Model Viva Questions for Electrical & Electronic measurement Common to: ET&T III SEM

Title of the Practical: Study of different parameters and Errors in measurement

Q1 what is measurement? The process of determining the amount, degree, or capacity by comparison (direct or indirect) with the accepted standards of the system units being used.

Q2 what is accuracy of a measurement? The degree of exactness (closeness) of a measurement compared to the expected (desired) value.

Q3 what is Resolution of a measurement? The smallest change in a measured variable to which an instrument will respond.

Q4 what is precision of a measurement? A measure of the consistency or repeatability of measurement, i.e. successive reading do not differ.

Q5 what is Sensitivity of a measurement? The ratio of the change in output (response) of the instrument to a change of input or measured variable.

Q6 what is Error of a measurement? The deviation of the change in output (response) of the instrument to a change of input or measured variable.

Q7 what are the three major categories of error? Error may arise from different sources and are usually classified as 1 Gross error 2. Systematic error 3. Random error

Q8 what is gross error? These errors are mainly due to human mistakes in reading or in using instruments or error in recording observations. This error also occurs due to incorrect adjustment of instruments and computational mistakes these errors can not treat mathematically.

Q9 what is Systematic error? These error occur due1. Instrumental error 2. Environmental error 3. Observational error

Q10 what is instrumental error? Ans: The instrumental errors arise due to three main reasons:

1. Due to inherent shortcoming in the instrument 2. Due to misuse of the instruments 3. Due to loading effect of instrument

Title of the Practical: Study of CRO.

Q1 what is Cathode ray oscilloscope (CRO)? Ans: Cathode ray oscilloscope is a instrument used for display, measurement and analysis of waveforms and other phenomenon in electrical and electronic circuit

Q2 what are the basic component of a CRO? Ans: CRO Circuit consists of following components: 1. Vertical deflection system 2. Horizontal deflection system 3.Synchronization circuit 4. Blanking circuit 5 Intensity modulation 6position control 7. Focus control 8. Cathode ray tube 9. Calibration circuit

Q3 what is the function of probe in CRO? Ans: The probe performs the very important function of connecting the test circuit to oscilloscope without altering, loading or otherwise disturbing the test circuit.

Q4 how many types of probe used in CRO? Ans: There are three types of probe used in CRO: 1 Direct probe 2. Isolation probe 3. Detector probe

Q5 what are the functions of different probes used in CRO? Ans: Direct Probe: direct probe avoids stay-pick up which may create problems when low level signals are being measured. It is used for low frequency or low impedance circuit. Isolation probe: Isolation probe is used in order to avoid the undesirable circuit loading effects of the shielded probe. Detector probe: when analyzing the response to modulated signals used in Communications equipment like AM, FM and TV receivers, the detector probe functions to separate the low frequency modulation component from the high frequency carrier.

Q6 what is the function of Attenuator in CRO? Ans: The voltage in input terminal of the vertical amplifier causes the beam to deflect off the CRT screen, is quite low in amplitude. So that high amplitude signals may be displayed, an attenuator network is placed between the vertical input terminals of the vertical amplifier. The main function of the attenuator is to reduce the amplitude of the vertical input signal before applying it to vertical amplifier.

Q7 which device is used for the source of emission of electrons in a CRT? Ans: A barium and strontium oxide coated cathode is used for the source of emission of electrons in a CRT.

Q8 what is the function of Aquadag in a CRO? Ans: An Aqudage is used in a CRO to collect secondary emission electrons.

Q 9 what is the function of electron gun assembly used in CRT? Ans: The electron gun assembly produces a sharply focused beam of electrons which are accelerated to high velocity .this focused beam of electrons strikes the fluorescent screen with sufficient energy to cause a luminous spot on the screen.

Q 10 what is the function of electron gun used in CRT? Ans: The source of focused and accelerated electrons beam is the electron gun. The electron gun emits electrons and forms them into a beam consist of a heater, a cathode, a grid, a pre-accelerating anode, a focusing anode and an accelerating anode.

Title of the Practical: Study of Recorders.

