Table of Contents
Advertisement
Model 3465 B
Section IV
lator and applies the seven-segment code to the display
drivers. The display drivers apply the seven-segment code to
all digits simultaneously. Digit strobe lines activate the digit
corresponding to the seven-segment code at that point in
time. Scanning of the digits is from the most significant to
the least significant digit. To complete the display, the
proper decimal point is enabled by range switching.
4-37. Power Supply.
4-38. Figure 4-4 is a block diagram of the power supply.
The power supply develops three output voltages from a
single dc output voltage (+ Vb)- This dc input voltage is
applied to a dc-to-dc converter which develops output
voltages of -t 11 V dc and - 7 V dc. A series regulated
+ 10 V output is developed from the + 11 V converter
output. This + 10 V is used as the reference voltage in the
A-D converter and to develop the reference current in the
ohms converter and as the reference voltage for the
converter regulator. The converter regulator controls the
converter and regulates the - 7 V and -t 11 V outputs of the
converter. A discussion of the operation and regulation
process of the dc-to-dc converter is presented in the
detailed theory.
SERIES
VOLTAGE
REGULATOR
dc-to-dc
CONVERTER
REGULATOR
CONVERTER
Figure 4-4. Block Diagram, Power Supply.
4-39. DETAILED THEORY.
4-40. This portion of the theory of operation provides a
detailed discussion of the circuits in the Multimeter. The
circuits described here are the ohms converter, ac—dc
converter, monopolar reference supply, data accumulator
of the logic section, display and the power supply. A
discussion of the precision resistor pack (R75) is also
provided. The detailed discussion makes use of the schema
tics in Section Vll.
4-41. Precision Resistor Pack (R75).
4-42. The precision resistor pack, R75, is a laser trimmed
substrate providing high precision resistances. A diagram of
R75 is shown on Figure 7-2. The input attenuator, power
supply, ohms reference supply. A—D reference supply and
the input amplifier require highly accurate resistances to
maintain the accuracy of the Multimeter. These resistances
are part of the resistor pack. The advantage to the resistor
pack is high precision resistors and good temperature
tracking. As resistance values of the resistor pack change
due to temperature changes, the ratio of the resistors
remains the same.
4-43. Ohms Converter.
4-44. Refer to Figure 7-2 for this discussion. Both ends of
the ohms converter are floating with respect to the
instrument ground. The unknown resistor, Rx, becomes the
feedback loop of the ohms amplifier. The ratio of Rx to
10" determines the gain of the ohms amplifier, Q25 and
U15. 10" is a variable resistance between 10 kfl and 10 MS2
selected by the range switching. The ohms converter input
is the reference voltage provided by the ohms reference
supply. This reference voltage times the amplifier gain is the
ohms converter output supplied to the input amplifier
during the run-up interval. Full-scale ohms converter gain
and output values are provided in the ohms converter table
located on Figure 7-2.
4-45. The
HI LEAD of the ohms converter is connected
to the reference supply through 10" of the resistor pack
R75. The fl HI LEAD is clamped by protection diodes
CR15 and CR25 to prevent the destruction of FET Q25
and R75 by the application of large voltages. These diodes
clamp the
HI LEAD to about 1.2 V positive or 0.7 V
negative.
4-46. With the SI HI LEAD clamped, over-voltage protec
tion must be provided to protect the ohms amplifier from
excess voltage. The over-voltage protection circuit is located
between the ohms amplifier and the LO terminal point and
is shown in Figure 4-5. During normal operation < 2 mA of
current flows through Q30, R94 and Q32. If a large voltage
is applied to the ohms terminals, the current through this
circuit will try to exceed 2 mA. This current will cause a
large enough voltage drop across R94 to turn on Q31. When
Q31 is on, it removes the base drive from Q30, which turns
off, disconnecting the LO terminal point from the ohms
converter. Since Q30 is a high voltage transistor, large
voltages are not applied to the ohms converter.
iOV
R82
LO TERMINAL POINT
FROM OHMS_
AMPLIFIER
T
1
OHMS
OUTPUT
.y.CR28
V
.?465-B-4l72
Figure 4-5. Over-Voltage Protection Circuit.
4-5
Advertisement
Table of Contents
