What is Resistor?

In this post, we are going to learn about fixed resistors from top to bottom. We will be learning, what is a resistor? Where it is used? How to read resistor color codes and discuss some practical circuits using (fixed) resistor.

What is a (Fixed) Resistor?

Before we jump into fixed resistors, let’s understand what a resistor is? A resistor is a passive electronic component, which is used to resist the flow of current through a circuit. The unit of resistance is ohm.

So, what does the above sentence mean? Let’s imagine, we are applying electric current through a resistor from one of its terminal, the current flow through the other terminal will be less than the applied current.This is the function of any resistor, it just reduce the flow of current in the circuit.

Now let’s understand what a fixed resistor is?

As the name suggests, the value of resistor is fixed and won’t change. It comes in wide range of values from few ohms to few hundred mega ohms.

The values of the resistance is color coded on resistor body, this is because resistors are tiny components, writing and reading values on its body is quite difficult job and may get erased easily. The user may find trouble in determine its resistance. A typical resistor has 4 colored rings around its body; we will see how to read color codes on later part of this article.

A fixed resistor is a two terminal passive device, comes with wattage rating also, a resistor with thicker body have higher wattage rate. An experienced hobbyist or technician will easily able to say its wattage rating by simply looking at the resistor. If you are not sure about the wattage rating, just take a look at the manufacturer data sheet or see specification card of resistor from where you purchased.

Why Wattage rating is an important parameter?

The ohm’s law says that higher the resistance, higher the heat get dissipated. If we apply current more than a resistor can handle, we’ll end up in burned resistor. So, wattage ratings play a crucial role in high voltage and high current circuits.

Illustration of high wattage wire wound resistor:

Hi-wattage Resistor
Hi-wattage Resistor

Here is a higher wattage resistor which is used in ceiling fan’s speed regulators. Since the resistor work at high voltage situation, it dissipate huge amount of heat. These kinds of resistor are capable of handling high thermal output, but with its boundary. The part of the energy is converted to heat due to its resistance. This is not an efficient way to regulate current to a circuit.

In these kinds of resistors the value is printed on its body, since it is easily visible and easy to write value on its body.

Now you know quite a bit about fixed resistors, now let’s see where resistors are used?

Where Resistors are used?

Resistors are used all electronic gadgets from a toy which runs on batteries to a satellite which is floating around the earth. We can say that there is no circuit/gadget without resistors.

As an electronics enthusiast let’s dive deep into resistor based circuits. A resistor is can be used to glow an LED at high voltage situation. Let’s simulate a scenario and find solution for it.

Consider a voltage source of 9V is given to light up an LED, but the LED just needs around 2.2V, the excess voltage could kill the LED in blink of an eye. So the voltage needs to be lowered.

So, we connect a resistor in series with LED. The excess current is dissipated as heat. Since a LED consumes few milli-amperes a quarter watt resistor is enough.

From the two diagrams, we can observe that one with a resistor connected with LED glows properly. But the one without resistor got burned. This is very simple example where resistor can be used.

The above resistor value is calculated for the LED. The same resistor can’t be used for LED with different specification. To determine correct resistance value we have to do some math.

LED Resistance Calculation:

The correct resistor for any LED circuit can be calculated with help of this formula:

R = (Vs – Vd)/ Id

Where, R is the resistance to be calculated.

Vs is the supply voltage.

Vd is the voltage of LED/ voltage drop across LED.

Id is the current consumed by LED.

If the calculated value is not available, you can always choose a resistor value which is near to calculated value.

The resistors are used in timer circuit, where a resistor determines the time period of pulses length. This concept will be discussed in future articles.

By now, you know how to uses resistor to light up an LED without blowing it.

Resistor in Series and Parallel configuration:

Resistors can be connected in series and in parallel, in order to get new resistance value. This can be done if you don’t have a desired resistor value. For example: if you have two 4.7K ohm resistors, but you need a value equivalent to 10K ohm.

For this we need to connect two 4.7K resistor in series, the two resistance values gets add up and gives a new value.

Let’s consider another scenario: you have two 10K resistors, but you need resistance equivalent to 5K. You just need connect two resistors in parallel which gives you 5K ohm.

Resistors in Series and parallel
                     Resistors in Series and parallel

This above situations may sound like a piece of cake, but when you connect several resistor is connected in parallel and series configuration, all the stuffs get complicated. We can find the resistance of series and parallel network by resistor formula.

For resistor in series:

R = R1 + R2 + R3 + …………….Rn

Where, R is the total resistance of the network.

R1, R2….Rn is individual resistance value.

For resistance in parallel is given by:

R = 1/R1 + 1/R2 + 1/R3 + ………….1/Rn

Where, R is total resistance of the network

 R1, R2….Rn is individual resistance value.

The resistance formula is not only used for calculating resistor networks, but it can also be used to calculate the total resistance of circuit, where you need to combine two or more different circuits.

Now you know how to calculate resistance for your requirements.

Resistor color code:

There are two ways to determine value of resistor, by reading the color code or by measuring resistance using a multimeter. The handy method for determining resistance is by reading color code, so that, you no need multimeter every time you check resistance.

Here is the colour code:

Black       – 0

Brown      -1

Red          – 2

Orange    -3

Yellow      -4

Green       -5

Blue          -6

Violet        -7

Grey          -8

White       -9

OK! How to decode this value?

10K ohm resistor
10K ohm resistor

Let’s consider an above resistor; we have to place the gold or silver ring to the right hand side.

Now, the first ring brown color, represents ‘1’, the second ring black color represents ‘0’, the third ring orange color represents number of zeros which need to be added at end of the value, here three zeros.

So the final value is 1 0 000, which results to 10,000 ohm or 10K ohm resistor

Let’s consider another example:

100K ohm Resistor
100K ohm Resistor

First ring brown: 1

Second ring black: 0

Third ring yellow: 0000 (value is 4 but, we need to add number of zeros)

So the resistance is 1 0 0000 = 100,000 ohm or 100K ohm

The gold or silver ring on resistor represents the tolerance of the resistor. The resistors fabricated don’t have exact same value, but there will be minor variations.

The gold ring represents +/-5% variation in the value, silver represents +/-10% variation in the value. Lower tolerance value, higher the accuracy.

Now, you are familiar with basic of fixed resistor, where it is used, how to use them, and how to read resistance color code.

If you have any questions, quires regarding this post, feel free to ask, you can anticipate a response from us.


My nick name is blogthor, I am a professional electronics engineer specialized in Embedded System. I am a experienced programmer and electronics hardware developer. I am the founder of this website, I am also a hobbyist, DIYer and a constant learner. I love to solve your technical queries via comment section.