Q1 what do you mean by a recorder? Ans: A recorder is a device which records an electrical and non electrical quantity as a function of time.

Q2 what are the types of recorder? Ans: there are two types of recorder: 1 Analog recorder 2. Digital recorder

Q3 what is an analog recorder? Ans: if the system has analog output then analog recorders are used. There are three types of analog recorder: 1. Graphic recorder 2. Oscillographic recorder 3. Magnetic tape recorder

Q4 what is the function of a graphic recorder? Ans: Graphic recorder records the history of some events, information in graph format. It uses a chart for displaying and storing the recorded information and a stylus which is moving in the chart with proper relationship to source of information

Q5 what are the types of graphic recorder? Ans: graphic recorders are two types: 1. Strip chart recorder 2. X-Y recorder

Q6 what is the function of a Strip Chart recorder? Ans: A strip chart recorder records one or more variables with respect to time. In a strip chart recorder a long roll of graph paper is moving vertically and a system for driving the paper at some selected speed, a stylus for making marks on the moving graph paper. The stylus moves horizontally in proportional to be quantity being recorded.

Q7 what is the function of a X-Y Chart recorder? Ans: In X-Y recorder one variable is plotted against other variables i.e. an X-Y recorder records one variable as a function of second variables. In X-Y recorder one self balancing potentiometer circuit moves a recording pen in the X direction while another self balancing potentiometer circuit moves the recording pen in the y direction at right angle to X-directions.

Q8 what is the function of a Magnetic tape recorder? Ans: Magnetic tape recorder is used to record high frequency signals. the magnetic tape recorder records the data in such a way that they can be reproduced in electrical form again.

Q9 what are the basic component used in a magnetic tape recorder? Ans: the basic component of a magnetic tape recorder is: 1. recording head 2. Magnetic head 3. Reproducing head 4. Tape transport mechanism 5. Conditioning devices

Q10 what is the function of recording head? Ans: recording head convert the electrical input signal into magnetic information. The information which have to record is first converted into electrical form (voltage or current) when this current is passes through the coil it magnetized. A tape which is coated from magnetic material is passes through the air gap of the recording head .when magnetic tape is magnetized and the current (information) is stored in the magnetic tape.

Title of the Practical: Study of Instrument transformer

Q1. What do you mean by an instrument transformer? Ans: transformer which is used with measuring instrument is called instrument transformers. Instrument transformer is used as a protection circuit of power system for the operation of over current, under voltage, Earth fault etc.

Q2 what are the types of instrument transformer? Ans: there are two types of instrument transformer: 1. Current transformer 2. Voltage transformer

Q3. What do you mean by a Current transformer? Ans: The transformers which are used for measurement of current is called current transformer.

Q4 what do you mean by a Voltage transformer? Ans: The transformers used for voltage measurement are called voltage or potential transformer.

Q5 what are the Advantages of instrument transformer? Ans: Advantages of instrument transformer are:

1. Instrument transformers are moderate in size and used for 5A current and 100 to 200 V measurement.

2. Instruments using instrument transformer are low cost. 3. Replacement of damage parts is Easy. 4. The metering circuit is isolated the high voltage power circuit hence insulation and safty is

assured for operation. 5. There is low power consumption in metering circuit.

Q6 what do you mean by a phase Angel of instrument transformer? Ans: The angle between secondary current and primary current of a current transformer is called phase angle of instrument transformer.

Q7 what do you mean by transformation ratio of instrument transformer? Ans: It is the ratio of primary winding current to the secondary winding current of an instrument transformer.

Q8 what do you mean by Nominal ratio of instrument transformer? Ans: It is the ratio of rated primary winding current to the rated secondary winding current of an instrument transformer.

Q9 what do you mean by Turns ratio of instrument transformer? Ans: It is the ratio of number of turns in secondary winding to the number of turns in primary winding of an instrument transformer.

Q10.what do you mean by ratio correction factor of instrument transformer? Ans: It is the ratio of the transformation ratio to the nominal ratio of an instrument transformer

Title of the Practical: Study of different Digital instruments.

Q1 what do you mean by a display device? Ans A device which is used to display the output of a measuring instrument in a measurement system is called display device.

