Megohmmeters (also a trademarked device Megger) is used to measure large values of resistance
Ques 12. A dc voltmeter has a sensitivity of 1000 Ω/volt. When it measures half full scale in 100 V range, the current through the voltmeter will be? (SSC-JE 2015)
50 mA
1 mA
0.5 mA
100 mA
Answer.3. 0.5 mA
Explanation:-
Sensitivity is 1000 ohms / Volt
Full-scale current is going to be 1V/1000ohms = 1 mA.
That means 1/2 of the full scale is going to be 0.5 mA.
Hence correct answer is 0.5 mA.
Ques.13. A DC voltmeter has a sensitivity of 1000Ω /volt. When it measures half full scale in 100 V range, the current through the voltmeter will be (SSC-2014, Set-1)
50 mA
100 mA
1 mA
0.5 mA
Answer.4. 0.5 mA
Explanation:-
Sensitivity of voltmeter (s) = full scale deflection of voltmeter/current passing through voltmeter
For 1 volt resistance is 1000 ohm as given in the question.
For 100 volt resitance = 1000 x 100 ohm
And thus full current in the meter = v/r
= (100)/(1000*100) =1ma
And therefore half full scale 1/2= 0.5ma
Ques 14. As compared to full-wave rectifier using two diodes the four diode bridge rectifier has the dominant advantage of (SSC-2014, Set-1)
Higher efficiency
Higher current carrying capacity
Lower peak inverse voltage requirement
Lower Ripple factor
Answer.3. Lower peak inverse voltage requirement
Explanation:-
The peak inverse voltage (PIV) of four diode bridge is one half that of a full-wave rectifier using two diodes. Therefore four bridge diode rectifiers required low peak inverse voltage.
Ques 15. In a 7/2 digit voltmeter, the largest number that can be read is: (SSC-2014, Set-1)
9999
0999
1999
5999
Answer.3. 1999
Explanation:-
A 3 1/2 (7/2) digits digital multimeter has three full digits that can change from 0 to 9 and a one-half digit that can change to either 0 or 1. That means 3 1/2 digits digital multimeter can show values from 0 to 1,999 or 0 to -1,999.
Ques 16. To increase the range of a.c ammeter you would use: (SSC-2014, Set-1)
A condenser across the meter
Current transformer
A potential transformer
An inductance across the meter
Answer.2. Current transformer
Explanation:-
Instrument transformers are used in conjunction with ammeter and voltmeter to extend the range of meters. In dc circuit shunt and multipliers are used to extend the range of measuring instruments. The shunt is used to extend the range of ammeter whereas multiplier is used to extend the range of voltmeters. But in the case of AC circuits instruments, it is not convenient to use shunt and multiplier for extend the range of ammeter and voltmeter.
The current transformer is used in conjunction with the current measuring device (such as an ammeter, wattmeter, energy meter, etc.). The current transformer is used with low-range ammeters to measure currents in high voltage alternating current circuit. Its primary winding consists of one or more turns of thick wire connected in series with the line whose current is to be measured.
Ques 17. For a half-wave rectified sine wave the ripple factor is: (SSC-2014, Set-1)
1.00
1.65
1.45
1.21
Answer.4. 1.21
Explanation:-
Ripple Factor (r): It is the ratio of root mean square (RMS) value of AC component to the DC component in the output and is given by
For half-wave rectifier
Irms = Im/2
Idc = Im/π
For the half-wave rectifier, RMS Current is given as Im/2 and DC current Im/π which results in the ripple factor of 1.21.
Ques 18. Which one of the following bridges are generally used for measurement of frequency and also capacitance? (SSC-2014, Set-1)
Wein Bridge
Hay’s Bridge
Owen’s Bridge
Schering Bridge
Answer.1. Wein Bridge
Explanation:-
The Wien bridge is one of many common bridges. Wien’s bridge is used for precision measurement of capacitance in terms of resistance and frequency. It is also used to measure audio frequencies
Ques 19. Two voltmeters of (0-300 V) range are connected in parallel to an A.C. circuit. One voltmeter is moving iron type reads 200 V. If the other is PMMC instrument, its reading will be (SSC-2014, Set-1)
127. 4 V
Slightly less than 200 V
Zero
222 V
Answer.3. Zero
Explanation:-
PMMC instrument is used only for DC current and voltage. For AC supply, no deflection will be there due to instantaneous opposite torques.
Ques 20. The least number of 1Φ wattmeters required to measure total power consumed by an unbalanced load fed from a 3Φ, 4 wire system is (SSC-2014, Set-1)
4
1
2
3
Answer.4. 3
Explanation:-
According to Blondel, a French engineer, when power is supplied by the N-wire AC system, the number of wattmeters required to measure power is one less than the number of wires, i.e(N- 1) regardless the load is balanced or unbalanced.
Hence for 3 phase 4 wire system number of wattmeter required is 3.
