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Dynamometer Type Wattmeter This is also similar in design and principle to the dynamometer type ammeter and voltmeter. Pic 1 Construction: In the dynamometer type instrument is used as a wattmeter, the fixed coil, which is divided into two equal portions, the fixed coils are connected in series with the load and carry the circuit (or load) current, therefore it is also called as current coil. The moving coils is connected across the load and carries the current proportional to the voltage across the circuit. Therefore, it is known as pressure coil or potential coil. A high non-inductive resistance ‘R’ is connected in series with the moving coil to limit current through it. The magnetic field of the fixed and moving coils react on one another causing the moving coil to turn about its axis. The movement of the coil is controlled by hair springs also lead the current into and out of the moving element. Damping torque is provided by the air friction. Working: When the instrument is connect...

Function Generator The function generator is a versatile instrument that gives a number of different waveforms at the output with their frequencies that can be adjusted over a wide range. The most commonly used waveforms are sine wave, square wave, triangular wave and the saw tooth wave. Different output signals from the function generator may be available simultaneously. Picture of Function Generator The fig shows the block diagram of a function generator. Here two constant current sources are used to feed the integrator circuit. When the circuit is switched ON, the upper constant current source sends a constant current into the integrator and the output of the integrator is connected to comparator. When the integrator output reaches a predetermined level, the comparator changes the state i.e. the upper constant current source is cut OFF while the lower source is switched ON. This current source passes a current in the reverse direction and the voltage at the integrator output decreas...

Cathode Ray Tube (CRT) A cathode ray tube is the heart of the CRO. The below fig shows the various parts of CRT. 1.        Glass Envelope: It is conical highly evacuated glass housing maintains vacuum inside and supports the various electrodes. The inner walls of CRT between neck and screen are usually coated with the conducting material called aquadag. This coating is electrically connected to the accelerating anode so that electrons which accidently strike the walls are returned to the anode. This prevents the walls of the tube from charging to a high negative potential. Pic 1. Cathode Ray Tube 2.        Electron Gun Assembly: the arrangement of electrodes which produce a focussed beam of electrons is called electrongun. It essentially consists of an indirectly heated cathode, a control grid, a focussing anode and an accelerating anode. The control grid is held at negative potential with respect to cathode whereas the anode a...

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 du...

Extending the Range of Ammeters and Voltmeters The ranges of electrical measuring instruments (whether ammeter, voltmeter or any other type of meters) are limited by the currents, which be carried by the coils of the instruments safely. For example, the moving coils of the instruments can carry maximum current of about 50 mA safely and the potential drop across the moving coil is about 50 mV. Hence, it becomes necessary that the current and voltage being measured be reduced and brought within the range of instrument. There are four common devices used for extending the range of the instruments, namely \ 1.        Shunts 2.        Multipliers 3.        Current transformers 4.        Potential transformers The let us discuss the first two topics i.e. shunts and multipliers 1.        Shunts: The range of an ammeters can be extended by connect...

PWM is used in all sorts of power control and converter circuits. Some common examples include motor control, DC-DC converters, DC-AC inverters and lamp dimmers. There are numerous PWM controllers available that make the use and application of PWM quite easy. One of the most popular of such controllers is the versatile and ubiquitous SG3525 produced by multiple manufacturers – ST Microelectronics, Fairchild Semiconductors, On Semiconductors, to name a few. SG3525 is used extensively in DC-DC converters, DC-AC inverters, home UPS systems, solar inverters, power supplies, battery chargers and numerous other applications. With proper understanding, you can soon start using SG3525 yourself in such applications or any other application really that demands PWM control. Before going on to the description and application, let’s first take a look at the block diagram and the pin layout. Pins 1 (Inverting Input) and 2 (Non Inverting Input) are the inputs to the on-board error amplifier. If you a...