Electrical and Electronics Engineering

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ELECTRICAL INSTRUMENTS  MULTIMETER: Multimeter is one of the most important and common (instrument) aids. It is nothing but a multipurpose meter, in which all electrical parameters combined in a single unit with a common movement and a common scale known as volt-ohmmilliammeter (VOM).  D’ Arsonval Movement (PMMC) As A Galvanometer: i) The d‟ Arsonval movement (PMMC) ii) Iron vane type iii) Electrodynamometer type, but most of the Multimeter use the first type, i.e. d‟ Arsonval movement.

Fig. (3.1) shows the construction of the movement. d‟ Arsonval is the name of scientist, it is also known as PMMC “ Permanent Magnet Moving coil” mechanism (Galvanometer). It consists of a soft iron core mounted in bearings, between two poles of a permanent magnet, which are in horseshoe type. The coil is suspended so that it can rotate freely in the magnetic field. An aluminium pointer is attached to the coil assembly, one at the top and the other at the bottom. These springs serve two purposes. First, they proved a path for the current to reach the coil, secondly, when current is zero they keep the pointer at the lower end of the scale and they provide a restoring torque ( refer fig. ) when current flows through the coil. The iron core helps in making the field radial, which uniform magnetic field throughout the coil.

When current passes through the coil, the magnetic flux is produced. This flux interacts with the flux produced by the permanent magnets. Torque is produced on the coil known as a deflection torque, which results in deflection of the coil with pointer, so the pointer moves on the scale. As the coil rotates, the restoring force caused by spring goes on increasing. The coil stops its deflection at a point, where the deflection torque is equal to restoring torque. This deflection produced is proportional to the amount of current passing through the coil.

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 Basic Milliammeter: Now let us see how we can use the same movement for current measurement having I FSD  0.1mA and RM  500 . Let us convert this movement into a milliammeter.

It is obvious that maximum current through the movement is 0.1 mA for full-scale deflection, for measurement of higher current it is necessary to provide other path for excess current.

VMovement  VShunt I FSD  RM  I SH  RSH  RSH 

I FSD  RM I SH

and ( I SH  I Range  I FSD )  RSH 

I FSD  RM I Range  I FSD

In this equation for any required current range, substituting I Range the value of RSH can be easily calculated. Selecting different values of shunt resistance, a multirange millammeter facility can be produced. Fig (3.4) shows the construction of a multirange milliammeter.  Multirange Milliammeter: The circuit consists of a PMMC movement. R1, R 2, R3 are the necessary shunt resistance and „S‟ is the function switch for selecting the range. The main drawback of this method is that when switch changes its position from one range to another, during this intermediate time, no shunt resistance is present in the circuit and there is a possibility of passing an excess amount of current through the meter. It may damage the meter refer fig. (3.4)

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 Ayrton Shunt Method: To overcome the possibility of damage to the meter this method is employed.

Another speciality of this method is the advantage for higher current range; shunt resistance need not be low because the value of RM , becomes high due to the circuit arrangement. Fig. (3.5) shows Ayrton shunt or universal shunt method of a multirange milliammeter.

When the switch is at 1 mA range position, ( R1  R 2  R3 ) are in parallel with meter.

Since I FSD is 100  A (0.1 mA) the shunt resistor required to pass the current will be 1 mA-0.1 mA=0.9mA. Thus we get the equation as R1  R2  R3 

I FSD  Rm ........( I ) I Range  I FSD

For 10 mA range shunt is ( R2  R3 ) and now meter resistance is in series with R1

 R2  R3 

 R3 

I FSD   RM  R1  .......( II ) I Range  I FSD

I FSD   RM  R1  R2  .......( III ) I Range  I FSD

By solving these three equations R1 , R2 R3 (fig. 3.5) can be calculated.

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 Basic Dc Voltmeter:

Which is greater than I FSD and this should not happen, so current flowing through the meter must be reduced or excess voltage should be dropped across series resistance. i.e. across multiplier ( RS )to convert the basic movement into a voltmeter.

The basic construction of such a voltmeter is shown in fig. (3.6) Figure shows that RS and RM are in series.

Derivation:

By Kirchhoff‟s voltage law, VRange  Voltage across RS  Voltage across RM V  VS  VM

I FSD Will be the current through the circuit when voltage connected across the

terminal is 10V. RS Should be calculated for I FSD and 10V, which is the range.

