(Aktu Btech) Electronic Instrumentation and Measurements Important Unit-2 Electronic Instruments

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Important Questions For Electronic Instrumentation and Measurements:
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Q1. Write a short note on : 

i. Op-amp voltage-follower voltmeter 

ii. Op-amp amplifier voltmeter. 

Ans. i. Op-amp voltage-follower voltmeter: 

• 1. The circuit diagram of Op-Amp voltage-follower voltmeter is shown in Fig.

• 2. The input resistance of the voltage-follower is significantly larger than that of the emitter-follower. There is no base-emitter voltage drop from input to output, unlike the emitter follower.
• 3. The voltage-follower input (E_B) is supplied to the Op-non-inverting Amp’s terminal in this circuit, and the feedback from the output is applied to the terminal on the inverting input.
• 4. The inverting terminal voltage tends to remain exactly identical to that at the non-inverting terminal thanks to the Op-extremely Amp’s high voltage gain and negative feedback. Hence, output voltage follows input voltage.
• 5. The attenuator is used for selecting the range of voltmeter. 

ii. Op-Amp amplifier voltmeter:

• 1. An IC Op-Amp can also be used to amplify low voltages to levels, measurable by a deflection meter. Op-Amp amplifier voltmeter circuit is Used for this purpose as shown in Fig.
• 2. The input voltage (E) is applied to the non-inverting terminal and voltage V_R3 is feedback to the inverting terminal of the Op-Amp. The internal voltage gain of Op-Amp causes V_R3 to be always equal to E. The output voltage is  

Q2. Explain the working of precision rectifier based voltmeters.

Ans. A. Working principle of electronic voltmeter: 

• 1. An Op-Amp voltage-follower AC voltmeter can be converted into an AC electronic voltmeter by connecting a rectifier in series with the meter circuit. Such a circuit is shown in Fig. 

• 2. The Op-non-inverting Amp’s terminal input is applied with an AC input signal that will be monitored. Unwanted DC voltages are blocked in this circuit by coupling capacitor C₁.
• 3. The voltage follower’s output is an exact replica of the AC input signal.
• 4. The rectifier receives this output voltage and produces a half-wave rectified version of the input signal. The metre circuit receives this rectified voltage.
• 5. The limitation of this circuit is that the voltage drop (V_F) across the rectifier acts as an error which must be taken into account in design calculation when the instrument is indicating full scale.  
• 6. This error can be avoided by using a precision rectifier. 

B. AC electronic voltmeter using precision rectifier: 

• 1. A precision rectifier is used to improve the accuracy of AC electronic voltmeter. 

• 2. Fig. shows an AC electronic voltmeter using precision rectifier. 
• 3. In this circuit, the cathode of rectifier D₁ is used as the voltage-follower feedback connection to the inverting terminal rather than the amplifier output.
• 4. As a result, there is no rectifier voltage drop from input to output since the half-wave rectified output precisely matches the positive half-cycle of the input voltage.

Q3. What do you mean by loading effect ? How does electronic voltmeter help in minimizing the loading effect ?

Ans. A. Loading effect: 

• 1. In order to measure the voltage in a circuit, a voltmeter is always connected across or in parallel with the measurement points.
• 2. If its resistance is too low, it may change the voltage in the circuit. The voltmeter loading effect is the term for this.
• 3. Where low levels are to be detected in electronic and communication circuits, loading effect arises.

B. Electronic voltmeter:  

• 1. By supplying the necessary power for measurement by employing external circuits like amplifiers, electronic voltmeters avoid the loading effects.
• 2. The amplifiers enable low level signals in addition to providing power for operation.
• 3. As vacuum tubes and transistors are used as amplifiers in electronic voltmeters, an additional source of power can be used to operate the deflecting element of the PMMC instrument.
• 4. As a result, although the circuit whose voltage is being measured controls the voltmeter’s sensing element, the power required from the circuit is very little or even insignificant.
• 5. This indicates a very high input impedance in the voltmeter circuit.
• 6. Voltage measurements in many high impedance circuits, such as those found in communication equipment, depend on this property of electronic voltmeters.  

