Introduction

They are present in almost every circuit and you will also find this in Ohm’s law. To know what is Ohm’s law click here

In simple words, resistors are materials which have resistance. They control the amount of current flowing in a circuit.

What makes up a Resistor

Most common resistors are made up of alloys and carbon.

Resistor in a circuit

A resistor has two terminals(legs). In circuit, they are represented by zig-zag lines. The terminals of the resistor are each of the lines coming out from the zig-zag shaped lines. These lines connect to the rest of the circuit.

Symbol for a resistor

Here is a circuit in which you can see many resistors, each resistor is named as R followed by a number(which are sequential) and the value of the resistor is also written with the resistor.

How resistor works

Resistors have a fixed value of resistance which never changes.[Link to Ohm’s Law].

Their job is to control the amount of current flowing in a circuit. They control current by converting some power into heat. This heat is released out of the circuit.

The resistance of a resistor is measured in ohms.

The symbol for an ohm is Ω.

How to use resistors

If you want to keep any component safe in your circuit. If you want to prevent it from burning out. Take a resistor. Connect one end of resistor to the positive terminal of battery. Connect the other end to the component’s terminal. This is the most basic way in which you can connect the resistor.

Do you know that batteries have polarity. They have a positive(+ve) terminal and a negative (-ve) terminal. But resistors don’t have any polarity. You are free to connect any end of a resistor to another component’s +ve or -ve terminal.

Ways in which you can connect resistors

Resistors are generally connected together in electronics, usually in either a series or parallel circuit. When resistors are combined in series or parallel, they create a total resistance, which can be calculated as follows-

In Series

When connected in series resistor values simply add up.

Total Resistance in circuit, R = R1 + R2 + R3

In Parallel

Calculating the total resistance when resistors are connected in parallel is not easy but you can do it…this is how.

The total resistance of all the resistors connected in parallel is the inverse of the sum of all inverse resistances.

Total Resistance in this circuit, 1/R = 1/R1 + 1/R2 + 1/R3

Types of resistors

There are many types of resistors are available in the market, such as:

Carbon composition

Carbon film

Metal film

How to purchase resistors?

Buying resistors is easy.

You don’t need to worry about the types of resistors you choose. Metal film resistors are widely used these days and they can be a perfect choice for your hobby project.

You have to take care about the following parameters before choosing a resistor.

1. Value of a resistor

How will you decide what value of resistor you have to buy? This question is easy to solve. Just look carefully at the circuit diagram that you are making and you can see the value of resistor written there. The values will be written like this-

Note the value of each of the resistor in your circuit.

2. Tolerance

In simple words, tolerance of a resistor tells you how much the resistor value will shift from its original value. Tolerance value of a resistor can be identified by looking at the resistor. Common colour bands for tolerance in a resistor are silver and gold. The tolerance colour band is at a gap from other colour bands. You can see this in the image below-

So, you have to choose resistors with very less tolerance like 1%(Brown), 2%(Red) and 5%(Gold). The tolerances of 10%(Silver) and 20%(No colour) are large and should be avoided while buying a resistance. Large tolerances mean you cannot depend on your resistors(used in circuit), as their values can vary too much in practical.

3. Power Handling

To understand what is power handling capacity of a resistor, you can do a simple activity. For this activity you need –

A 18Ω resistor rated 0.25 Watt

0.25 Watt rated resistors are small in size, like this

A 9V battery

Touch the resistor, it should at normal temperature. Now connect both the terminals of resistor to the terminals of battery(don’t worry about +ve and -ve here). After few seconds,just touch the resistor (Be careful!!), it is now very very hot.

You must be surprised, how can this happen!!

Let us do some maths to get the answer.

From Ohm’s law we know, I = V/R. So, your resistor is 18Ω and V is 9V(battery).

I = 9/18 = 0.5 Amps = 500 milliamps

Now, Power through resistor = V x I = 9 x 0.5 = 4.5 Watts (Unit of power is watts).

Power is amount of energy transferred per unit time. So, you can see that 4.5 watt power is given to a resistor rated with 0.25 watt. So much energy is passing through a small resistor. This is the reason it becomes too much hot.

So, before buying resistors you have to take care that how much power you are going to give them (you can calculate and see). The power that you are going to give to the resistor should not exceed its power rating value. For small circuits where we use small voltages like 2, 4, 5, 6 etc, 0.25 watt resistors can be used. If you are going to work with larger voltages then you have to use higher power ratings of resistors like 1 watt or more than that. Supplying more power to a resistor than its power rating, can damage the resistor and make it less reliable.

1 watt resistor are quite bigger in size and looks like this –

Here is an image of resistors with different power ratings.

You can observe the difference in sizes of these resistors.

Some interesting facts

The value of a resistor is measured in Ohms, named after the german scientist Simon Ohm.
There are some resistors whose value can be varied, they are called variable resistors or potentiometers.