T ERM PAPER

OF

ELECTRICAL SCIENCE-2

TOPIC:- Electrical and electronics measuring instruments and cathode ray oscilloscope.

SUBMITTED BY SUBMITTED TO

NAME- BITTU KUMAR LECT. NITIKA WADHWA

SECTION-E2801 (DEPARTMENT OF ELECTRICAL SCIENCE)

ROLL NO. – 46

REGISTRATION NO. -10808479

CONTENTS

CHAPTER-1.ELECTRONICS AND ELECTRICAL MEASURING INSTRUMENT.

1.1.INTRODUCTION

1.2.INDICATING INSTRUMENTS

1.2.1 ESSENTIAL OF INDICATING INSTRUMENT

1.3 TYPE OF INDICATING INSTRUMENT

1.3.1 PERMANENT MAGNET MOVING COIL INSTRUMRNT

1.3.2 ELECTRODYANOMETER TYPE WATTMETER

1.3.3 INDUCTION TYPE ENERGY METER

1.4 MEASUREMENT OF RESISTANCE

1.4.1 WHEATSTONE BRIDGE

CHAPTER-2. CATHODE RAY OSCILLOSCOPE

2.1 INTRODUCTION

2.2 CRO OPERATION

2.3 CRO CONTROLS

REFERENCES

CHAPTER1. ELECTRICAL AND ELECTRONICS MEASURING INSTRUMENTS

1.1. INTRODUCTION

The instrument which are used to measure the electrical quantities like current voltage, power, energy , etc. are known as electrical measuring instrument can be divided broadly as absolute and secondary instruments. Absolute instrument are those which give the value of the quantity to be measured in term of the deflection and instrument constant. The tangent galvanometer and Rayleigh’s current balance are few example of absolute instrument. These instrument are used in the laboratories and institution as standardizing instrument.

Secondary instrument are those, in which the value of the quantity to be measured in determine form from the deflection of the pointer, as the scale is calibrated in the term of the unit quantity. These instruments are commonly used in the laboratory, institution, industries, and the power station. They are further classifying into three group, that is indicating instruments, recording instrument, and integrating instruments.

Indicating instrument are those which indicate and the magnitude of the measure quantity at a time. Instrument like ammeter, voltmeter, etc. are the some example of the indicating instruments. recording instruments are those which give the continuous record of the variation of the measuring quantity over a specified period of the time.

Integrating instrument are those which measure the total amount of the electrical quantity supplied over a period of time. Energy meter and ampere hour meter are few example of the integrating instrument.

Indicating instrument, their essential devices and the types of the indicating instrument are discussed. Permanent magnet moving coil and moving iron

instrument are explained in detail. Electrodynamometer wattmeter induction type meter, meggar and multimeter are discussed, so that students get conversant with all the instrument to be used in the laboratory for experimentation.

1.2 INDICATING INSTRUMENTS

An indicating instrument indicates the magnitude of a quantity being measured on a graduate scale. The moving system of such an instrument is fitted with a pointer that moves over the calibrated scale indicate the reading.

The weight and the inertia of the pointer is kept low in order to reduce the load on the bearing and necessary damping torque. The swing of the pointer on the scale is limited buffer or stop to a little more than range og the scale. To avoid bending of the pointer, when it strikes the stop sharply on sudden overloads or reversal current, the stop are constructed as very light springs. The scale of these instruments is normally printed on the enamelled surface of the metal plate. Measuring instrument of indicating type should not alter the circuit condition, when these are connected in a particular circuit for measuring a certain quantity. They should draw minimum possible power for their operation. The moving system of the instrument should be light which is possible by using aluminium. The frictional forces of these instrument are reduces to minimum by using a spindle, mounted between jewel bearing and also by balancing the system properly.

1.2.1 ESSENTIAL OF INDICATING INSTRUMENT

Indicating instrument must process the following three essential devices for their satisfactory operation

1. A deflecting device producing a mechanical forces by electric current, voltage or power to move the pointer from its zero position

2. A controlling devices to produce a mechanical force equal and opposies to the deflecting force.

3. A damping device prevents oscillation of the moving system and enable it to its equilibrium position rapidly.

A). DEFLECTING DEVICE

A deflecting device produce the deflecting force for moving the pointer from its zero position. The deflecting force can be produced by any one of the following effects.

1. magnetic effect – generally for ammeter and voltmeter

2. Heating effect - for ammeter and voltmeter

3.Electrodynamic effect – for ammeter voltmeter, and wattmeter

4.Electromagnetic effect – for dc ampere hours

5.Electrostatic effect – for voltmeter only

The deflecting devices measuring instrument converts the quanity to be measured (electric current or potential) into a mechanical force generally known as deflecting force which is responsible for the deflecting of the pointer.