Q2 what are the types of display device? Ans: there are two types of display device: 1. Analog display 2. Digital display

Q3 what do you mean by analog display device? Ans: A device which display the result of a measurement with the help of a pointer deflection. This display is more error full.

Q4 what do you mean by Digital display device? Ans: A device which displays the result of a measurement in the form of decimal number digitat display work on the principal of quantization.

Q5 what are the advantages of digital display over analog display device? Ans: the main advantages of digital display device over analog display device are:

1. Digital device display the result of any measurement in direct decimal form hence the probability of error is less.

2. Result of digital display is stored in memory devices. 3. The requirement of input power is less in digital display as compare to analog display. 4. The accuracy of digital display is high.

Q6. What do you mean by seven segment display? Ans: Seven segment displays is a form of electronic display device for displaying decimal number system. Seven segment display are widely used in digital clock, electronic meter etc.

Q7. What do you mean by Liquid crystal diode? Ans: Liquid crystal diode is used to display numeric and alphanumeric character in dot matrix and segmental display.

Q8. What are advantages of LCD? Ans: The main advantages of LCD are:

1. Low power consumption 2. low cost

Q9.What do you mean by a digital millimeter? Ans: Millimeter is an electronic instrument which is used to measure voltages, current and resistances.

Q10. What do you mean by a digital Voltmeter? Ans: Digital voltmeter is an indicating device which displays D.C. or A.C. voltages as discrete numerals

Title of the Practical: Resistance measurement using different methods.

Q1 what do you mean by a resistance? Ans: when a potential difference is applied across a conductor, the free electrons start moving in particular directions. While moving through the materials these electrons collide with other atoms and molecules .they oppose this flow of electrons trough it, this opposition is called resistance

Q2 Explain the law of resistance? Ans: The resistance of a wire depends upon the various factors:

1. It directly proportional to its length L. 2. It is inversely proportional to its area of cross section A. 3. It depends upon the nature of the material. 4. It also depends upon the temperature of the wire.

Q3 what is the range of low resistance? Ans: All resistances of the order of 1 ohm and under may be classified as low resistances.

Q4 what is the range of Medium resistance? Ans: All resistances of the order of 1 ohm upwards to about 0.1 M Ohm are classified as medium resistances.

Q5 what is the range of High resistance? Ans: All resistances of the order of 0.1 MOhm and upwards may be classified as high resistances.

Q6 what are the different methods used for measurement of medium resistances? Ans: the different methods used for measurement of medium resistances are:

1. Ammeter- voltmeter method 2. Substitution method 3. Wheatstone bridge method 4. Ohmmeter method

Q7. Explain Wheatstone bridge? Ans: The Wheatstone bridge is an instrument for making comparison measurements and operates upon a null indication principal.

Q8. What are the factors affects the measurements of medium resistance? Ans: The following factors affect the measurements of medium resistance:

1. Resistance of contact leads 2. Thermoelectric effects 3. Temperature effect 4. Contact resistance 5. Change in ratio arms 6. using a high resistance

Q9. What are the limitations of Wheatstone bridge? Ans: The use of Wheatstone bridge is limited to the measurement of resistances ranging from a few Ohm to several Mega Ohm. The upper limit is set by the reduction in sensitivity to unbalance caused by high resistance values.

Q10. What are the methods used to measure high resistances? Ans: The different methods employed for measurement of high resistance are:

1. Direct deflection method 2. Loss of charge method 3. using mega Ohm method 4. Using megger

Title of the Practical: Study of different AC Bridges.

Q1. Why Maxwells bridge is used? Ans: Maxwells bridge is used to measure the inductance by comparison with a standard variable capacitance. Q2. What are the Advantages of Maxwell Bridge? Ans: the main Advantages of Maxwell Bridge are: 1 The two balance equations are independent if we choose R4 and C4 as variable elements. 2 The frequency does not appear in any of the two equations. 3 The Maxwells inductance-capacitance bridge is very useful for measurement of a wide range of inductance power and audio frequency. Q3. What are the Disadvantages of Maxwell Bridge? Ans: the Disadvantages of Maxwell Bridge are: 1. This bridge requires a variable standard capacitor which may be very expensive if calibrated to a

high degree of accuracy. 2. The bridge is limited to measurement of low Q coils.

