Test: Electrical & Electronic Measurements- 1 - Electrical Engineering (EE) MCQ

• The Wein's bridge is primarily known as a frequency determining bridge. It may be employed in a harmonic distortion analyzer, where it is used as notch filter, discriminating against one specific frequency. The Wein's bridge also finds applications in audio and HF oscillators as the frequency determining device.

• The Hay's bridge is suited for the measurement of high Q inductors, especially those inductors having a Q greater than 10. For inductors having Q values smaller than 10, the term (1/Q)² in the expression for inductance

becomes rather important and thus cannot be neglected. Hence this bridge is not suited for measurement of coils having Q less than 10 and for these applications a Maxwell's bridge is more suited.

• Thermo-couple instrument has a "squarelaw response" so it does not have a linear scale.

• Their accuracy can be as high as 1% for frequencies upto 50 MHz and can be classified as RF instruments. Above this frequency the effective resistance of heater wire is increased on account of skin effect, thereby reducing the accuracy. Tubular designs are used for heater wire above 3A to reduce errors on account of skin effect produced by high frequencies. For small currents upto 3A, the heater wire is solid and very thin.

• At normal rated current, the heater attains a temperature of 300 °C. If we pass twice the rated current, the heater would give a temperature of nearly 4 times the normal temperature i.e., 1200 °C. It is obvious that square law rate will bring the heater to nearly its burn out temperature. Thus it is imperative that the heater be protected against damaging over loads. The fuses do not provide any protection as due to overload the heater wire may burn out before fuse blows out. Thus the overload capacity of thermo-couple instruments is small as compared with other instruments and is about 150 percent of the full scale current.

  on solving bridge circuit.

There will be two deflecting torques, one acting on coil A and the other on coil B. The coil windings are so arranged that the torques due to the two coils are opposite in direction. So, there is no controlling torque.

The error caused by pressure coil inductance is VI sin ϕ tan β. W ith low power factor, the value of ϕ is large and therefore, the error is correspondingly large.

For full-wave rectifier type of instruments 

Braking torque T_D ∝ R² where R is the radial position of braking magnet with respect to the centre of the disc.