B). CONTROLLING DEVICES:-

The controlling devices of an indicating instrument serves the following purposes

1. Produces a force equal and opposites to the deflecting forces, so that the pointer deflects to a definite position for the particular magnitude of current . in the absence of a controlling devices, deflecting of the moving system would be indefinite.

2. The bring the moving system back to its zero position, when the cause is removed. In the absence of the controlling devices the Pointer of the instruments once deflecting will not back to its zero position an the removing the current.

Contolling devices can be broadly classify into two parts

1. Spring control (commonly used in the modern instrument).2. Gravity control (not much used modern instrument).

1.3 TYPE OF INDICATING INSTRUMENTS:-

1.3.1 PERMANENT MAGNET MOVING COIL INSTRUMENT

Permanent magnet moving coil (PMMC) ammeter and voltmeter are used for measuring current and voltage respectively in dc system. These instrument are most accurate for dc measurement.

The compass and conducting wire meter can be considered a fixed-conductor moving-magnet device since the compass is, in reality, a magnet that is allowed to move. The basic principle of this device is the interaction of magnetic fields-the field of the compass (a permanent magnet) and the field around the conductor (a simple electromagnet).

A permanent-magnet moving-coil movement is based upon a fixed permanent magnet and a coil of wire which is able to move, as in figure . When the switch is closed, causing current through the coil, the coil will have a magnetic field which will react to the magnetic field of the permanent magnet. The bottom portion of the coil in figure will be the north pole of this electromagnet. Since opposite poles attract, the coil will move to the position shown in figure.

A permanent-magnet moving-coil movement is based upon a fixed permanent magnet and a coil of wire which is able to move, as in figure.

When the switch is closed, causing current through the coil, the coil will have a magnetic field which will react to the magnetic field of the permanent magnet. The bottom portion of the coil in figure will be the north pole of this electromagnet. Since opposite poles attract, the coil will move to the position shown in figure.

Figure. 1- A movable coil in a magnetic field (no current).

Fig.1.a movable coil in magnetic field fig 2. coil in moving magnetic field

The coil of wire is wound on an aluminum frame, or bobbin, and the bobbin is supported by jeweled bearings which allow it to move freely. This is shown in figure2.

Figure 3. - A basic coil arrangement.

To use this permanent-magnet moving-coil device as a meter, two problems must be solved. First, a way must be found to return the coil to its original position when there is no current through the coil. Second, a method is needed to indicate the amount of coil movement.

The first problem is solved by the use of hairsprings attached to each end of the coil as shown in figure 3. These hairsprings can also be used to make the electrical connections to the coil.

With the use of hairsprings, the coil will return to its initial position when there is no current. The springs will also tend to resist the movement of the coil when there is current through the coil. When the attraction between the magnetic fields (from the permanent magnet and the coil) is exactly equal to the force of the hairsprings, the coil will stop moving toward the magnet.

Figure 4. - Coil and hairsprings.

As the current through the coil increases, the magnetic field generated around the coil increases. The stronger the magnetic field around the coil, the farther the coil will move. This is a good basis for a meter.

But, how will you know how far the coil moves? If a pointer is attached to the coil and extended out to a scale, the pointer will move as the coil moves, and the scale can be marked to indicate the amount of current through the coil. This is shown in figure 4.

Figure 5. - A complete coil.

Two other features are used to increase the accuracy and efficiency of this meter movement. First, an iron core is placed inside the coil to concentrate the magnetic fields. Second, curved pole pieces are attached to the magnet to ensure that the turning force on the coil increases steadily as the current increases.

The meter movement as it appears when fully assembled is shown in figure 6.

Figure 6. - Assembled meter movement.

This permanent-magnet moving-coil meter movement is the basic movement in most measuring instruments. It is commonly called the d'Arsonval movement because it was first employed by the Frenchman d'Arsonval in making electrical measurements. Figure 1-10 is a view of the d'Arsonval meter movement used in a meter.

Figure 7. - A meter using d'Arsonval movement.

ADVANTAGE AND DISADVANTAGE OF PMMC INSTRUMENTS

A). ADVANTAGE

The major advantage in using coil instrument for the measurement of current and voltage are

1.High sensitivity

2. Uniform scale

3. Well sheielded from any stray magnetic field

4. Low power consumption

5. No hystreresis

B). DISADVANTAGE

1.More expensive than the moving coil instrument.

2. Can be used only for dc measurement.

3. Develop error due to ageing of control springs and permanent magnet.

1.3.2 ELECTRODYANOMETER TYPE WATTMETER

Electric power is measured by means of a wattmeter. This instrument is of the electrodynamometer type. As shown in figure 3-18, it consists of a pair of fixed coils, known as current coils, and a moving coil, called the voltage (potential) coil. The fixed current coils are wound with a few turns of a relatively large conductor. The voltage coil is wound with many turns of fine wire. It is mounted on a shaft that is supported in jeweled bearings so that it can turn inside the stationary coils. The movable coil carries a needle (pointer) that moves over a suitably graduated scale. Coil springs hold the needle at the zero position in the absence of a signal.