Q4. . Why Hays bridge is used? Ans: Hays bridge is a modification of Maxwells bridge. Hays bridge is used to measure an unknown inductance of a high coil. Q5. What are the Advantages of Hays Bridge? Ans: the main Advantages of Hays Bridge are:

1. This bridge gives very simple expression for unknown inductance for high coils and is suitable for coils having Q >10.

2. This bridge also gives a simple expression for Q factor.

Q6. What are the Disadvantages of Hays Bridge? Ans: the Disadvantages of Hays Bridge is that it is suitable for the measurement of high Q inductors having a Q greater than 10. For inductor having Q values smaller than 10 hence this bridge is not suited for measurement of coils having Q less than 10. Q7. Why Andersons bridge is used? Ans: This Bridge is modification of Maxwells bridge. In this bridge method the self inductance is measured in terms of a standard capacitor. This method is applicable for precise measurement of self-inductance over a very wide range of values.

Q8. What is the Disadvantages of Andersons bridge? Ans: the main Advantages of Andersons Bridge are:

1. Balance condition is obtained easily. 2. A fixed capacitor can be used instead of a variable capacitor as in the case of Maxwells bridge. 3. This bridge may be used for accurate determination of capacitance in terms of inductance.

Q9. What are the Disadvantages of Andersons bridge? Ans: the main Disadvantages of Hays Bridge are:

1. The Anderson Bridge is more complex than its prototype Maxwells bridge. The Anderson Bridge has more parts and is complicated to set up and manipulate. The balance equations are not simple and in fact are much more tedious.

2. An additional junction point increases the difficulty of shielding the bridge.

Q10. Why Schering Bridge is used? Ans: Schering Bridge is used to measure the unknown capacitance.

Q11. Why Wien Bridge is used? Ans: Wien Bridge is used to measure the unknown frequency.

Title of the Practical: Study of Extension of range Voltmeter & Ammeter.

Q1 what do mean by an electrical instrument? A1: The instruments which are used to measure electrical quantities are called electrical quantity are electrical instruments.

Q2 Explain the types of electrical instruments? A2: The electrical instrument may be classified as:

1. Absolute instruments 2. Secondary instruments

Q3 Explain Absolute instrument? A3: The instruments which give the value of the quantity to be measured in terms of the constants of the instruments are called the Absolute instruments.

Q4. Explain Secondary instrument? A5: The instruments which determine the electrical quantity to be measured directly in terms of deflection are called secondary instruments.

Q5. Explain the types of Secondary instruments? A5: The secondary instruments are further classified as: 1. Indicating instruments 2. Integrating instruments 3. Recording instruments

Q6. Why Wheatstone bridge can not be used for precision measurement? A6: A Wheatstone bridge can not be used for precision measurement because errors are introduced into on account of: 1. Resistance of connecting leads 2. Thermoelectric effects 3. Contact resistance

Q7. Why guard terminal are used in high resistance measurement? A7: guard terminal are used in high resistance measurement because it bypass the leakage current.

Q8. What do you mean by a digital Frequency meter? A8: A digital frequency meter measures the frequency of an unknown signal.

Q.9. what are basic component of a digital Frequency meter? A9: The basic component of a digital Frequency meter is 1. Unknown frequency source 2. Amplifier 3. Schmitt trigger 4. Start-stop gate 5. Counter

Q10. Why Campbells Bridge is used? A10: This Bridge measures an unknown mutual inductance in term of a standard mutual inductance.

Title of the Practical: Digital Instruments Digital Voltmeter, Digital Frequency Meter, Digital Panel Meter, Digital Storage Oscilloscope.

Q1: What do you mean by measurement? A1: Measurement is a process by which the property of an object is compared to an accepted standard unit.

Q2: What do you mean by Errors? A2: Errors are define as it is the deviation of outputs to the input OR The difference between inputs and outputs is called errors

Q3: What is Voltmeter? A3: A voltmeter is an instrument used for measuring the electrical potential difference between two points in an electric circuit.

