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TM 9-254
CHAPTER 8
ELECTRONIC TEST EQUIPMENT
Section I. METER CHARACTERISTICS
8-1.
Types of Meters.
a.
Multimeter. The multimeter is a combination of a voltmeter, ohmmeter, and ammeter combined in one unit
using only one meter movement. Each multimeter consists of a basic direct current (DC) meter movement combined with
additional devices to serve a specific purpose. Shunt resistors are used for the ammeter, multiplier resistors for the
voltmeter, and resistors and batteries for the ohmmeter. By proper arrangement of these devices along with switches and
jacks (plug-in connections) the multimeter can be built into a small, compact, portable unit. However, multimeters have
two distinct disadvantages over other types of meters.
(1)
Circuit loading effect. The input impedance of a nonsolid state multimeter is considered low as
compared to the solid state multimeter. This is due to the shunt resistors and multiplier resistors used in
the meters internal circuitry. When the multimeter is placed into the test circuit, the low input impedance
alters the test circuit impedance. The change in the test circuit impedance will cause the measurements
to be incorrect.
(2)
Accuracy. An analog type meter movement is used in most multimeters as the indicating device. This
type of meter movement uses test circuit current to cause the pointer of the meter to move and register
a reading on the scale. Operator error can be induced into the meter indication if care is not taken to
observe the pointer from a straight forward position.
b.
Transistorized Voltmeter. The transistorized voltmeter (TRVM) is also a combination of a voltmeter,
ohmmeter, and ammeter contained in one unit. The transistorized voltmeter has two major advantages compared to a
multimeter.
(1)
Circuit loading effect. The TRVM contains four transistors arranged to form an input amplifier referred to
as a differential amplifier. This type of input amplifier gives the TRVM a high input impedance compared
to that of a multimeter. When placed into the test circuit, the high input impedance of the TRVM has
little effect on the test circuit impedance thus providing a more accurate reading.
(2)
Accuracy. Unlike the multimeter, the TRVM contains a power supply to supply current for operating the
internal meter circuitry and meter movement. However, the same analog type meter movement, that is
used in the multimeter, is also used in the TRVM. Operator error can be induced into the meter
indication if care is not taken to observe the pointer position from a straight forward position.
8-1
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