24 questions and answers
How can potentiometer be used for
The ammeter to be calibrated is connected in series with a variable resistance R and a standard resistance S. The standard resistance, S should be such of magnitude that with the current to be passed through it, the voltage drop across it does not exceed the range of the potentiometer.
Draw the circuit diagram of DC potentiometer & explain how it works.
Let the standard cell SC be connected with its negative terminal through selector switch S, galvanometer G and key K to the sliding contact J. Since slide wire AB is of uniform cross section, the resistance per unit length r will be constant and equal to R/L. Where R is resistance of whole wire and L is the length of wire.
Deduce the expression of torgue of electrodynamometer type instrument. Why multiplier is used with PMMC instrument? What do you mean by sensitivity of PMMC instrument? Why sensitivity of electrodynamometer type instrument is low? Why the scale of moving iron instrument is cramped at lower end?
Let e= instantaneous voltage across load =\(E_m\) sin (ωt-φ) \(i_t\) =instantaneous load current
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.
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.
Explain the worming of Anderson’s bridge with a neat sketch. Derive the required expression for obtaining the unknown inductance.
Let, \(L_1\) = Self-inductance to be measured \(R_1\) = Resistance of self-inductor C = Fixed Capacitor
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 =\(\frac{15}{5^2}=0.6\Omega\)
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.
Write short notes on
The Q-factor for a series L–C–R circuit is the voltage magnification at resonance, i.e. \(Q-factor = \frac{voltage\ across\ capacitor}{supply\ voltage}\)
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-
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