V  I FSD RS  I FSD RM I FSD RS  V  I FSD RM Rs 

V I FSD

 RM ( whereV  VRange )............. formula.......(3.2)

For another voltage range we connect another multiplier Rs in the circuit.

Multirange DC Voltmeter In a Multimeter different multipliers are connected in series with the movement for different voltage measurement. Fig. illustrates the idea of multirange voltmeter.

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When the switch is,



At 10V range:

RS  R1 

V I FSD

 RM 

10  500 R1  99.5K 0.1mA

At 50V range  V  RS  ( R1  R2 )     RM  I FSD   V  R2      RM  R1   I FSD   50    500  99.5K    500 K   100 K  3    0.110  R2  400 K   At 100 V range:

 100V  RS  R1  R2  R3      RM   0.1mA  R3  1000 K   ( RM  R1  R2 ) 100  500  400 K   99.5K   0.1103  1000 K  500 K    500 K  R3 

Observe the calculations carefully, higher the voltage range the higher is the multiplier value. i.e. more voltage is dropped across the multiplier.  MULTIRANGE AC VOLTMETER:

To convert a basic d‟ Arsonval movement into a multirange AC voltmeter a rectifier circuit is required in order to convert an AC into DC. The rest part of the circuit is same as in multirange DC voltmeter. Generally, a full wave bridge rectifier circuit is preferred.

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 RESISTANCE MEASUREMENT (OHMMETER): The PMMC movement can be used to measure the resistance. Three common methods are used for the measurement of resistance known as an Ohmmeter.

A) We can connect unknown „R‟ in series with meter to test the ability of unknown R to prevent the current through it. This type of ohmmeter is known as „Series type ohmmeter‟. It is suitable for medium „R‟.

B) By connecting unknown „R‟ in shunt with meter and determining the ability of unknown R to bypass the current through the shunt path. This method is known as „Shunt type ohmmeter‟. This method is suitable only for low – value Resistance.

C) Third method is to apply a known voltage across unknown „R‟ and then to determine the current through it. The ratio of voltage to current gives the value of „R‟. This method is called as „Megger Type‟. This ohmmeter is used to measure very high value of R.  SERIES TYPE OHMMETER: Fig. (3.11) shows the construction of series type of Ohmmeter. IN this type an unknown resistor (Rx) is kept in series with the meter. For Ohmmeter; a battery is necessary which supplies a fixed current. Actually it measures current, but on the Ohmmeter scale, it is calibrated in Ohm. The scale is marked in reverse dir3ection to the scale of current and it is non-linear.

Now let us observe how this circuit measures unknown resistance. Supposes if the probes are shorted (x-x) i.e. RX  0 . The series resistor RS is adjusted suitably so that I FSD flows through the meter and it shows 0  on the scale. This adjustment is known

as “Zero adjustment”. If the probes are open it shows infinite resistance, no current flows through the movement, which is marked as  on the scale. RS is for zero adjustment because the internal battery discharges with time. So it is necessary to adjust I FSD . Multirange ohmmeter is same as current meter, a number of RS are required to get multirange ohmmeter.

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 SHUNT TYPE OHMMETER: In shunt type ohmmeter current scale and resistance scale are in the same direction i.e. zero deflection shows 0  .

But it requires an ON/OFF switch to disconnect the battery when meter is not in use.

Fig (3.12) shows a typical shunt type ohmmeter. The meter, battery and unknown resistance are in parallel. When Rx = 0  or when the probes are shorted, the meter current will be zero because whole current flows through the probes. When Rx =  or when the probes aren‟t shorted current flows through the meter only it is I FSD . Usually this type of ohmmeter is rarely used. But it is useful for determining low value resistance. IN shunt type Ohmmeter; the zero deflection corresponds to zero Ohms and full deflection to infinite Ohms. The scale is not inverted like series type.  COMPARISON: Series Type Ohmmeter

Shunt Type Ohmmeter

1. Unknown Resistance, battery and galvanometer are in series.

1. They are in parallel.

2. Current and Ohm scale are in opposite direction.

2. Ohm scale is in the same direction with Current scale.

3. It has right hand zero scale.

3. It has left hand zero scale.

4. Switch is not required.

4. Switch is required to disconnect the battery when meter is not in use.

5. Circuit diagram

5. Circuit diagram

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