1. A voltmeter has a sensitivity of 1000 Ω/V reads 200 V on its 300 V scale. When connected across an unknown resistor in series with a millimeter. When the milliammeter reads 10 mA. The apparent resistance of the unknown resistor will be?
a) 20 kΩ
b) 21.43 kΩ
c) 18.57 kΩ
d) 22.36 kΩ
Explanation: RT = \( \frac{V_T}{I_T}\)
VT = 200 V, IT = 10 A
So, 20 kΩ.
2. A voltmeter has a sensitivity of 1000 Ω/V reads 200 V on its 300 V scale. When connected across an unknown resistor in series with a millimeter. When the milliammeter reads 10 mA. The actual resistance of the unknown resistor will be?
a) 20 kΩ
b) 18.57 kΩ
c) 21.43 kΩ
d) 22.76 kΩ
Explanation: Resistance of voltmeter,
RV = 1000 × 300 = 300 kΩ
The Voltmeter is in parallel with an unknown resistor,
RX = \(\frac{R_T R_V}{R_T – R_V} = \frac{20 × 300}{280}\) = 21.43 kΩ.
3. A voltmeter has a sensitivity of 1000 Ω/V reads 200 V on its 300 V scale. When connected across an unknown resistor in series with a millimeter. When the milliammeter reads 10 mA. The error due to the loading effect of the voltmeter is ________
a) 3.33%
b) 6.67%
c) 13.34%
d) 13.67%
Explanation:RT = \( \frac{V_T}{I_T}\)
VT = 200 V, IT = 10 A
So, RT = 20 kΩ
Resistance of voltmeter,
RV = 1000 × 300 = 300 kΩ
Voltmeter is in parallel with unknown resistor,
RX = \( \frac{R_T R_V}{R_T – R_V} = \frac{20 × 300}{280}\) = 21.43 kΩ
Percentage error = \(\frac{Actual-Apparent}{Actual}\) × 100
= \(\frac{21.43-20}{21.43}\) × 100 = 6.67%.
4. A 500 A, 50 Hz current transformer has a bar primary. The secondary burden is a pure resistance of 1 Ω and it draws a current of 5 A. If the magnetic core requires 250 Ampere-turn for magnetization, the percentage ratio error is __________
a) 10.56%
b) -10.56%
c) 11.80%
d) -11.80%
Explanation: IM = 250/I = 250 A
or, R = \(\frac{V^2}{R} \)
∴ R = ± (2 × 1.5 + 5) = ± 8%.
5. In the simplest form, an AC bridge consists of ____________
a) arms, source and a detector
b) arms and source
c) source and detector
d) arms and detector
Explanation: In its simplest form, an AC bridge consists of four arms, a source for excitation and a null detector. The source is connected across a pair of arms while the detector is connected to the pair of opposite arms.
6. Source is ________
a) dc supply
b) ac supply
c) mixed mode supply
d) high voltage supply
Explanation: For an AC bridge we require an AC supply as the source of voltage. It supplies AC voltage at the required frequency
7. At high frequency, source consists of ________
a) amplifiers
b) regulators
c) oscillators
d) op amps
Explanation: Op amps are basically differential amplifiers. Amplifiers are used in analog circuits for increasing the strength of the signal. Electronic oscillators form sources at high frequencies
8. Commonly used balance detectors for AC bridges are headphones, tuned amplifiers and vibration galvanometers.
a) True
b) False
Explanation: Headphones, tuned amplifier circuits and vibration galvanometers are used for detecting the balance condition in AC bridges
9. What is the frequency range for a headphone as a detector?
a) 20 Hz to 20 kHz
b) 10 kHz to 1 MHz
c) 10 MHz to 1 GHz
d) 250 Hz to 4 kHz
Explanation: Headphones can be used as detectors in AC bridges in the low audio frequency range. Low audio frequency range varies from 250 Hz to 4 KHz
10. For single frequency value, the most sensitive detector is ________
a) tuned detector
b) vibration galvanometer
c) headphone
d) oscillator
Explanation: Vibration galvanometer is used for detecting the balance condition. Oscillator is used as a source of supply voltage. Tuned detector is the most sensitive detector for a single frequency value.