Sunday, 27 January 2013

Rheostats Lecture Notes

Rheostats Lecture Notes: Rheostats are high power variable resistors. Rheostats have sliding contacts, the resistance value is varied by the sliding contacts. By winding suitable wire over a ceramic former coated with enamel, rheostats are manufactured. The sliding contact moves along a line with linear variation in resistance. The end portions of the movable contacts and the resistance wire are connected to a three lug type terminal.
The power ratings of rheostats are ranging from 10 to 100 Watts. The maximum withstanding temperature of rheostats are up to 3000C. The resistance value of rheostats varies from 10Ω to 5KΩ.

Figure 1
Figure 1 shows the circuit diagram of a rheostat. A rheostat can be used in an electronic circuit for the variation of currents through the circuit. In figure 1, as the value of the rheostat changes, the current through the lamp also changes.

Capacitors Lecture Notes

Capacitors Lecture Notes: Capacitor is an important passive component used in electronic circuits. The principle of a capacitor is to store charge (electrical energy) and release it at the required time. A capacitors ability to store charge is its capacitance. In other words the property of a circuit to oppose sudden change in voltage in the circuit is its Capacitance. Two conducting plates, separated by an insulating material (dielectric) constitute a capacitor. The capacitance of a capacitor is given by: 
c= εA/d

Where ‘ε’ is the dielectric constant of the dielectric medium used in the capacitor, ‘A’ is the area of the plates and‘d’ denotes the spacing between the plates.
Applications of capacitors in electronic circuits:
Dc voltages are blocked in circuits by using capacitors.
In cascade amplifiers for coupling capacitors are used.
As a bypass capacitor in amplifier circuits
For charge storage in electronic circuits

Action of a capacitor
Capacitor stores electrical energy by accumulating electrons on a metallic surface. The working principle is shown in the figure:


When the switch is opened, there will not be flow of electrons and no electrostatic field in dielectric.
When the switch is closed, electrons from the negative battery terminal moves to the plate.  The electrons repel the electrons on the other plate towards the connected battery V. This continuous process makes the one side of the capacitor negatively charged and the other side positively charged. No charging will takes place when the capacitor voltage reaches the applied voltage. The capacitor will get discharged when the applied voltage is removed from the circuit. At this time the excess electrons from the negatively charged plate will rush to the other plate and neutralize the positive charge on it. This will constitute a discharge current that will flow till the positive and negative charges are neutralized.  At this time the capacitor is completely discharged.
Charge stored in a capacitor is directly proportional to the given applied voltage and the proportionality constant is the capacitance.
Q = C*V
Where, Q is the charge stored ( in coulombs)
C is the capacitance in farads and
V is the voltage across the capacitor in volts.

Classification of Capacitors :

The main classification of capacitors is fixed capacitors and variable capacitors. Depending up on the dielectric material used, the classification of capacitors is again sub classified. Dielectric material is the physical parameter that effects the variation in capacitance.

Fixed capacitors:

According to the dielectric used, fixed capacitors are classified. Mica, paper, ceramic and electrolyte are the most commonly used dielectric materials.

Mica Capacitors:

Mica sheets are brittle. Mica sheets cannot be rolled up like a paper dielectric. So mica should be used as sheets. The mica should be selected carefully to be free from cracks and folds, uniformly thick and free from holes. Mica capacitors are manufactured by thin sheets of mica interleaved with foils of aluminium or tin.  The separate terminals are connected by two sets of metal plates. The important features of mica capacitors are small capacitance values and high voltage ratings. They have low loss factor and high stability. They are used for high frequency applications.