Q4. Write a short note on multimeter probes.

Ans.

• 1. A connection is made between the test circuit and the measurement device using a probe.
• 2. Any conductor, such as a piece of bare wires, a multimeter lead, or an unterminated coaxial cable, may be used as a probe.
• 3. A probe simply transfers the least amount of energy possible from the circuit being tested to a measuring device with the highest degree of accuracy.
• 4. The ideal probe provides the following key attributes:  
  • i. Ease of connection.  
  • ii. Absolute signal fidelity. 
  • iii. Zero signal source loading. 
  • iv. Complete noise immunity. 
• 5. There are many probes for use with multimeters that can extend the range of measurement. Some of these are given as:

i. High voltage probe:

• 1. In essence, a high voltage probe is a potential divider.
• 2. The voltage to be measured is often divided by a factor of 1000, essentially multiplying the instrument scales by 1000. A 10 V scale becomes a 10 kV scale as a result.

ii. High current probe: 

• 1. By using a current transformer, the AC current probes lower the high levels of alternating current.
• 2. The conductor carrying the current to be measured is close to and open to the transformer’s core.
• 3. The transformer’s secondary windings control the amount of measured current. Hence, by using the current probe, a multimeter’s 1 mA AC scale is changed to a 1 A scale.

Q5. How are digital voltmeters classified ? What are the operating and performance characteristics of a digital voltmeter ?  

Ans. A. Digital voltmeter: 

• 1. Digital voltmeters basically function as analog-to-digital converters that display the measured voltage on a digital screen.
• 2. Unlike analogue instruments, which display pointer deflections on a continuous scale, digital voltmeters (DVMs) display measurements of DC or AC voltage as discrete digits.
• 3. It is a flexible and precise tool used in numerous laboratory measuring applications.

B. Classification of digital voltmeter:

• 1. Ramp-type DVM.
• 2. Dual-slope type DVM.
• 3. Integrating type DVM.
• 4. Successive approximation DVM. 

C. Characteristics features of DVM:

• 1. Input range: From ± 1.000 V to ± 1000 V with automatic selection and overload indication.
• 2. Absolute accuracy: As high as ± 0.005 % of the reading.  
• 3. Resolution: 1 part per million. 
• 4. Stability: Short term, 0.002 % of the reading (for a 24 hour period), long term, 0.008 % of the reading (for a 6-month period). 
• 5. Input resistance: Typically 10 M𝛺. 
• Input capacitance: 40 pF.
• 6. Calibration: Internally from stabilized reference sources, independent of measuring circuit. 
• 7. Output signals: In BCD form, for print output and further digital processing. 

Q6. What is the importance of calibration in instrumentation ? Sketch the circuit for calibrating a wattmeter and explain the calibration procedure.

Ans. Importance of calibration:

• 1. Assurance of accurate measurements.
• 2. Ability to trace measurements to international standards.
• 3. Correct diagnosis of illness (medical reports).
• 4. Consumer protection (legal metrology). 

B. Circuit of calibration a wattmeter:

• 1. The calibration of a wattmeter combines the calibration of an ammeter and a voltmeter.
• 2. In Fig., a wattmeter is depicted with its voltage coil linked to one DC power source and its current coil attached to another.
• 3. The device as a whole calculates the power utilized by the load by using the ammeter and voltmeter to measure current and voltage, respectively.
• 4. The wattmeter’s two coils are the stationary coil and the moving coil. These two coils are connected in various circuits to measure power.
• 5. To measure the current in the circuit, the fixed coil is linked in series with the conventional ammeter. As a result, the current coil refers to the fixed coil.
• 6. When the moving coil is wired across the normal voltmeter, it measures a current inversely proportional to the voltage.
• 7. To reduce the current to a tiny amount, a large non-inductive resistance is connected in series with the moving coil.

• 8. The moving coil is also known as the pressure coil or voltage coil since it transmits current that is proportional to voltage.
• 9. P = VI is used to compute power. After the correct voltage and current for a particular wattmeter reading have been established, P = VI can be used to calculate the precise power.
• 10. The maximum voltage and maximum current must be avoided in order to protect the instrument. ‘

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