Learn The Basics of a Resistor [Easy And Complete Explanation]

Resistor Basics

Hi. Hope you are having a good life. In this post, I am going to share with you my knowledge about the resistor basics. A resistor is the part of almost every electronic circuit. I personally did not see any circuit without it. And this makes it a special and interesting component to learn about. In fact, to be honest, a resistor is the very first thing I learned on my first day in engineering university.

What is a Resistor?

A resistor is, in general, anything that provides resistance or opposition. It behaves just like some hurdle in your path. The more technical definition of a resistor is:

Resistor is a two-terminal electronic component which provides resistance to electric current flow

The straightforward definition is a little bit confusing. let me take you step by step, with simple examples to make it clear what basically resistor is. To have an insight about the resistor, you need to understand the following two terms first, resistance and electrical resistance.

What is resistance?

Resistance is a general term, defined as “the measure of a property of anything to oppose the flow or movement“. Consider two pipes, one is narrow and other is wide. Suppose that high-pressure water is flowing through both the pipes. The flow is smooth through the wider pipe, while it is restricted in the narrow one. This happened because of the fact that the narrow pipe is opposing the flow of water hence is providing resistance. While, there is less opposition offered in the wider pipe, hence water flows smoothly through the wider pipe.

Did you notice one thing here?

The higher the resistance, the more restricted is the movement.

Keep this in mind, it will help you later.

What is Electrical Resistance?

An electrical resistance is an opposition offered to the flow of current through a conductor and is denoted by R. By definition, it is the ratio of the voltage applied across the conductor to the current flowing through it (Ohm’s Law).

R=\frac{V}{I}

Its unit in electrical terms are Ohm represented by Greek letter omega.

Now I think you understand the term “Resistance”, let’s revise the definition of resistor:

A two-terminal electronic component which provides resistance to electric current flow.

The next question you might have asked yourselves would be, how does this device look like?  You’ll get the answer next.

Electrical Symbol of Resistor

In circuit theory, it is very important to know and be able to identify the electrical symbols of circuit elements. Use of symbols make it easy to draw schematic of complex circuit. Just like other circuit symbols resistor has got its own symbol. And personally I like it very much. The electrical symbol of a resistor is like a zig-zag thing as given below:

Resistor symbol

I like it very much because its very easy to remember. And I have very strange imagination for it. I think the symbol as mountain range. Just like going through mountain range is hard. Similarly it hard for electric current to flow through it. By the way, this is just my personal imagination. You can have your own imaginations about it and share it with me. It would be fun to see the resistor according to your imaginations as well.

The next time you need to identify a resistor in a circuit diagram, search for a zig-zag pattern in the diagram, and there you know is your resistor. The resistor might have some labeling as well, it will tell you what its resistance is.

How can you identify resistor in a circuit?

You might have asked yourself another question at this point. Does a resistor physically look like that too i.e. a zig-zag type of thing?

The answer is NO!

Then how does it look like?

Let’s not keep you wondering. Shown below is the physical appearance of a resistor:

Resistor

It just looks like a sausage with some colored lines on it. It is just a tiny device that you might miss to notice it in the first attempt. Carefully observe a sausage-shaped thing with colored lines on it, and I’m sure you will find a resistor without difficulty. Other important thing to learn is resistors come in many packages, for beginners it is good to just know through hole and surface mount resistors.Resistor Basics

Measuring resistance of a Resistor

Most important in learning resistor basics is measuring the value of a resistor. Resistance of a resistor is measured in Ohm. But how we can measure this Ohm value is a very interesting question. Traditionally, most people categorize their resistors by simply buying them in categories, storing them separately and labeling them.

Sounds funny … Right?

I did that too until I learned ways to measure it. While this is one way, it not the standard and efficient one since you may get them mixed and there you know you are in trouble.

