Mutchus

Weston (M.I) Type Frequency Meter Principle:  It is a moving iron instrument which operates on the principle of variation of impedance in an inductive circuit with variation of frequency. Weston Frequency Meter Construction: It consists of two coils A and B mounted perpendicular to each other. Each coil is divided into two sections. The connections are as shown in the above fig. The branch of coil has a resistance R 1 connected in the series with it while coil B has a reactance coil L 2 in series with it. Circuit of coil A is in parallel with a reactance coil L 1 while that coil B is in parallel with a resistance R 2 . The moving element is a soft iron needle. The needle is pivoted on a spindle which is also carries a pointer and damping vanes. There is no controlling torque. The series reactance L 0 acts to suppress higher harmonics in the current of instrument and therefore tends to minimize the waveform errors in its indication. Working: When the meter is connected across th...

Dynamometer Type Instrument ElectroDynamometer Instrument The electrodynamometer type instrument is a transfer instrument. A transfer instrument is one which is calibrated with a D.C source and used without any modifications for A.C measurements. Such a transfer instruments has same accuracy for A.C and D.C measurements. The electro-dynamometer type instruments are often used in accurate A.C voltmeters and ammeters, not only at the power line frequency but also in the lower audio frequency range with some little modifications, it can be used as a wattmeter for the power measurements. Principle: Electro-dynamometer type instruments are very similar to PMMC type instrument in which the operating field is produced, not by a permanent magnet but by another fixed coil (usually two fixed air cored coils are used). The PMMC instrument cannot be used on A.C currents or voltages. If A.C supply is given to these instruments, an alternating torque will be developed. Due to moment of inertia of th...

Errors in Moving Iron Instruments There are two types of error which occur in moving iron instruments. They are 1.        Errors with both A.C and D.C work:               (a)      Hysteresis error               (b)      Stray magnetic field error.               (c)      Temperature error.               (d)      Friction error. 2.        Errors with A.C work only:               (a)      Frequency error.                (b)      Error due to reactance of the instrument coil.                (c)      Error due to eddy current.     ...

Uni Junction Transistor (UJT) Basically, it is a three terminal silicon diode. As the name indicates, it has only one pn junction. It differs from an ordinary diode in that it has three terminals and it differs from a FET in that it has no ability to amplify. However, it has the ability to control a large a.c. power with a small signal. It also exhibits a negative resistance characteristics which makes it useful as an oscillator. It consists of an n-type silicon bar with an electrical connection on each end. The leads to these connections are called Base 1 B 1 and Base 2 B 2 . Partway along the bar between the two bases, nearer to B 2 than B 1 a pn junction formed. The lead of this junction is called emitter E the below Fig shows the basic structure and the circuit symbol of unijunction transistor.