What Is A Resistor And What Does It Do?

The resistor is a component that resists the flow of current. It doesn’t do anything actively, so it’s called a passive component.

Sounds boring and pretty useless, but it’s actually an extremely useful component. Once you know the basics of the resistor – and how it works with currents and voltages in a circuit – you’re in charge 😎.

Here’s something that’s important to know about the resistor:

The amount of current flowing into a resistor is exactly the same as the current flowing out of it.

That’s something that a lot of beginners will argue against. “If the resistor reduces current, there must be less current coming out of it, no?” And because they get this wrong, they have a hard time understanding circuits in general.

But here’s the thing:

The resistor reduces the current in the whole circuit. If you add a resistor in series with a circuit – the current in the whole circuit will be lower than without the resistor.

What Is a Resistor?

A resistor is nothing magic. Take a long wire and measure the resistance, and you will realize that resistance is just a normal property of wires.

Some resistors are made up of just that – a long wire (the wire-wound resistor)

But you can also find resistor types that are made of other types of materials. Like carbon or metallic film.

What is a resistor like on the inside?

A material with no resistance at all lets current flow freely. The more resistance a material has, the harder it is for current to flow through it. So resistance is just a measurement of “how bad” the resistor is at letting current flow through it.

How Do Resistors Work?

The resistor is a passive device and doesn’t do anything actively to your circuit.

It’s actually a pretty boring device. If you add some voltage to it, nothing really happens. Well, maybe it gets warm, but that’s it.

But, by using resistors, you can design your circuit to have the currents and voltages that you want to have in your circuit. And in electronics, everything happens because of currents and voltages:

If you set the right voltage on the wires of a motor it will start spinning. If you set the correct current through a Light-Emitting Diode (LED) it will light up.

So the resistor gives you, the designer of circuits, control over what happens in the circuit! How about that?

Using different resistor values to control the brightness of an LED
A current-limiting resistor controls the brightness of an LED

Learn To Work With Resistors

When I started learning electronics, it seemed to me like resistors were just randomly placed around a circuit and I thought that you didn’t really need them.

I first started understanding that maybe there was something to these resistors when I tried connecting an LED directly to my 9V battery. At first, it seemed to work fine. The LED lit up brightly. But then, after a few seconds, the LED turned really hot. So hot I almost burned my fingers. Then it died.

Learning to work with resistors is important in electronics. One fundamental skill you should learn is how to use Ohm’s law. It describes the relationship between the voltage, resistance, and current:

V = R * I

  • V is the symbol for voltage.
  • I is the symbol for current.
  • R is the symbol for resistance.

It is the only formula in electronics that I use pretty much every time I design a circuit.

You can switch it around and get R = V/I or I = V/R. As long as you have two of the variables, you can calculate the last. Instead of memorizing it, you can use Ohm’s law triangle to find the three formulas:

Use your hand to cover the letter you want to find. If one of the remaining letters is above the other, it means dividing the top one by the bottom one. If they are next to each other, it means multiply one with the other.

For example to find the formula for current, place your hand over the I. Then you’ll see the V over the R, which means you find current by dividing V by R.

Resistors in Series

If two or more resistors are connected in series it means that they are connected after one another, like this:

Resistors in series

You can look at resistors in series as one resistor if you find the combined resistance of the two. This is useful for example if you want to find the current flowing in this circuit.

To find the combined resistance, simply find the sum of all the resistance values in series.

Resistors in Parallel

If two or more resistors are connected in parallel it means they are connected next to each other, with both pins of one connected to each pin of the next, like this:

Resistors in parallel

Resistors connected in parallel can also be simplified into one resistance value, but they are a little more complicated to calculate than resistors in series. Check out the article series and parallel circuits to learn more.

Resistor Circuit: The Voltage Divider

The voltage divider uses two resistors to convert – or divide – a voltage into a smaller voltage. It’s used in both simple and advanced circuits all the time. Here’s the basic structure:

What is a resistor used for? A basic voltage divider circuit

It is useful for example for reading sensors like thermistors and photoresistors since it converts an unknown resistance into a voltage. Or to reduce the volume of an audio signal via a potentiometer.

