Resistor and Its types
An array of axial-lead resistors:
Electronics symbol:
| Type | Passive |
|---|
| Working principle | Electric resistance |
|---|
A resistor is a passive to two-terminal electrical component that implements electrical resistance as a circuit element.
In electronic circuits ,resistor are used to reduce current flow, adjust signal levels, to divide voltage,bias active elements and terminate transmission line, among other uses. High power resistors that can dissipates many watts of electrical power as heat, may be used as part of motor controls, in power distribution system or as a test load for generator. Fixed resistors have resistance that only change slightly with temperature, time or operating voltage. Variable resistor can be used to adjust circuit elements ( such as a volume control or a lamp dimmer), or as sensing device for heat, light, humidity, force, or chemical activity.
Resistors are common elements of electrical networks and electronic circuits and are ubiquitous in electronic equipment. Practical resistors as discrete component can be composed of various compounds and forms. Resistors are also implemented within an integrated circuit.
Theory of operation with symbol of resistor
The ohm is obtained represented by the Omega (Ω) symbol: The symbol of resistance is a zigzag line as shown in below.
The letter R is used in equations.
Ohm's law(Ω):
The behavior of an ideal resistor is dictated by the relationship specified by ohm's law.
V=I.R.
ohm's law states that the voltage (V) across a resistor is proportional to the current (I) where the constant of proportionality is the resistance (R). For example, if 300Ω resistor is attached across the terminal of 12 volt battery, then the current of 12/30=0.04 amperes flows through that resistor.
practical resistors also have some inductance and capacitance which affect the relationship between voltage and current In alternating current circuits.
Ohms(Ω) is the SI unit of electrical resistance, named after George Simon ohm. An ohms is equivalent to a volt per ampere. Since resistors are specified and manufactured over a very large range of values, the derived unit of mΩ(1 mΩ = 10−3 Ω), kiloΩ (1 kΩ = 103 Ω) and MegaΩ (1 MΩ = 106 Ω) are also in common usages.
Serial and parallel resistors:
The total resistance of resistors connected in series in the sum of their individual resistance values.
The total resistance of resistor connected in parallel in the reciprocal of the sum of the reciprocals of the individual resistors.
For example, a 10 ohm resistor connected in parallel with a 5 ohm resistor and a 15 Om resistor produce 1/1/10 + 1/5 + 1/15 ohms of resistance, or 30/11= 2.727 ohms.
Resistor network that is a combination of parallel and series connection can be broken up into smaller part of that are either one or the other. Some complex network of register cannot be resolved in this manner, requiring more sophisticated circuit analysis. generally the Y-∆ transform or Matrix method can be used to solve such problems.
Power dissipation:
At any instant the power P(watt) consumed by a resistor of resistance R ohms is calculated as: 
where V(volts) is the voltage across the registers and I (ampere) is the current flowing through it. using ohm's law, the two other forms can be derived. this power is converted into heat which must be dissipated by the registers package before its temperature rises excessively.
Resistors are rated according to their maximum power dissipation. They usually absorb much less than a watt of electrical power and require little attention to their power rating.
Resistors required to dissipate substantial amount of power, particularly used in power supplies, power conversion circuits, and power amplifier, are generally referred to as a power resistors; the designation is loosely applied to resistors with power rating of 1 watt or greater. power registers are physically larger and may not use the preferred values, color code and external packages described below.
If the average power dissipated by a resistor is more than power rating,damage to the resistor may occur, permanently altering its resistance; this is distinct from the reversible change in resistance due to its temperature coefficient when it warms. Excessive power dissipation may rise the temperature of the resistor to a point where it can burn the circuit board or adjacent component or even cause a fire. There are flameproof resistors that fail (open circuit) before they overheat dangerously.
since poor air calculation, high altitude, or high operating temperature may occur resistors may be specified with higher rated dissipation than is experienced in service.
All resistors have a maximum voltage rating. this may limit the power dissipation for higher resistance values.
Types of resistors
- Carbon composition resistor
- Thermistor
- Wire wound resistor
- Metal film resistor
- Carbon film resistor
- Variable resistor
- Varistor
- Light dependent resistor
Carbon composition resistor
A carbon composition resistor is a very commonly used resistor. These resistors are low cost and easy to construct.
