what is a resistor?
A resistor is an electrical component with a known specific value of resistance.It is probably the most common component in all kinds of electronic equipment ranging from a small radio to a colour television receiver.As its name suggests ,a resister resists or opposes the flow of current through it.Resistance is necessary for any circuit to do useful work.In fact,without resistance,every circuit would be a short circuit!
Some of the common uses of resistors are:
- to establish proper values of circuit voltage due to IR drops
- to limit current and
- to provide load
The two main characteristics of a resister are its resistance and power rating.Resistors can be connected in the circuit in either direction because they have no ‘polarity’.
Resistors are mainly of two types and can be either of fixed or variable value.
- Wire wound resistor
- Carbon resistors
- Carbon composition type
- Carbon film type
- Cermet film type
Another type is called metal thin film resistor.
Wire wound resistors:
They are constructed from a long fine wire(usually nickel-chromium wire) wound on a ceramic core.The length of the wire used and its resistivity determine the resistance of the unit.The wire is bare but the entire assembly is covered or coated with a ceramic material or special vitreous enamel.
Such resistors are generally available in power ratings from 5 W to several hundred watts and resistance values from 1 Ω to 100 k.These can be of either fixed value or variable type.Wire wound resistors are used where:
(a):Large power dissipation is necessary
(b):Precise and stable resistance values are required as for meter shunts and multipliers
Carbon composition Resistors:
They are made of finely divided carbon mixed with a powered insulating material in suitable proportion.Often,the resistance element is a simple rod of press carbon granules which is usually enclosed in a plastic case for insulation and mechanical strength.The two ends of the carbon resistance element are joined to metal caps with with leads of tinned wire for soldering its connections into a circuit.
Such resistors are available in power ratings of 1/10 ,1/8 ,1/4,1/2,1,2 watt and in resistance values ranging from 1 Ω to 20 MΩ.Where power dissipation is 2 W or less,such resistors are preferred because they are smaller and cost less .Carbon resistors with power rating of 1 W or less are most common in electronic equipment.
Carbon film resistors:
They consists of a high grade ceramic rod or core (called the substrate) on which is deposited a thin resistive film of carbon.They are cheaper than composition resistors.
Cermet film resistors:
They consists of thin carbon coating fired on to a solid ceramic substrate.The main purpose is to have more precise resistance values and greater stability with heat.Very often ,they are made in a small square with leads to fit into a printed circuit board.
Metal film Resistors:
They are also referred to a as thin film resistors.They consists of a thin metal coating deposited on a cylindrical insulating support.The high resistance values are due to thinness of the film.Because it is difficult to produce films of uniform thickness,it is not possible to control their resistance values accurately as in the case of wire wound resistors.Such resistors are free of trouble some inductance effects so common in wire wound resistors particularly at high frequencies.
The power rating of a resistor is given by the maximum wattage it can dissipate without excessive heat.Since it is current which produces heat,power rating also gives some indication of the maximum current a resistor can safely carry.If the current exceeds this value ,more heat will be produced than can be carried safely and the resistor will burn out,A 1/2 watt resistor,for example ,can dissipate 1/2 watt of heat without damage various a 1 W resistor can throw off twice as much heat.In a circuit , you may substitute 1 watt resistor of same resistance value for a 1/2 watt resistor but not vice versa.
The physical size of a resistor is no indication of its resistance through it thus give some indication of its wattage rating. For a given value of resistance,greater the physical size,higher the power rating.Also higher wattage resistors can operate at higher temperatures,moreover ,higher power rating allows a higher voltage rating.This rating gives at highest voltage that may be applied across the resistor without internal arcing.
By tolerance is meant the posible variations from the nominal or marked resistance value of a resistor.It means that actual resistance of resistor may be greater or lesser than its indicated value.All resistors are manufactured and sold with a specified tolerance.F or example 1000 Ω resistor with a tolerance of 10 % will have a actual resistance anywhere between 900 Ω and 1100 Ω i.e 100Ω more or less then rated value.
Carbon composition resistors have tolerance of ±5% ,±10 % and ± 20 % whereas general purpose wire wound resistors usually have tolerance of ± 5%.
Variable resistors are designed so that their resistance values can be changed easily with a manual or an automatic adjustment.Two basic uses for variable resistors are to divide voltage is called a potentiometer.
The variable resistor used to control current is called a rheostat .
Two types of automatically variable resistors:
A thermistor is a type of variable resistor that is temperature sensitive .When its temperature coefficient is negative,the resistance changes inversely with temperature. When its temperature coefficient is positive the resistance changes directly with temperature.
The resistance of a photo-conductive cell changes with a change in light intensity.This cell also has a negative temperature coefficient. Symbols for both of these devices are shown in below figure.