Rectifier type instrument measures the alternating voltage and current with the help of rectifying elements and permanent magnet moving coil type of instruments.
The rectifier type of instruments is calibrated in terms of r.m.s. values of sinusoidal currents and voltages
The markings on the instrument are 1.11 and 2.22 times the average current for full-wave and half-wave rectification respectively
These instruments are subject to waveform errors as other waveforms may not have a form factor of 1.11 (in the case of full-wave rectifier) or 2.22 (in the case of half-wave rectifier)
Effect of Temperature Change: The resistance of the rectifying element varies with the change in temperature. And this property of the rectifying component causes the error in the instruments.
Frequency Error: As the PMMC instrument is used in rectifier instruments, frequency error is absent.
12. What is the basic range of an Electronic Voltmeter?
Causes maximum deflection
Causes minimum deflection
Causes zero deflection
Causes medium deflection
Answer.1. Causes maximum deflection
Explanation:
In an Electronic voltmeter, the value of the input which causes maximum meter deflection is the basic range of the meter. This is generally the lowest range on the range switch in nonamplified models.
High ranges can be obtained by using an input attenuator. The input attenuator is a calibrated front panel control in the form of the resistance voltage divider. The full-scale voltage appears across the divider hence the voltage at each tap is an increasing lower fraction of the full input voltage.
13. The Q-meter works on the principle of
A mutual inductance
Self-inductance
Series resonance
Parallel resonance
Answer.3. Series resonance
Explanation:
Q meter is an instrument that is designed to measure the value of quality factor directly and it is useful in measuring the characteristics of coils and capacitors.
Q meter works on the principle of series resonance.
At series resonance, the voltage across the capacitor is equal to Q times of applied input voltage. Thus, we can measure the value of Q directly by connecting a voltmeter across capacitors.
It is used for the measurement of quality factor, inductance, self-capacitance, inductance, bandwidth, and capacitance of a coil.
14. High range can be obtained in a basic D.C. electronic voltmeter by ________
A transformer
An attenuator
A transducer
A resistor
Answer.2. An attenuator
Explanation:
In DC electronic voltmeter the circuit consists of an input attenuator source follower and d.c. coupled amplifier. The attenuator is basically a voltage divider circuit with a predefined front panel.
The purpose of the input attenuator is to provide input voltage levels that can be accepted by the d.c. amplifier. A FET is used to serve as a source follower at the input stage to obtain high input impedance. FET effectively isolates the meter circuit from the circuit under measurement.
15. In a thermocouple element, heat energy transferred to the hot junction is converted back to electrical energy by
Johnson’s effect
Seebeck effect
Hall effect
Faraday’s effect
Answer.2. Seebeck effect
Explanation:
A thermocouple is an electrical device containing junctions of two dissimilar metal joints. It is used as a temperature sensor.
It works on the principle of the Seebeck effect which states that the temperature difference between two dissimilar electric conductors produces a voltage difference between them. This potential difference is used to measure temperatures.
16. The sensitivity of Electronics Voltmeter is ______
High
Low
Medium
Negative
Answer.1. High
Explanation:
The conventional PMMC instruments have low sensitivity and low input impedance. The conventional PMMC instrument has a sensitivity of 20 kΩ/V with a 0-0.5V range and an input impedance of only 10 kΩ at its 0.5V range. The sensitivity of electronic voltmeters is much higher than that of analog instruments. The sensitivity is very high as much as 100 times the normal PMMC voltmeter.
17. A multimeter cannot measure
Current
Potential difference
Charge
Resistance
Answer.2. Charge
Explanation:
A multimeter is an electronic instrument. It is mainly used to measure the three basic electrical characteristics of voltage, current, and resistance.
It can also be used to test continuity between two points in an electrical circuit.
Multimeters cannot be used to measure electrical quantities such as frequency, charge, etc.
18. Overloading in Electronic Voltmeter cause ________
Damages the meter
Increases the temperature
Doesn’t affect the meter
Decreases the sensitivity
Answer.3. Doesn’t affect the meter
Explanation:
The input overload does not burn the electronic voltmeter because the amplifier saturates, limiting the maximum current through the meter. As a result, overloading does not affect the meter and it remains safe.
19. A rectifier is used in an instrument for the purpose of ________.
Measuring high voltage values
Measuring high current values
Converting AC into DC
Making the instrument more stable
Answer.3. Converting AC into DC
Explanation:
A rectifier is used in an instrument to convert AC into DC whereas an inverter is used to convert DC into AC.
These are mainly of two types
1) Full-wave rectifiers
2) Half-wave rectifiers
20. The input impedance of Electronic Voltmeter ranges from
10Ω – 100Ω
10kΩ – 100kΩ
10 MΩ – 100 MΩ
Infinite Impedance
Answer.3. 10 MΩ – 100 MΩ
Explanation:
The above electronic voltmeter has input resistance ranging from 10 MQ to 100 MQ, thus producing less loading of the circuit under test another advantage of an electronic voltmeter (EVM) is that its input resistance remains constant in overall ranges.
