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RESISTANCE MEASUREMENTS
Figure 5 shows a simplified diagram of the resistance measurement function.
A simple series circuit is formed by the voltage
source, a reference resistor from the voltage divider
(selected by range switches), and the external
unknown resistor. The ratio of the two resistors is
equal to the ratio of their respective voltage drops.
Therefore, since the value of one resistor is known,
the value of the second can be determined by using
the voltage drop across the known resistor as a
reference. This determination is made directly by
the A/D converter.
h
MEASUREMENT
FE
Figure 6 shows a simplified diagram of the h
measurement function. Internal circuits in the 7106
IC maintain the COMMON line at 2.8 volts below
V+. When a PNP transistor is plugged into the
transistor socket, base to emitter current flows
through resistor R49. The voltage drop in resistor
R49 due to the collector current is fed to the 7106
and indicates the h
of the transistor. For an NPN
FE
transistor, the emitter current through R50 indicates
the h
of the transistor.
FE
CAPACITANCE MEASUREMENT
The capacitor circuit consists of four op-
amps. IC3 D& A form an oscillator, which is
applied to the test-capacitor through the test
leads. The capacitor couples the oscillator
to pin 6 of IC3B. The amount of voltage
developed at pin 6 is indicative of the
capacitors ESR value. IC3B and C amplify
the signal which is seen at pin 8. The AC
signal is then converted to a DC voltage and
displayed on the meter.
100
900
9k
20k
External
Resistor
90k
200k
900k
2M
9M
20M
Figure 5 Simplified Resistance Measurement Diagram
Low Pass
Filter
2k
200
100mV
Ref
Voltage
Source
Overall operation of the A/D converter during a
resistance measurement is basically as described
earlier in this section, with one exception.
reference voltage present during a voltage
measurement is replaced by the voltage drop
across the reference resistor.
voltage across the unknown resistor to be read
during the read period. As before, the length of the
read period is a direct indication of the value of the
unknown.
FE
220k
COM
-4-
7106
V+
PNP
NPN
E
C
R50
220k
B
B
C
E
10
Figure 6
Figure 7
The
This allows the
Low Pass
Filter
7106
100mV
Ref
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