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Meter Usage
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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