Here’s an example circuit where a voltage divider is used to convert light into a voltage:

Voltage divider for sensing light based on resistance

How To Learn About Using Resistors?

Resistors help you set the brightness of an LED, add time delays together with capacitors, convert sensor values such as light into voltage, and much more. The more you learn about electronics, the more obvious it becomes how important this component is.

To learn how to use resistors in circuits, I recommend you build several basic circuits that use resistors, and then experiment with different resistor values. If you combine that with learning the basics of electronics, you’ve come a long way.

Then continue by learning to simplify complex circuits, and finally read up on Kirchoff’s laws.

If you have any questions, let me know in the comments below!

More Resistors Tutorials

37 thoughts on “What Is A Resistor And What Does It Do?”

    • You see, a resistor is just like a paneer roti that my mamaji makes. It resist the flow of the daal that is supposed to go in the rice. So to put into real perspective the resistor stops the current from passing through. Although, sometimes the roti has many holes in it because my mamaji heats it a lot. Thus some of the daal passes through the paneer roti. So the rice or in my language we call it bhath it gets wet form the daal. So the current sometimes passes through the resistor so it doesn’t stop it completely. Now you may be wondering how I know this things, this is because my mamaji had taught me the ways of the science on rotis. You may feel like I am wrong or trolling you, but this is false. You see my mamaji had taken a roti course in the university of Bhangda it is a small university in the city of Puranik. There he learned that all cases of life can directly be related to roti and rice. To put this into an example of an everyday struggle like “I can’t tie my shoelaces”. This is like how many new chefs that can’t make the loaf properly because they don’t know how to. Thank you for your time and I am very sorry that I replied you late, but I am also taking the roti course so I was very busy.

  1. Lol, it’s pretty much a “tap of the breaks” against higher voltage to slow down the current so it doesn’t burn up whatever is getting the electricity.. in this case, the LED light…

  2. The explanation above is actually wrong or misleading in many areas.

    A resistor has a voltage drop proportional to current through the resistor and resistance. E=I*R

    A resistor has a current proportional to voltage across the resistor and resistance. I = E/R

    A resistor develops heat based on current through the resistor and resistance. P = I^2 * R

    These formulas can be manipulated to solve different problems, but in the strictest terms it all comes down to voltage, resistance, and current. A resistor technically just drops voltage based on the current flowing through it and the resistance.

  3. Used a 1/4 watt resistor on a 5 volt adaptor to power a 3 volt LED. But the volt meter still showed 5 volts past the resistor. Is that right??

  4. We are a group of volunteers and starting a new scheme in our community.
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  5. In “Learn To Work With Resistors” section, I had a similar experience, but just that I burnt my LED as I had not put any resistor and then I learnt something
    Resistor is an important component just like a slight break you apply to control your car speed.

  6. What are the color bands on the resistors? I see a lot of different colors. Is there a key chart I can look at that would help me figure out what the different color bands means? Also what is the difference between 4,5 and 6 bands?

  7. Wait what the heck is going on lol. What exactly does a resistor do?

    What’s happening “under the hood” ?

    Does a resistor slow down current? how so? A denser material that slows drift velocity?
    But then there’s also the conservation of charge so the current entering the resistor and leaving is the same?!?

    • I don’t think it slows down the current but it makes it travel a longer distance cause it is a coiled wire. So I guess it could be just a extension to the circuit. For the longer the distance traveled the, I guess, thinner or smaller current gets. Also resisters do get warmed up so some of the current is leaking out . So If I am wrong let me know and teach me.

    • Current is the amount of charge flowing through a certain point.

      You can say that the resistor slows down the flow of charge.

      But a circuit is always completely full of charge carriers – even when there is no battery connected. When you connect the battery, the voltage in the battery starts to push those charge carriers around the circuit. When one charge carrier is pushed out of the battery the charge carrier at the end of the circuit (right before the minus of the battery) is pushed into the battery.

      So when you insert a resistor into the circuit that slows down the charge carriers, the whole system slows down.

      Not sure if that makes sense, I tried to explain in a way I have never done before…




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