A). ADVANTAGE

1.Accurately measure both ac and dc power.

2. Uniform scale

3. Can be used as standard meter calibration.

B). DISADVANTAGE

The inductance of a moving coil can cause error specially at low power factors.

1.Effects of stray magnetic fields.

1.3.3 INDUCTION TYPE ENERGY METER

Energy meter is an integrating meter which measure electrical energy consumed by a load. Induction type energy meter are very commonly used to measure electrical energy consumed in domestic, commercial and industrial installation. These energy measure in KWh.

The magnetic field produced by shunt electromagnet is pulsating in character cuts through the rotating disc and induces eddy currents there in, but normally does not in itself produce any driving force. Similarly series electromagnet induces eddy currents in the rotating disc, but does not in itself produce any driving force. In order to obtain driving force in a single phase meter, phase displacement of 900 between the magnetic field set up by shunt electro-magnet and applied voltage V is achieved by adjustment of the copper shading band,

also known as power factor compensator or compensating loop. The reaction between these magnetic fields and eddy currents set up a driving torque in the disc which would revolve at very high speed in the absence of any opposing force.

1.4 MEASUREMENT OF RESISTANCE:-

1.4.1 WHEATSTONE BRIDGE METERA Wheatstone bridge is a measuring instrument invented by Samuel Hunter Christie in 1833 and improved and popularized by Sir Charles Wheatstone in

1843. It is used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. Its operation is similar to the original potentiometer except that in potentiometer circuits the meter used is a sensitive galvanometer.

Wheatstone's bridge circuit diagram.

In the circuit on the right, Rx is the unknown resistance to be measured; R1, R2

and R3 are resistors of known resistance and the resistance of R2 is adjustable. If the ratio of the two resistances in the known leg (R2 / R1) is equal to the ratio of the two in the unknown leg (Rx / R3), then the voltage between the two midpoints (B and D) will be zero and no current will flow through the galvanometer Vg. R2 is varied until this condition is reached. The direction of the current indicates whether R2 is too high or too low.

Detecting zero current can be done to extremely high accuracy (see galvanometer). Therefore, if R1, R2 and R3 are known to high precision, then Rx

can be measured to high precision. Very small changes in Rx disrupt the balance and are readily detected.

At the point of balance, the ratio of R2 / R1 = Rx / R3

Therefore,

Alternatively, if R1, R2, and R3 are known, but R2 is not adjustable, the voltage difference across or current flow through the meter can be used to calculate the value of Rx, using Kirchhoff's circuit laws (also known as Kirchhoff's rules). This setup is frequently used in strain gauge and resistance thermometer

measurements, as it is usually faster to read a voltage level off a meter than to adjust a resistance to zero the voltage.

First, Kirchhoff's first rule is used to find the currents in junctions B and D:

Then, Kirchhoff's second rule is used for finding the voltage in the loops ABD and BCD:

The bridge is balanced and Ig = 0, so the second set of equations can be rewritten as:

Then, the equations are divided and rearranged, giving:

From the first rule, I3 = Ix and I1 = I2. The desired value of Rx is now known to be given as:

If all four resistor values and the supply voltage (VS) are known, the voltage across the bridge (VG) can be found by working out the voltage from each potential divider and subtracting one from the other. The equation for this is:

This can be simplified to:

SIGNIFICANCE

The Wheatstone bridge illustrates the concept of a difference measurement, which can be extremely accurate. Variations on the Wheatstone bridge can be used to measure capacitance, inductance, impedance and other quantities, such as the amount of combustible gases in a sample, with an explosimeter. The Kelvin double bridge was specially adapted from the Wheatstone bridge for measuring very low resistances.

CHAPTER#2. CATHODE RAY OSCILLOSCOPE

2.1 INTRODUCTION:-

The cathode-ray oscilloscope (CRO) is a common laboratory instrument that provides accurate time and aplitude measurements of voltage signals over a wide range of frequencies. Its reliability, stability, and ease of operation make it suitable as a general purpose laboratory instrument. The heart of the CRO is a cathode-ray tube shown schematically in Fig. 1.

The cathode ray is a beam of electrons which are emitted by the heated cathode (negative electrode) and accelerated toward the fluorescent screen. The assembly of the cathode, intensity grid, focus grid, and accelerating anode (positive electrode) is called an electron gun. Its purpose is to generate the electron beam and control its intensity and focus. Between the electron gun and the fluorescent screen are two pair of metal plates - one oriented to provide horizontal deflection of the beam and one pair oriented ot give vertical deflection to the beam. These plates are thus referred to as the horizontal and vertical deflection plates. The combination of these two deflections allows the beam to reach any portion of the fluorescent screen. Wherever the electron beam hits the screen, the phosphor is excited and light is emitted from that point. This coversion of electron energy into light allows us to write with points or lines of light on an otherwise darkened screen.

