Analog Multimeter

What is a Multimeter?

Analog multimeter or simply called a multimeter, is an electronic instrument used for testing of electrical or electronic circuits of an installation. Multimeters are commonly used by all electronic technicians and engineers for testing of batteries, household wiring, electric motors, power supplies etc.

Types Of Multimeters

All multimeters can be broadly classified in three categories –

  1. Analog multimeters.
  2. Digital multimeters.
  3. Clamp meters.

Analog Multimeter

Analog multimeter is a multi functional device that operates based on electromechanical movement. Basically, it is constructed of a moving coil galvanometer. The galvanometer coil is wound around an aluminium core which is pivoted on two jeweled bearings. The drum is placed between the poles of two permanent magnets. The drum is connected with a pointer to indicate the reading on a graduated scale.

There are two spiral springs attached to the coil assembly at the top and bottom, which provide a path for the flow of current and controlling the torque.

An analog multimeter is shown in figure with details of probe lead sockets and range selection dial.

When a current passes through the coil, a magnetic field is induced in the coil. This field reacts with the magnetic field of permanent magnet. This interaction produces a resultant force causing the pointer attached to the drum to deflect on the scale.

The drum is also attached to a returning spring which provides an opposing force to the motion of the drum to control the deflection of the pointer. The pointer moves as a result of flow of a current through the coil.


Use of Analog Multimeter

A multimeter is capable of versatile use. It can measure voltage, current, and resistance for which its galvanometer is converted as a voltmeter or an ammeter or an ohmmeter. This conversion is possible with the help of suitable circuits and elements incorporated inside it. A multimeter is mainly used for the following purposes –

  1. To measure the AC or DC voltage between two relevant points in a circuit. Thus, a multimeter can be used as a voltmeter.
  2. For measuring of AC or DC current in a wire in the circuit. Thus, a multimeter can be used as an Ammeter.
  3. To measure the resistance of an element in a circuit. Thus, a multimeter can be used as an Ohmmeter.
  4. A multimeter is also used to test continuity between two points in an electrical circuit.

As per their functions, a multimeter is also known as AVO (i.e., Amperemeter-Voltmeter-Ohmmeter) or VOM (i.e., Voltmeter-Ohmmeter-Milliammeter) etc.

A block diagram showing internal elements and connections of a typical analog multimeter is shown in figure.


Voltage Measurement

Galvanometer of the multimeter has a current sensitivity of the order of 0.1 \ mA and a small internal resistance of R_{Shunt} = 500 Ω ohms.

To measure a high voltage, meter range is required to extended by connecting a high resistance in series with the galvanometer coil using resistors R_{Multiplier-1}, \ R_{Multiplier-2}, \ R_{Multiplier-3} etc. as shown in figure. A rotary switch is provided to select different ranges of parameters to be measured. Some sockets indicating the parameter range are provided in a multimeter for inserting probe leads.

While making, measurement one lead is inserted in the Common Socket and the other lead is inserted in the required voltage or current range socket. If DC voltage has to be measured, then the rotary switch is set to DC voltage range. The probe lead is inserted in the voltage socket.

The multimeter can also measure AC. For this purpose, a rectifier is incorporated in the circuit. The rectifier converts AC into DC before application to the galvanometer coil.

Current Measurement

For measuring a current, the galvanometer is converted into an ammeter by connecting a small resistance in parallel with the meter, as shown in figure.

The range of ammeter is extended within limits by reducing the value of shunt resistance R_{Shunt} . For this purpose, some low resistances are connected in parallel with the meter through a selector switch. The desired range can be selected by moving the selector switch to a particular position.

Resistance Measurement

For measuring resistance, the instrument is used as an ohmmeter. In this circuit, an internal battery is connected in series with the meter through an adjustable resistance and the fixed resistances.

The resistance to be measured is connected between test leads. The current flowing through the circuit depends upon the resistance of the test piece.

To measure the resistance, a suitable range is selected by using R_{Multiplier} . Then the meter leads are shorted and variable resistance is so adjusted to give full-scale deflection. Under this condition, the resistance between test leads is found zero. Then the resistance under measurement is connected between test leads.

Sensitivity of Analog Multimeter

The sensitivity of a multimeter may be defined as the resistance offered per unit volt of full scale deflection. If the sensitivity of a multimeter is high, it means it has high internal resistance R_{Shunt} .

When a high sensitive meter is used to measure a voltage, it will draw negligible current from the circuit. Thus, it will measure the correct voltage. The sensitivity of a multimeter varies from 8000 Ω per volt to 20000 Ω per volt.

Disadvantages of Analog Multimeter

Use of a analog meter require great mathematical skills because user require to make quick calculations. Thus it takes more time for doing mathematical calculations.