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What is a Resistor and how does it works.


What is a Resistor.

A resistor is a device which oppose the flow of current through it in a circuit. A resistor can also be defined as a device designed to offer a known value of resistance in 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 value of resistance of a resistor, the more opposition given to flow of current by a resistor.

Resistance of a resistor is due to bonds between electrons and protons and also collision between electrons themselves as they move through a conducting material.

The following is the general symbol of resistors although it is also related to fixed resistors symbol.

General symbol

resistor symbol

Laws of Resistance

When designing a resistor they are factors which are considered so that a resistor of known value of resistance can be manufactured.

We have explained these factors in detail in the article - Factors affecting resistance.
The factors are:

factors affecting resistance

1.Cross sectional area.

Cross sectional area affect the resistance of a resistor, resistance varies inversely with cross sectional area that is when cross sectional area is increased resistance decreases.


The length of a resistor affect its value of resistance, the resistance of a resistor varies directly with length that is if the length of a resistor is increased its resistance also increases.

3.Type of material.

Aslo known as resistivity. Resistance of a resistor depends on type of material used. Materials such as copper offers low resistance while materials such as mica, ceramic offers high resistance.

To come out with the actual resistance value of a resistor to manufacture we combine the three factors in the formula:

formula of resistance

Resistor Specification.

When selecting a resistor for a particular circuit, they are some factors which are considered so that the selected resistor for a particular circuit 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 size will anable proper operation of the circuit.

If a resistor is wrongly selected maybe of smaller size than required, it will cause to much current to flow in a circuit resulting in damage of circuit components and itself or if of larger size than required, too much current will be limited results in circuit components not operating well.


Is the minimum and maximum deviation of a resistor from its normal value. For example if a resistor of 20 Ohm has a tolerance of 5%, it means 20 Ohm minus 5% of 20 Ohm is the minimum deviation and 20 Ohm plus 5% of 20 Ohm is maximum deviation.

If measuring the resistance of a resistor and find the value between its minimum and maximum deviation values, it mean the resistor is live but if not it means the resistor is dead.

3.Power rating

Is the maximum power that a resistor can dissipate without the temperature rise being such that damage occurs to the resistor.

Power is the rate of doing work or is the amount of work done in one second. In context of resistors, it is the amount of voltage × current passing through it in every second.

When current pass through a resistor it produces heat depending with the amount of current, so power rating takes into account the current in relation to supply voltage which can flow through a resistor without causing heat which result in damage of a resistor.

4.Power Disipation

it is actual power a resistor disipate in the circuit. Power disipation is different from power rating in that when selecting a resistor we first find the power disipation a resistor is supposed to disipate in a particular circuit and then go for power rating.

Power disipation is found by multiplying the square of current which is supposed to flow in a circuit and the resistance value of a resistor.

Usually power disipation is of lower value than power rating, if power disipation exceed power rating, damage to resistor occurs

As you are now familiar with resistor specification, resistor specification is very important, it anable proper selection of a resistor for a particular circuit.

In circuits like of instrumentation, where we need accurate measurements, the resistors used there should have a low tolerance value so that their deviation from normal value does not affect our measurements.

Types of Resistors

They are various types of resistors we found in electrical circuits, these resistors are grouped into linear and nonlinear resistors.

Linear resistors

These are resistors which obeys Ohm's law.

Ohm's law state that current (I) in a circuit is directly proportional to the circuit voltage when temperature of the circuit is not changing or remain constant.

A change in temperature affect resistance of most materials, if a circuit maintain its resistance when its temperature is kept constant, it obeys ohms law.

If a circuit's resistance changes even when its temperature is kept constant due to other factors it does not obey ohms law.

Linear resistors are said to obey Ohm's law because when connected across a voltage source, current passing through them increases or decreases proportionally with applied Voltage.

It should also be noted that a resistor to be called linear it should have a negligible change in its resistance even when its temperature changes between acceptable limits.

In linear resistors we have fixed and variable resistors.

Variable Resistors.

variable resistors symbol

These are resistors with resistance value which can be varied or tuned between their minimum and maximum values for a example rheostat.


A rheostat consist of a resistive carbon track and a wiper which is moved along the track inorder to vary the resistance.

It has three contacts, but only two contacts are used as shown in the following diagram. If all contacts are used it works as a potentiometer.


Fixed Resistors.

fixed resistor symbol

These are resistors with resistance value which cannot be changed or altered. Examples of fixed resistors include carbon composition resistors, carbon firm resistors, metal film resistors ,wire wound resistors etc.

Fixed resistors are color or digit letter coded which represents their resistance value and also not every value of fixed is manufactured only prefered values are manufactured.

After calculating the value of resistor you then go to groups of prefered values and then pick the resistor close to the one you have calculated. We have covered color coding and prefered values in detail in our youtube video.

1.Carbon composition resistors

These are resistors formed from a mixture of carbon and a binding resin with different proportions for producing a desired resistance.

The resistors has tinned coper leads for soldering a resistor into a circuit. The resistors are enclosed inside a plastic case to prevent them from moisture and others

When current pass through them they tend to produce an electric noise. They has a low power rating which goes up to 2W, 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 deposing a caborn firm on a ceramic rod. These are an improvement of carbon composition resistors.

3.Metal firm resistors

Metal firm resistors are resistors made from deposing metal on a ceramic rod. These resistor have an excellent tolerance value.

4.Wire wound resistors

Wire wound resistors are resistors which consist of resistive wire wound on a ceramic rod. Different wire alloys are used for providing different resistance ranges.

These resistors have highest power rating and stability. Due to their high cost they are not suitable for low cost applications.

Non-linear resistors

These are resistors which do not obey Ohms law. Non-linear resistors are resistors with resistance which changes due to other factors even when their temperature is kept constant.

Examples of Nonlinear resistors include light dependent resistors, temperature coefficient resistors, voltage dependant resistors, magnetic field dependant resistors e.t.c.

1.Light dependent resistors

Light dependent resistors symbol

These are resistors which are sensitive to light intensity falling on their surfaces, their resistance decreases with increase in light intensity falling on them as shown in the diagram below.

On the graph the vertical axis shows values of resistance and the horizontal axis shows the intensity of light from left to right.

Characteristic curve


These are resistors which are sensitive to heat. We have positive and negative temperature coefficient resistors.

i. Positive temperature coefficient resistors - PTC

Positive temperature coefficient resistors symbol

These are resistors with resistance which increase with increase in temperature as shown in the following diagram.

Characteristic curve

ii. Negative temperature coefficient resistors - NTC

negative temperature coefficient resistor symbol

These are resistors with resistance which decreases with increase in temperature as shown in the following diagram

Characteristic curve