Measuring resistance of a Resistor using the multimeter

Using multimeter for measuring resistance is the most technical way for an engineer or for any one who wants to learn basic electronics. This a huge topic and I have decided to write a separate post for it. The moment I write it I will update a link here. The current post is about to teach you the resistor basics. And in resistor basics some topics are those which I want to make a separate post for it.  Actually my purpose is, if you are really into basic electronics, my job is to provide you every detail about it the best way possible. I want to share my knowledge of basic electronics with you to just help you out as a brother.

Measuring resistance of a Resistor Using color code

This is an interesting way of measuring the resistance of a resistor. It comes in handy when you don’t have a multimeter with you and you need to measure the resistance of your resistor. Here you will need to have some theoretical knowledge. A resistor has some colored lines on it. These colored lines are the color code of the resistor and they tell you what value each resistor is carrying. Compare these color with the following color code table and calculate you resistor value in second, or you can use online color code calculators for doing so. I love using this website. Have a fun with color code online calculator.

Color code table of resistor

You might be thinking how can you remember the above table, since its really long, right? Here’s the solution. Cram this sentence here:

  BB ROY of Great Britain is a Very Good Woman

Each capital letter in the sentence represents the colors in the table in the same order.

Color code of resistor

Source: Allspectrum

Usually, the resistors are coded using 4 color bands. The 1st to 3rd bands represent the resistance value, while the last band is the tolerance value. The 4th band usually constitutes the gold or silver color. From the 1st to 3rd bands, the bands 1 and 2 represent numbers and the 3rd band represents the multiplier i.e. a power of ten. For the third band, the actual color value becomes the power of ten.

The steps for reading the bands
Identify the tolerance band. It will be silver or gold in case there are 4 bands on your resistor and will be slightly farther from the rest of the bands. The reading direction is then from left to right, keeping the tolerance band to your right.

Combination of resistors

Resistors are only available in some standard values. While these range over large values, there are certain values which are not commercially available. If you need such a value, what would you do? There is a simple solution. You combine resistors to achieve the desired values. The combination can be series or parallel, or combination of both. But the basic categories in which the combinations fall are only series and parallel. Following is just introduction to series and parallel resistances. If you really want to go in depth of it see my this post.

Series Combination of Resistors

As the name depicts, the resistors in this combination are in series i.e. connected end to end, hence the same current would flow through them. When I say same, I’m not referring to the same amount current only, not the voltage, because the amount of voltage will depend upon how large or small the value of resistance is. The series combination is as given:

Series connected resistors

In series combination, the resistive effect is additive, i.e the values are linearly added. The more resistors you add, the more is the cumulative resistance in the circuit. Hence,

In series, the output resistance is always greater than the single resistor values.

To understand, let’s consider a circuit with 1Ohm resistor. Now let us add another 1Ohm resistor in series to it. By adding the resistance, the total resistance of the circuit becomes 2Ohm which is greater than either of the resistors present in the circuit.

The general formula for series connected resistor is:

R_{eq}=R_{1}+R_{2}+R_{3}+R_{4}+\,.\,.\,. \,+R_{N}

Parallel Combination of resistors

In this combination, the resistors are connected in parallel. So, when the current flows through the circuit, the current is divided into the branches. The amount of current through each branch depends on the amount of resistance connected to each branch. More the resistance, lesser will be the amount of current flowing through it.

To understand the concept, consider water flowing in a pipe which splits into 2 at some point. When water reaches the junction, the water divides in 2 directions.If the water is flowing with pressure, more water will flow through the pipe having a wider diameter since there is less opposition to water flow and vice versa. The parallel combination is shown in the figure below:

Parallel connected resistors

The parallel combination of resistors is calculated as:

The more resistors you add, the lesser is the output resistance of the circuit. So,

The output resistance of the parallel combination of resistors is always less than the minimum value of resistor that is present in your circuit.

