Construction and Working of Multimeter

Construction and Working of Multimeter Points : construction and working of multimeter, definition of multimeter The ammeter, the voltmeter and the ohmmeter all use a basic D. Arsonval movement. The difference between these instruments is the circuit in which the basic movement is used. It is therefore, obvious that an instrument can be designed to perform these three measurement functions. This instrument which contains a function switch to connect the appropriate circuits to the D, Arsonval movement, is called a “Multimeter” or Amper-volt-ohm meter. Some times it is also called volt-ohm-milli-ammeter (V.O.M.).

This is a multi-purpose instrument as its name indicates, it is used for measurement of current (d.c. and a.c), voltage (d.c. and a.c.) and resistance. With the external source, high resistance above (1 mΩ), inductance and capacitance can also be measured with this instrument. Construction & Working The “heart” of the meter consists of a d.c. moving coil instrument taking very low current of the order of 1 mA for full scale deflection. On d.c. a tapped universal shunt is used for current measurement and a tapped series resistance for voltage measurements. On a.c. a transformer feeds into a metal rectifier, the output from which goes to the d.c. movement. Thus, the same scales are used for d.c. and a.c. measurements. The resistance are measured by applying a set of voltage from an internal battery to the resistance under test measuring the resultant current. Since the resultant current is, by “Ohm’s Law”, inversely proportional to the resistance, so the scale can be calibrated directly in Ohms, Kilo and mega-ohms.