What is a Resistor and how does it works?
A resistor is a device that opposes the flow of current through it in a circuit. A resistor is also defined as a device designed to introduce a known value of resistance into a circuit.
Resistance of a resistor is the ability of a resistor to oppose or hinder the flow of current through it. Resistance is measured in ohms. The larger the resistance value of a resistor, the more opposition it gives to flow of current.
The resistance of a resistor is due to bonds between electrons and protons and also collisions amomg electrons themselves as they move through a conducting material.
The following is the general symbol for resistors, although it is related to the symbol fixed resistors.
General symbol
Laws of Resistance
When designing a resistor, there are factors that are considered so that a resistor with a known value of resistance is manufactured.
We have explained these factors in detail in the article, Factors Affecting Resistance.
The factors are:
1.Cross-sectional area.
Cross-sectional area affects the resistance of a resistor. Resistance varies inversely with cross-sectional area, meaning that when the cross-sectional area increases, resistance decreases.
2.Length.
The length of a resistor affects its resistance value. The resistance of a resistor varies directly with length, meaning that if the length of the resistor increases, its resistance also increases.
3.Type of material.
Also known as resistivity. The resistance of a resistor depends on type of material used. Materials such as copper offer low resistance, while materials such as mica and ceramic offer high resistance.
To come up with the actual resistance value of a resistor, we combine the three factors in the formula:
Resistor specifications.
When selecting a resistor for a particular circuit, there are factors that are considered so that the selected resistor will operate as desired.
The factors are:
1.Value of a resistor in ohms
For example, 10 Ohms. Resistor value is important since the right value anables proper working of the circuit.
If a resistor is wrongly selected, maybe of smaller value than required, it will causes too much current to flow in a circuit, resulting in damage to the circuit components and itself. If the value is larger than required, the current will be limited, resulting in circuit components not operating well.
2.Tolerance.
Is the minimum and maximum deviation of a resistor from its nominal value. For example, if a resistor of 20 ohms has a tolerance of 5%, it means 20 ohms minus 5% of 20 Ohm is the minimum deviation and 20 Ohms plus 5% of 20 Ohms is maximum deviation.
If you measure the resistance of a resistor and find the value within its minimum and maximum deviation values, it means the resistor is fuctional. However, if the measured value falls outside this range, it indicate that the resistor is faulty or dead.
3.Power rating
Power rating is the maximum power that a resistor can dissipate in a circuit without a temperature increase that could causes damage to the resistor.
Power rating = Current × Voltage
4.Power Disipation
It is actual power a resistor disipates in the circuit. Power disipation is different from power rating. When selecting a resistor, we first determine the power disipation a resistor is expected to handle in a particular circuit, and then choose a suitable power rating.
Power dissipation = (Current)² × Resistance
Power dissipation is of a lower value than power rating. If power dissipation exceeds power rating, damage to resistor occurs
Resistor specifications are very important, they anable proper selection of a resistor for a particular circuit.
In circuits such as instrumentation, where accurate measurements are needed, the resistors used there should have a low tolerance value so that their deviation from nominal value does not affect the measurements.
Types of Resistors
There are various types of resistors found in electrical circuits, these resistors are grouped into linear and nonlinear resistors.
Linear resistors
These are resistors that obey Ohm's law.
Ohm's law states that current is directly proportional to voltage when temperature is kept constant.
Linear resistors obey Ohm's law, current through them proportionally increases or decreases with voltage when the temperature surrounding them is not changing.
Linear resistors are grouped into fixed and variable resistors.
Variable Resistors.
These are resistors with resistance value that can be varied or adjusted between their minimum and maximum values.A rheostat is an example of variable resistors.
Rheostat
A rheostat consists of a resistive track and a wiper that is moved along the track to vary the resistance as shown in the diagram below.
Fixed Resistors.
These are resistors with resistance values that cannot be changed or altered. Examples of fixed resistors include carbon composition resistors, carbon film resistors, metal film resistors and wire-wound resistors.
Fixed resistors are color-coded or digitally coded, which represents their resistance value.
1.Carbon composition resistors
These are resistors formed from a mixture of carbon and a binding resin in different proportions to produce a desired resistance.
The resistors have tinned coper leads for soldering them into a circuit. The resistors are enclosed in a plastic case to protect them from moisture and other factors.
When current passes through them, they tend to produce electric noise. They have a low power rating, which goes up to 2W and a high tolerance value. If properly used, their failure rate is low. They are most common in low-cost applications.
2.Carbon firm resistors
These are resistors made from depositing a carbon film on a ceramic rod. These are an improvement over carbon composition resistors.
3.Metal firm resistors
Metal film resistors are resistors made from depositing metal on a ceramic rod. These resistors have an excellent tolerance value.
4.Wire-wound resistors
Wire-wound resistors are resistors that consist of resistive wire wound on a ceramic rod. Different wire alloys are used to provide different resistance ranges.
These resistors have the highest power rating and stability. Due to their high cost, they are not suitable for low-cost applications.
Non-linear resistors
These are resistors that do not obey Ohm's law. Non-linear resistors are resistors with resistance that change due to other factors even when the temperature surrounding them is kept constant, which is why the current through them do not vary directly with voltage.
Examples of nonlinear resistors include light-dependent resistors, thermistors (temperature-dependent resistors), voltage dependent resistors and magneto-resistors (magnetic field-dependant resistors).
1.Light dependent resistors
These are resistors that are sensitive to light intensity falling on their surfaces. Their resistance decreases with an increase in light intensity falling on thier surfaces, as shown in the diagram below.
On the following graph, the vertical axis shows the values of resistance. The horizontal axis shows the intensity of light from left to right.
2.Thermistors
These are resistors that are sensitive to heat. We have positive and negative temperature coefficient resistors.
i. Positive temperature coefficient resistors - PTC
These are resistors with resistance that increases with an increase in temperature, as shown in the following diagram.
ii. Negative temperature coefficient resistors - NTC
These are resistors with resistance that decreases with an increase in temperature, as shown in the following diagram