Q4: What is analog voltmeter? A4: Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; General purpose analog voltmeters may have an accuracy of a few per cent of full scale, and are used with voltages from a fraction of a volt to several thousand volts. Digital voltmeters give a numerical display of voltage by use of an analog to digital converter.

Q5: Why we use digital meter? A5: Digital meters can be made with high accuracy, typically better than 1%. Specially calibrated test instruments have higher accuracies, with laboratory instruments capable of measuring to accuracies of a few parts per million.

Q6: What is moving coil galvanometer type voltmeter? A6: A moving coil galvanometer can be used as a voltmeter by inserting a resistor in series with the instrument. It employs a small coil of fine wire suspended in a strong magnetic field. When an electrical current is applied, the galvanometer's indicator rotates and compresses a small spring. The angular rotation is proportional to the current through the coil. For use as a voltmeter, a series resistance is added so that the angular rotation becomes proportional to the applied voltage.

Q7: What is sensitivity of voltmeter? A7: The sensitivity of such a meter can be expressed as "ohms per volt", the number of ohms resistance in the meter circuit divided by the full scale measured value. For example a meter with sensitivity of 1000 ohms per volt would draw 1 milliampere at full scale voltage.

Q8: What is the principle of voltmeter? A8: Voltmeters operating on the electrostatic principle use the mutual repulsion between two charged plates to deflect a pointer attached to a spring. Meters of this type draw negligible current but are sensitive to voltages over about 100 volts and work with either alternating or direct current.

Q9: What are average reading AC voltmeters? A9: Normally ac voltmeters are average responding type and the meter is calibrated in terms of the rms values for a sine wave. Since most of the voltage measurements involve sinusoidal waveform so this method of measuring rms value of ac voltages works satisfactorily and is less expensive than true rms responding voltmeters. However, in case of measurement of non-sinusoidal waveform voltage, this meter will give high or low reading depending on the form factor of the waveform of the voltage to be measured.

Q10: How can measure power in dc circuit? A10: The result of this arrangement is that on a dc circuit, the deflection of the needle is proportional to both the current and the voltage, thus conforming to the equation P=VI.

Title of the Practical: Ammeter range extension using shunts

Q1: What is Ammeter? A1: An ammeter is a measuring instrument used to measure the electric current in a circuit. Electric currents are measured in amperes (A), hence the name. Smaller values of current can be measured using a millimeter or a micrometer.

Q2: What is tangent galvanometer? A2: The tangent galvanometer was used to measure currents using this effect, where the restoring force returning the pointer to the zero position was provided by the Earth's magnetic field. This made these instruments usable only when aligned with the Earth's field. Sensitivity of the instrument was increased by using additional turns of wire to multiply the effect - the instruments were called "multipliers".

Q3: What is D'Arsonval galvanometer? A3: The D'Arsonval galvanometer is a moving coil ammeter. It uses magnetic deflection, where current passing through a coil causes the coil to move in a magnetic field Moving iron ammeters use a piece of iron which moves when acted upon by the electromagnetic force of a fixed coil of wire. This type of meter responds to both direct and alternating currents (as opposed to the moving coil ammeter, which works on direct current only)

Q4: What is hot-wire ammeter? A4: In a hot-wire ammeter, a current passes through a wire which expands as it heats. Although these instruments have slow response time and low accuracy, they were sometimes used in measuring radio-frequency current

Q5: What is Digital ammeter? A5: Digital ammeter designs use an analog to digital converter (ADC) to measure the voltage across the shunt resistor; the digital display is calibrated to read the current through the shunt.

Q6: How to measure large current? A6: To measure larger currents, a resistor called a shunt is placed in parallel with the meter. Most of the current flows through the shunt, and only a small fraction flow through the meter. This allows the meter to measure large currents

Q7: What are Zero-center ammeters? A7: Zero-center ammeters are used for applications requiring current to be measured with both polarities, common in scientific and industrial equipment Zero-center ammeters are also commonly placed in series with a battery. In this application, the charging of the battery deflects the needle to one side of the scale (commonly, the right side) and the discharging of the battery deflects the needle to the other side.

