Resistors

A resistor offers resistance to the flow of current. The resistance is the measure of opposition to the flow of current in a resistor. More resistance means more opposition to current. The unit of resistance is ohm and it is represented as Ω. When one volt potential difference is applied across a resistor and for that one ampere of current flows through it, the resistance of the resistor is said to be one Ω. Resistor is one of the most essential passive elements in electrical and electronics engineering.

                                                 

Types of Resistor

            There are different types of resistor depending upon their construction, power dissipation capacities and tolerance of the value. Such as

1. Carbon Composition Resistor
2. Metal Film Resistor
3. Carbon Film Resistor
4. Non Linear Resistor
5. Varistor
6. Thermistor .

Carbon Composition Resistor

                    These types of resistor are very commonly used low cost resistor. The construction of carbon composition resistor is very simple. It is also commonly referred as carbon resistor. It is mainly made of carbon clay composition covered with a plastic case. The lead of the resistor is made of tinned copper.

                  The main disadvantage is that they are very much temperature sensitive. These resistors are available in wide range of values. It is available in as low as 1 Ω value and it is also available in as high as 22 Mega Ω value. The tolerance range in resistance of carbon composition resistor is of ± 5 to ± 20 %.

Wire Wound Resistor:-

                    The construction of this type of resistor is also very simple. In wire wound resistor a wire of manganin or constantan is wound around  cylinder of insulated material. The temperature coefficient of resistance of these two materials is almost zero. So there would no resistance variation with temperature. The wounded wire is covered with an insulating material such as baked enamel. This cover of insulating heat resistible material is provided to resist the effect of ambient temperature variation.The range of resistance values varies from 1 Ω to 1 MΩ. Typical tolerance limit of these resistors varies from 0.01 % to 1 %. They can be used for high power applications of 5 to 200 W dissipation ratings.              

                                     

Metal Film Resistor and Carbon Film Resistor:-

            Basic structure of this type of resistor is constructed by means of film deposition technique of deposition a thick film of resistible material such as pure carbon or metal on to an insulating core. The desired value of resistance of metal film resistor or carbon film resistor can easily be obtained by either trimming the layer of thickness or by cutting helical grooves of suitable pitch along its length. That means for different resistance values, the length and depth of the helical grooves is maintained accordingly.

Metallic contact cap is fitted at both ends of the resistor. The caps must be in contact with resistible film or helical grooves. The lead wires are welded to these end caps. Metal Film Resistor or Carbon Film Resistor can be made up to a value of 10,000 MΩ and size of this type of resistor is much smaller than wire wound resistor resistor. Because of their constructional features these resistor are fully non - inductive. The accuracy level of metal film resistor can be of order ± 1 % and they are suitable for high grade applications.

Variable Resistor:-

           The variable resistor means its resistance value can be changed at will. There is a rotating shaft or sliding contact is provided so that by sliding the contact we can change the resistor value. Basically there we have a resistance coil and by sliding the contact we change the position of the coil and hence the resistance changes. Examples of such resistors are Potentiometer, Rheostat etc.

Symbol of Variable Resistor

Thermistor

      This is a special resistor that is used in electrical circuit. The word means that it is thermal resistor. Its resistance value changes with the change in the temperature value. Most of the thermistors have the negative temperature coefficient which means its resistance will fall down when the temperature value increases. The temperature coefficient may be positive as well. It is constructed by the use of various metals such as nickel, cobalt, copper etc. Also it can be prepared from the semiconductor devices as well.

Symbol of Thermistor

Non Linear Resistor

      They are also known as varistors. They are popular for having the non linear V-I characteristics curve. That is its resistance is not uniform and it does not obey OHM’s law. They are made of materials such as silicon carbides, zinc oxide. They are of three types-

1. Silicon carbide disc type varistor.
2. Silicon carbide rod type varistor.
3. Zinc oxide type varistor.

Light Dependent Resistor

    As the name suggests its resistance value depends on the intensity of light falling on it. This is made of cadmium sulfide containing a very small number of electrons when not illuminated. When light ray of a particular intensity falls on it electrons are ejected and hence the conductivity of it increases. That is it offers low resistance when light falls on it and offers high resistance in the dark or no light condition. They are only light dependent up to a wavelength of 680 nm. Mathematically we can say that, R = KE^-α Where, K and α are constant. R is the resistance value in ohm. E is the intensity of illumination in lux.

Symbol of Varistor