Carbon resistors are mainly made of carbon clay composition covered with a plastic case. The lead of resistor is made of tinned copper.
The main advantage of this resistor are that they are readily available, low cost, and they are very durable. these resistors are also available in a wide range of values, from as low as 1 ohms to as high as 22 megaohms. For this reasons, carbon composition resistor are often included in many of the best Arduino starter kits.
The main disadvantage of carbon composition resistor is that they are very temperature-sensitive. The tolerance range in resistance of carbon composition resistor is 5 to 20%. Although this is not a concern for the majority of Electronics project one would experiment with at home.
This type of register has a tendency to produce some electric noise due to the passage of electrical current from one carbon practical to others. where low cost is the main criterion for designing a circuit rather than its for perfection of performance, these resistors are normally used. carbon register are provided with a different colored band on their cylindrical body. these color band are code for the resistance value of resistors along with their tolerance range.
Thermistor
The word thermistor is a thermal resistor. Its resistance value change with the change in the temperature. Most thermistors have a negative temperature coefficient which means its resistance will fall down when the temperature increases. These are normally made of semiconductor materials A resistance upto a few megaohms can be obtained from thermistors. they are used to detect a small temperature change, when there is a temperature change, however small, there will be a large change in the value of the resistance.
Wire wound resistor
In wire wound resistor a wire is manganin constantan is wound around a cylinder of insulating material. the temperature coefficient of resistance of manganin and constantan is almost zero. So resistance variation with temperature of these resistor is negligible. the wounded wire is covered with an insulating cover such as baked enamel. This cover of insulating heat resistible material resist the effect of ambient temperature variation. different sizes and ratings of wire wound Resistors can easily be achieved by using different length and diameters of the wire.
these resistors are easily available for wide range of rating. This range of resistance values vary from 1 ohm to 1 mega ohm. The typical tolerance limit of these Resistors varies from 0.01% to 1%. They can be used for high power applications of 5 to 200W dissipation ratings. The cost of these resistor is much higher than carbon registers. Normally a wire wound resistor is used to where a carbon composition resistor cannot meet the purpose because of its limitations.
The main disadvantage of these resistors is the inductance that arises because of its coil like structure. at high frequency, the behavior of the circuit may be changed due to its reaction. This problem can be solved if one half of the wire is wound in one direction and another half in the opposite direction so that the inductance due to this two halves cancel each other hence the net inductive effect of the resistor becomes Nil. The non inductive wire wound resistor is ideal for high frequency circuit but it is costlier than an ordinary one.
Metal film resistor and carbon film resistor
The resistor is constructed by means of deposition a thin film of a conductive 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 the thickness or by cutting helical grooves of suitable pitch along the its length.
Metallic contact cap is fitted at both end of the resistor. The caps are in contact with the conductive film or helical grooves. the lead wire is welded to the end caps. Metal film resistor or carbon film resistor can be made up to a value of 10000 mega ohms and size of this type of Resistor is much smaller than a wire wound resistor. because of their constructional features, these register are only non inductive.
The accuracy level of metal film resistor can be of order 1% and they are suitable for high grade applications. carbon film resistor give lower tolerances and smaller values of electrical resistance than those available with metal film. however the carbon film posses a mildly negative temperature coefficient of resistance which is very useful for certain electronic circuits.
Variable Resistor
The variable resistor means its resistance value can be adjusted similar to a potentiometer. There are a rotating shaft and a wiping contact. Basically, there is a resistive semicircular bar or coil and by wiping the contact we change the effective length of the resistive element and hence the resistance gets changed. One example of such Resistors is rheostat.
The variable resistor or rheostat can also be a linear sliding type where the sliding contact move on the resistor element linearly for adjustment of the effective resistance of the resistor.
Nonlinear resistor or Varistor
They are also known as varistors. They are popular for having a non-linear V-I characteristic 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.
There are three types of varistors:
- Silicon carbide disc type varistor
- zinc oxide type Varistor
- Silicon carbide rod type varistor
Light dependent resistor
Light dependent resistor or LDR will vary in resistance depending on the intensity of light falling on it. This is made of cadmium sulphide which contain a smaller number of electrons when it is not illuminated. when a light rays falls on it, electrons get ejected and hence the conductivity of it increases. Hence it offers low resistance when light falls on it and offers high resistance in the dark.
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