Electronic measurements and instrumentation miscellaneous Difficult Questions and Answers | Page - 1

Electronic measurements and instrumentation miscellaneous

It is required to measure temperature in the range of 1300°C to 1500°C. The most suitable thermocouple to be used as a transducer would be:

1. Chromel-constantan
1. Iron-constantan
1. Chromel-alumel
1. Platinum-rhodium

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NA

Correct Option: D

NA

The resistance of two coils of a wattmeter are 0–0·1 ohm and 1000 ohms respectively and both are non-inductive. The load current is 20 A and the voltage across the load is 30 V. In one of the two ways of connecting the voltage coil, the error in the reading would be:

1. 0·1% too high
1. 0·2% too high
1. 0·15% too high
1. zero

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107. (c) Let the resistance of the current coil is
r_cc = 0·01Ω
(current coil is always low resistance as compare to voltage coil)
and the resistance of voltage coil is r_vc = 1000Ω
There are two ways in which the current coil and voltage coil can be connected. The wattmeter reads high by an amount equal to power loss in current coil in one case and in voltage coil in the other. The two possible power losses are:
(1) I_c² r_cc = 20² × 0·01 = 4W
(2)
V² = 30²
r_{v_c} 1000

= 0.9 W
The two percentage errors are
(i) 4/(20 × 30) × 100 = ·67%
(ii) 0·9/(20 × 30) × 100 = 0·15%

Correct Option: C

107. (c) Let the resistance of the current coil is
r_cc = 0·01Ω
(current coil is always low resistance as compare to voltage coil)
and the resistance of voltage coil is r_vc = 1000Ω
There are two ways in which the current coil and voltage coil can be connected. The wattmeter reads high by an amount equal to power loss in current coil in one case and in voltage coil in the other. The two possible power losses are:
(1) I_c² r_cc = 20² × 0·01 = 4W
(2)
V² = 30²
r_{v_c} 1000

= 0.9 W
The two percentage errors are
(i) 4/(20 × 30) × 100 = ·67%
(ii) 0·9/(20 × 30) × 100 = 0·15%

A 150μ-amp meter movement has 1000 ohm resistance, what shunt resistance is required to extend the range to 750μ-amp?

1. 222 ohms
1. 225 ohms
1. 250 ohms
1. 150 ohms

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Given 150μA, 1000Ω Ammeter. Let the shunt resistance R_sh is required to extend the range to 750μA as shown below in figure:

From the above figure
V = 1000 × 150 μA = R_sh × 600 μA
or
R_sh = 1000 × 150 μA
600 μA

or
R_sh = 250Ω

Correct Option: C

Given 150μA, 1000Ω Ammeter. Let the shunt resistance R_sh is required to extend the range to 750μA as shown below in figure:

From the above figure
V = 1000 × 150 μA = R_sh × 600 μA
or
R_sh = 1000 × 150 μA
600 μA

or
R_sh = 250Ω

If the current sensitivity of a galvanometer is very high then its critical resistances are:

1. High and low respectively
1. Low and high respectively
1. High and high respectively
1. Low and low respectively

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If the current sensitivity of a galvanometer is high then its critical resistances are High and high respectively.

Correct Option: C

If the current sensitivity of a galvanometer is high then its critical resistances are High and high respectively.

Find the shunt needed to extend the range of a 50Ω 1 mA movement to (i) 2 mA, (ii) 10 mA:

1. 50,5·5Ω
1. 50,5Ω
1. 50,40Ω
1. 55,5·5Ω

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The shunt resistance needed to extend the range of a 50Ω 1mA movement to
(i) 2mA
when I = 2mA
from the above figure it is clear that current in shunt resistance (R_sh) is 1mA.
So,
R_sh = 50 × 1mA = 50Ω
1mA

(ii) 10mA
In this case.
R_sh = 50 × 1mA
9mA

or
R_sh = 5·5Ω (approximately)

Correct Option: A

The shunt resistance needed to extend the range of a 50Ω 1mA movement to
(i) 2mA
when I = 2mA
from the above figure it is clear that current in shunt resistance (R_sh) is 1mA.
So,
R_sh = 50 × 1mA = 50Ω
1mA

(ii) 10mA
In this case.
R_sh = 50 × 1mA
9mA

or
R_sh = 5·5Ω (approximately)