltage
at the collector of A13Q2 changes. This change is
amplified by the Error Amplifier, A2Q12 and A2Q13.
The output from the collector of A2Q13 is fed to the
Heater Control transistor, Q3. Q3 controls the cur-
rent through the Oven heating element, HR1. The cur-
rent through HR1 is increased or decreased according
to the temperature Sensing Bridge output until the
oven tem perature stabilizes at 80
0
C, balancing the
bridge.
4 -90. The A13 circuit com ponents are factory matched
and aged. For this reason, the components inA13
are not separately replaceable.
4-91. REFERENCE DIVIDER (Figure 7 -14).
4-92. The Reference Divider supplies a 0 to + 1. 00000
V output in Standard mode; 0 to +0.99999 V output in
Differential Voltmeter mode. The divider consists of
the five front panel VOLTAGE SET switches, the
STANDARD VERNIER switch and some padding re-
sistors on AI.
4-93. The + 5. 9 V output of the 1 V Reference Supply
at Al pin 22 (Figure 7-13) is dropped across S3Rl and
S3R2. The 10kn resistance of the Xl VOLTAGE SET
divider is in parallel with S3R2, causing the parallel
resistance of S3R2 and the Xl divider to be 1 kn.
The voltage across the parallel resistance is a stable
and accurate 1 V.
4-94. Regardless of how the resistors in the Xl di-
vider are switched, there will be a total of 9 kn within
the Xl divider which will make from 0 to + 0.9 V in
0.1 V steps available at the Xl divider output. The
remaining 0.1 V is dropped across the parallel com-
bination of S4R5 and the XlO divider resistance.
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