Table of Contents
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DC VOLTAGE MEASUREMENT
Figure 4 shows a simplified diagram of the DC voltage
measurement function. The input voltage divider resistors
add up to 1 megaohm. Each step down divides the voltage
by a factor of ten. The divider output must be within the
range –0.199 to +0.199 volts or the overload indicator will
function. The overload indication consists of a 1 in the most
significant digit and blanks in the remaining digits.
AC VOLTAGE MEASUREMENT
Figure 5 shows a simplified diagram of the AC voltage
measurement function. The AC voltage is first rectified and
passed through a low pass filter to smooth out the
waveform. A scaler reduces the voltage to the DC value
required to give the correct RMS reading.
CURRENT MEASUREMENT
Figure 6 shows a simplified diagram of the current
measurement function. Internal shunt resistors convert the
current to between –0.199 to +0.199 volts which is then
processed in the 7106 IC to light the appropriate LCD
segments. When current in the range of 10A is to be read,
it is fed to the 10A input and does not pass through the
selector switch.
RESISTANCE MEASUREMENT
Figure 7 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 the selector
switches), and the test (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.
Overall operation of the A/D converter during a resistance
measurement is basically as described earlier with one exception. The
reference voltage present during a voltage measurement is replaced by
the voltage drop across the reference resistor. This allows the voltage
across the unknown resistor to be read during the read period.
200mV
Volts
900kΩ
2V
90kΩ
20V
9kΩ
200V
900Ω
600V
100Ω
Common
Figure 4
Simplified DC Voltage Measurement Diagram
Rectifier
Volts
Low Pass
Filter - Scaler
900Ω
100Ω
Common
Figure 5
Simplified AC Voltage Measurement Diagram
200μA
2mA
A
20mA
200mA
10A
10A
Common
Figure 6
Simplified DC Amps Measurement Diagram
Ω
Test
Resistor
Common
Simplified Resistance Measurement Diagram
-14-
Low Pass
Filter
100mV
REF
Low Pass
Filter
200V
100mV
REF
600V
900Ω
Low Pass
Filter
100Ω
200μA
2mA
20mA
9Ω
200mA
10A
.99Ω
.01Ω
Low Pass
Fuse
100Ω
900Ω
2MΩ/Dio
200Ω
9kΩ
20kΩ
90kΩ
200kΩ
2mΩ
900kΩ
Voltage
Source
Figure 7
7106
7106
7106
100mV
REF
Filter
7106
Reference
Voltage
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