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1

**Explain the relevant circuit diagram, the principle of measurement of low resistance by kelvin’s double bridge. Prove that the condition of balance is independent of the read resistance. Up to what low value it can be used?**

The kelvin bridge is a notification of the Wheatstone bridge with greater increased accuracy in measurement of low value resistance.
Consider the bridge circuit shown in the figure.

2

**Explain the functional block diagram of CRO with neat diagram.**

The cathode Ray Oscilloscope or mostly called as CRO is an electronic device used for giving the visual indication of a signal waveform.
It is an extremely useful and the most versatile instrument in the electronic industry.

3

**What is lissajous pattern? Explain how phase & frequency can be measured using this figures.**

Cathode Ray Oscilloscope (CRO) is very important electronic device. CRO is very useful to analyze the voltage wave form of different signals. The main part of CRO is CRT(Cathode Ray Tube). A simple CRT is shown in figure below-

4

** How that driving torque in a moving iron instrument is given by \(\theta = \frac{1}{2K}i^2\frac{\text{d}L}{\text{d}\theta}\) . Where the symbols have their usual meaning.**

Let ‘\(\theta\)’ be the deflection corresponding to a current of ‘I’ amp
Let the current increases by di, the corresponding deflection is ‘\(\theta+d\theta\)’
There is change in inductance since the position of moving iron change w.r.t the fixed electromagnets.

5

**In a certain current transformer, the following data is obtained. Nominal ratio = 25/5A, Turn ratio = 3, primary terns = 40, secondary turns = 120, secondary resistance = 0⋅16z, secondary reactance = 0⋅195Ω, secondary burden = 15 VA, Burden power factor = 0⋅7, secondary terminal voltage = 3V. **

**Find ratio & phase angle errors. The magnetising and loss ampere turns corresponding to an emf of 4⋅26V are13 & 10⋅1 respectively. **

Secondary burden=15VA, secondary winding current=5A=\(I_s\)
Secondary circuit impedance =\(\frac{15}{5^2}=0.6\Omega\)

6

**Explain the principle of operation of a digital frequency meter.**

A frequency meter has a small device which converts the sinusoidal voltage of the frequency into a train of unidirectional pulses. The frequency of input signal is the displayed count, averaged over a suitable counting interval out of 0.1, 1.0, or 10 seconds.

7

**Explain how a low resistance is measured by a potentiometer**

Let unknown resistor R is connected in series with a standard resistor S. The current through the circuit is controlled with the help of rheostat. A two-pole double throw switch is used.

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