In the most common use of the oscilloscope the signal to be studied is first amplified and then applied to the vertical (deflection) plates to deflect the beam vertically and at the same time a voltage that increases linearly with time is applied to the horizontal (deflection) plates thus causing the beam to be deflected horizontally at a uniform (constant> rate. The signal applied to the verical plates is thus displayed on the screen as a function of time. The horizontal axis serves as a uniform time scale.

The linear deflection or sweep of the beam horizontally is accomplished by use of a sweep generator that is incorporated in the oscilloscope circuitry. The voltage output of such a generator is that of a sawtooth wave as shown in Fig. 2. Application of one cycle of this voltage difference, which increases linearly with time, to the horizontal plates causes the beam to be deflected linearly with time across the tube face. When the voltage suddenly falls to zero, as at points (a) (b) (c), etc...., the end of each sweep - the beam flies back to its initial position. The horizontal deflection of the beam is repeated periodically, the frequency of this periodicity is adjustable by external controls.

To obtain steady traces on the tube face, an internal number of cycles of the unknown signal that is applied to the vertical plates must be associated with each cycle of the sweep generator. Thus, with such a matching of synchronization of the two deflections, the pattern on the tube face repeats itself and hence appears to remain stationary. The persistance of vision in the human eye and of the glow of the fluorescent screen aids in producing a stationary pattern. In addition, the electron beam is cut off (blanked) during flyback so that the retrace sweep is not observed.

2.2 CRO OPERATION:- A simplified block diagram of a typical oscilloscope is shown in Fig. 3. In general, the instrument is operated in the following manner. The signal to be displayed is amplified by the vertical amplifier and applied to the verical deflection plates of the CRT. A portion of the signal in the vertical amplifier is applied to the sweep trigger as a triggering signal. The sweep trigger then generates a pulse coincident with a selected point in the cycle of the triggering signal. This pulse turns on the sweep generator, initiating the sawtooth wave form. The sawtooth wave is amplified by the horizontal amplifier and applied

to the horizontal deflection plates. Usually, additional provisions signal are made for appliying an external triggering signal or utilizing the 60 Hz line for triggering. Also the sweep generator may be bypassed and an external signal applied directly to the horizontal amplifier.

2.3 CRO CONTROLS The controls available on most oscilloscopes provide a wide range of operating conditions and thus make the instrument especially versatile. Since many of these controls are common to most oscilloscopes a brief description of them follows.

A.)CATHODE-RAY TUBE

Power and Scale Illumination: Turns instrument on and controls illumination of the graticule.

Focus: Focus the spot or trace on the screen.

Intensity: Regulates the brightness of the spot or trace.

B). VERTICAL AMPLIFIER SECTION

Position: Controls vertical positioning of oscilloscope display.

Sensitivity: Selects the sensitivity of the vertical amplifier in calibrated steps.

Variable Sensitivity: Provides a continuous range of sensitivities between the calibrated steps. Normally the sensitivity is calibrated only when the variable knob is in the fully clockwise position.

AC-DC-GND: Selects desired coupling (ac or dc) for incoming signal applied to vertical amplifier, or grounds the amplifier input. Selecting dc couples the input directly to the amplifier; selecting ac send the signal through a capacitor before going to the amplifier thus blocking any constant component.

C). HORIZONTAL-SWEEP SECTION

Sweep time/cm: Selects desired sweep rate from calibrated steps or admits external signal to horizontal amplifier.

Sweep time/cm Variable: Provides continuously variable sweep rates. Calibrated position is fully clockwise.

Position: Controls horizontal position of trace on screen.

Horizontal Variable: Controls the attenuation (reduction) of signal applied to horizontal aplifier through Ext. Horiz. connector.

D).TRIGGER

The trigger selects the timing of the beginning of the horizontal sweep.

Slope: Selects whether triggering occurs on an increasing (+) or decreasing (-) portion of trigger signal.

Coupling: Selects whether triggering occurs at a specific dc or ac level.

Source: Selects the source of the triggering signal.

INT - (internal) - from signal on vertical amplifier EXT - (external) - from an external signal inserted at the EXT. TRIG. INPUT. LINE - 60 cycle triger

Level: Selects the voltage point on the triggering signal at which sweep is triggered. It also allows automatic (auto) triggering of allows sweep to run free (free run).

REFERENCES

1. BASIC ELECTRICAL ELECTRONICS ENGINEERING2. ELECTRICAL ELECTRONICS MEASURING INSTRUMENT3. ELECTRICAL ENGINEERING FUNDAMENTALS