ANALOG MULTIMETER

Multimeters as the name suggest meters that used to measure multiple quantities with the same instrument. The most basic multimeters measures voltage, current, and resistance,because it is used for measuring resistance(ohm), current(ampere) and voltage(volt) can be also called as AVO meter.

Multimeters can be divided into two parts,namely Analog multimeters and Digital Multimeter.

Analog Multimeters:-

Analog multimeters were first of its type to be manufactured,but due to latest technological development now it is of less use.However, despite of such developments, its importance can’t be neglected.

Basically ,an analog multimeter is a PMMC meter.Its working principle is based on the d’Arsonval galvanometer principle and measures current,voltage and resistance of any circuit.It consists a needle to indicate the measured value on the scale.Analog multimeter works on the principle that like magnetic poles repel each other.A coil moves in a magnetic field when current passes through the coil.The indicating needle is fastened to the coil.When current passes through the coil,a deflecting torque is produced due to which the coil rotates by some angle and the pointer moves over a graduate scale.A pair of hair springs attached to the moving spindle to provides the controlling torque .In a multimeter ,the galvanometer is left zero-type instrument ,i.e needle rests to the extreme left of the scale.                                     

To measure dc current ,the meter acts as an ammeter with a low series resistance .To measure high current a shunt resistor is connected in such a way that the current flow through the meter is not more than the maximum value .by adding a shunt resistance ,an analog multimeter can be used as milli-ammeter or ammeter.

For dc voltage measurement ,the basic instrument becomes a dc measuring instrument .By adding  a multiplier resistance ,an analog multimeter can measure the voltage from milli-volts to kilovolts and this meter works as millivoltmeter,voltmeter or kilo voltmeter.

By adding a battery and resistance network,this instrument works as ohm-meter.The range of the ohm-meter can be changed by connecting the switch to a suitable shunt resistance.By selecting different values of shunt resistance ,different ranges can be obtained.

Below the basic block diagram of an ammeter is given 

Here we are using two switches namely S₁ and S₂ to select the desired meter. Additional range-selector switches may be used to select individual ranges required in reading amperes,volts, and ohms.

To measure an ac voltage or current by measuring instrument a rectifier is used.

Advantages :

•       All measurements are possible by using one meter only.
•       Increase or decrease in signal levels can be observed.

Disadvantage:

• ·       Analog meters are bulky in size.
• ·       They are heavy and costly .
• ·       The pointer movement is slow ,can’t be used to measure voltages with frequencies greater than 50 HZ.
• ·       Inaccurate due to the effect of earth’s magnetic field.
• ·       They are vulnerable to shock and vibration.