Q8: What is AC/DC ammeter? A8: Accurate AC/DC non-contact ammeters have been constructed using Hall effect magnetic field sensors. A portable hand-held clamp-on ammeter is a common tool for maintenance of industrial and commercial electrical equipment, which is temporarily clipped over a wire to measure current.

Q9: What is analog wattmeter? A9: The traditional analog wattmeter is an electrodynamic instrument. The device consists of a pair of fixed coils, known as current coils, and a movable coil known as the potential coil. The current coils connected in series with the circuit, while the potential coil is connected in parallel.

Q10: Operation of analog wattmeter? A10: The potential coil carries a needle that moves over a scale to indicate the measurement. A current flowing through the current coil generates an electromagnetic field around the coil. The strength of this field is proportional to the line current and in phase with it. The potential coil has, as a general rule, a high-value resistor connected in series with it to reduce the current that flows through it.

Title of the Practical: 16. Study of Displays Cold cathode displays, Fluorescent displays, Light emitting, diodes, Liquid crystal diodes, Alpha-numeric display Recorders Analog Recorder, Graphic ,Recorder, Strip Chart Recorder, X-Y Recorder.

Q1: What are the basic elements of a measurement system? A1: There are three basic elements of measurement system:-

(a) Sensing elements: - these are basically transducers. (b) Signal conditioning elements: - these are generally filters and amplifiers circuit. (c) Outputs elements: - these are basically display devices, like LED or LCD display.

Q2: What is LED? A2: The Led is a PN Junction device which emits light when a current passes through it in the forward direction. Charge carrier recombination occurs at a pn junction as election cross form n-side & recombinicationa with holes on the p-side. When recombinicationa take place. The charge gives up energy in the form of heat &light. If the semi conducting material is translucent the light is emitted &the junction is source of light. This is the light emitting diode.

Q3: What materials are used in LED? A3: Semiconductor material used of manufacture of LED are GaAsP which emits red &yellow light of GaAs which emits red & green emission.LEDS are used extensively in segmental &dot matrix display of numeric & alphanumeric characters . LEDs are available in many colors like green yellow amber &red.

Q4: What is LCD? A4: We get the definition of LCD from the name Liquid Crystal itself. It is actually a combination of two states of matter the solid and the liquid. They have both the properties of solids and liquids and maintain. A little amount of heat can easily turn the liquid crystal into a liquid. We always use devices made up of Liquid Crystal Displays (LCDs) like computers, digital watches, thermometer and also DVD and CD players.

Q5: What is color liquid crystal display? A5: Color LCDs are those that can display pictures in colors. For this to be possible there must be three sub-pixels with red, green and blue color filters to create each color pixel. For combining these sub-pixels these LCDs should be connected to a large number of transistors.

Q6: disadvantages of LCDs? A6: One of the main disadvantages of these types of LCDs is the size. Most manufacturers try to reduce the height than gain it. This is because more transistors and greater pixels will be needed to increase the length. This will increase the probability of bad pixels. It is very difficult or also impossible to repair a LCD with bad pixels.

Q7: What is seven segment displays? A7: A seven-segment display, or seven-segment indicator, is a form of electronic display device for displaying decimal numerals. A seven segment display is composed of seven elements. Individually on or off, they can be combined to produce simplified representations of the Arabic numerals. Seven-segment displays are widely used in digital clocks, electronic meters

Q8: what is Cathode ray oscilloscope (CRO)? A8: Cathode ray oscilloscope is an instrument used for display, measurement and analysis of waveforms and other phenomenon in electrical and electronic circuit

Q9: What is the function of electron gun assembly used in CRT? A9: The electron gun assembly produces a sharply focused beam of electrons which are accelerated to high velocity .this focused beam of electrons strikes the fluorescent screen with sufficient energy to cause a luminous spot on the screen. Q10: which device is used for the source of emission of electrons in a CRT? A10: A barium and strontium oxide coated cathode is used for the source of emission of electrons in a CRT.