Measuring resistance of a circuit

You might have encountered situations where you are connecting two circuits or driving one circuit from another. In such cases, you might have felt the need to measure the resistance of the whole circuit. One thing which is important to mention here is that the resistance of a complete circuit is commonly termed as “impedance” because it also includes elements such as capacitors inductors etc. Another important thing to remember here is that there are two types of impedances that you can measure in a circuit an input impedance and an output impedance. Now if you are a person like me, you might have difficulties understanding and be distinguishing the terms.

Let’s understand it in simple words.

  • The input impedance is what is seen by the source or input.
  • The output impedance is seen at the output terminals.

To understand it in detail let’s take an example of a simple resistive circuit below:

Resistor circuit

The input resistance of this circuit will be the one that input will drive. In this case, the input is driving 3 resistors in series. So, the input resistance of the circuit will be 18K. The output resistance will be the one seen at the output terminals which in this case is the terminal 2 and 3. So the resistance measured at these points is R2 which is 10K. If this is still confusing, don’t worry. We can have a look at another example. Let’s assume you built the circuit as shown in the image below:

The input impedance or resistance of this circuit will be the impedance measured at the terminals Vin. The output impedance of the circuit will be the impedance of the load circuit i.e. circuit 2 in the above circuit. Now compare this complex circuit with the simple circuit above and try to relate circuit 1 with R1 and circuit 2 with R2. What you get at the end is 2 resistors in series.Now try to relate the resistance measurement of both circuits. The input resistance will be the sum of both resistors and output resistance will be the R2.

Advantages of using resistors

1.  Before you begin to read the advantages of the resistors, think for a while for yourself. Can you guess any advantages that a resistor may offer yourself? Resistor restricts current flow. This is an advantage of a resistor that it restricts current flow. You might be thinking how? Let’s think of something that makes it easy to understand. Imagine filling a balloon. It has the certain capacity to which it can expand. If you overfill it, what happens?

It bursts.

Now consider electronics components as balloons and current to be the air you are filling it with. You are guessing it right, more current will damage the devices. They may not burst like balloons do, but they will burn. So, to avoid such mishaps, you connect resistors in your circuit, so the currents do not exceed a certain limit and damage your precious circuit components.

2. Another advantage is voltage division. Sometimes it is required to provide a specific amount of voltage to next component in your circuit which is different from your source voltage. In such cases, these tiny components play their role. You connect them to your circuit and divide the source voltage accordingly. To understand clearly, let’s have a look at the following circuit:

The voltages across both resistors divide as:

V_{R_{1}}=IR_{1} \\ V_{R_{2}}=IR_{2}

Let the output is being taken from R2. Then changing the value of R2 will affect the voltage appearing across it. This is what we call voltage division. This is just like pouring water from a jug into glasses. The larger the glass, the more water it receives.

3. Resistors are used in feedback systems as well. Op amp uses resistors to limit their ideal infinite gain.

Where can you buy resistors?

You can purchase resistors of almost every standard value at an electronics store nearby. These are common devices and hence easily available. The fancy ones may not be available everywhere. For those, you might need to ask about some place in your area where they are sold. But if you are one lazy soul like me, you will grab your phone and order them online. Who wants to search the market when everything can come to your doorstep! So, for you guys here are some links where you can buy them and enjoy your laziness. 😊

Standard resistor values table

Source: vishay.com

Summary

Short summary for resistor basics
  • Resistors are two terminal, electronic devices that restrict current flow and have no polarity.
  • By definition, it is the ratio of the voltage applied across the conductor to the current flowing through it.
  • Its value can be measured using a multimeter and color codes.
  • They can be combined in series or parallel to achieve various values and build complex circuits.
  • The resistance of a complete circuit can be measured at both input and output ends and is termed as “Impedance” of the circuit.
  • They offer advantages in terms of safety, limiting current and dividing the voltage.

This was all about resistor basics. Hopefully, it has helped you. If you have any suggestion or question please free to comment or contact me. I really do appreciate that.

Thank you.


Special thanks to

Engr. Tuba Tanveer, MS Electrical Engineering from National University of Science and Technology (NUST), Islamabad

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