A voltage divider is a circuit that creates a smaller voltage from an input voltage by using two resistors. You’ll see it in both simple and advanced circuits all the time. Here’s the basic setup:

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.

You can find the output voltage by inserting the resistor values and the input voltage into the following formula:

Or you can use the calculator a little bit further down on this page.

Once you know how it works, it’s much easier to see how circuits work. And it will let you calculate voltages at many different points in a circuit – which is often needed to understand it.

## Voltage Divider Formula

This formula is one of the few electronics formulas (in addition to Ohm’s law) that I use on a regular basis.

It’s for finding the output voltage when you have two resistors connected like this:

The formula for calculating the output voltage is:

This can be a useful formula to remember. It will come in handy often. Or use the voltage divider calculator below if you prefer the easy way ;)

## Voltage Divider Calculator

Fill in the input voltage and resistor values in the voltage divider calculator below to find the output voltage:

_{1})

_{1})

_{2})

_{out})

*We’ve added this calculator to a practical voltage divider calculator page that you can bookmark and come back to in the future.*

## Where Do You Find The Voltage Divider?

One example of a voltage divider circuit is for reading analog sensors. For example, the *thermistor* is a temperature sensor. It changes its resistance based on the temperature. If you connect it with a known resistor value in a voltage divider setup, you’ll get a voltage that depends on temperature:

Or you can combine a known resistor with a *photoresistor*. The photoresistor changes resistance based on the amount of light it detects. This way, you have a circuit that increases or decreases the voltage based on light.

You can connect the output of any of these circuits into a comparator to check it’s above or below a certain voltage. Then do something based on that. For example, if the temperature is above 40 degrees, turn on a fan.

Or connect into an analog pin of an Arduino or a microcontroller and do cool stuff with it. Maybe turn on a light if the photocell indicates that it’s dark?

## Calculation Example: Different resistor values

Let’s say we have the following values:

By using the formula above we get

## Calculation Example: Equal resistor values

Now, let’s say R1 and R2 has the same value.

By using the formula above we get

This means that when the two resistors have the same value, the output is always half the input.

## Can You Use Voltage Divider as a Power Supply?

If you have a circuit that needs 4.5 V, could you use a voltage divider with two 500 Ω resistors to get 4.5 V from your 9V battery?

Unfortunately, it’s not that easy.

Any circuit you want to power will have an internal resistance. So from the perspective of the voltage divider, whatever circuit you connect to the voltage output can be seen as a resistor (R_{LOAD}) connected in parallel with R2.

If the internal resistance of the circuit (R_{LOAD}) is also 500 Ω, what happens?

Now, the R2 from the voltage divider formula becomes the parallel resistance of R2 and R_{LOAD}. Which is just 250 Ω. If you put this into the voltage divider formula, you get an output voltage of 3V instead of the 4.5V you wanted.

For a power supply, you want the voltage to stay at the chosen level no matter if the circuit you connect has a high or low internal resistance. That’s why the voltage divider is not commonly used for power supplies.

Instead, you need to use a voltage regulator.

## Questions?

What are your questions about the voltage divider? Let me know in the comment section below.

Well, I subscribed, and I enjoy the info attached to the emails, but where is the book???????

Hey Ted

You should have received a link right after signing up. If you can’t find it, just reply to one of my emails and I’ll send you the link again.

Cheers!

Oyvind

thanks for every thing , like learning.

No problem =)

thanks for every think.

I have been wondering about the book that was meant to be sent to me. Given that I live in south africa, was wondering if it got lost in the post.

Hey Delmaine

It’s an ebook. You’ll receive a link for download right after you sign up. If you can’t find the link, just reply to one of the emails and ask for it and I’ll send it again :)

Cheers!

Oyvind

Thank you I like what u are teaching! U are definetly out there!

Thanks Donald, I really appreciate it =)

Oyvind

Your emails are very useful and those inspired me to making circuits.

Cheers

Thanks Thanvir, it means a lot to me to hear that =)

Oyvind

Hey Oyvind, what’s up?

Great article: simple and right to the point.

I would like to ask you a question: suppose I have a circuit that runs on 5V (like Arduino), and I need to add a device that runs on 3V3, that will draw its current from the circuit. Supposing the circuit has enough Ampères to run and supply to the device, is it safe to set up this voltage divider to lower the tension level to 3V3? I am using a breakout from Sparkfun – but two résistors seem to me much cheaper that this breakout… :)

See ya,

Julio

Hey Julio!

Good question =)

Yes, you can use two resistors two go from 5V to 3.3V. But there are some things you need to keep in mind.

Let’s say you put a resistor in parallel with R2, with the same value as R2. The resulting resistance of the parallel connection will be only half the value of R2. And this will reduce the output voltage from the voltage divider.

If you want to power a circuit with the output from the voltage divider, you have to make sure that R2 in the voltage divider is much much lower than the resistance from 3.3V to ground in the circuit you want to power. (And this resistance might vary depending on what it going on in the circuit, so you can’t simply measure it)

You might be tempted to think that you can just use extremely small resistors for R1 and R2. But the problem with this is that then you would need extremely high precision on your resistors to get the right voltage.

Basically it all comes down to the circuit you want to power. So I recommend you to test it out and observe what happens. It might work. It might be too unstable.

Oyvind

THANKS FOR YOUR TEACHING. THEY ARE SO WELL PUT. THAT IS A GIFT. THEY ARE ENJOYABLE ALSO.

COULD YOU SEND ME THE LINK TO YOUR EBOOK I AM ALSO FROM SOUTH AFRICA.

THANKING YOU ONCE AGAIN

REGARDS

COLIN

You get the link automatically when you confirm your newsletter subscription. If you can’t find it, reply to your welcome email and I’ll help you.

Oyvind

hi

who can i get ldr

Hello Oyvind,

My name is Jason, I’m from England & asked for an Arduino for Christmas. I’ve been going through the starter kit guide. I understand all the code, logic and what the components do, but the explanations in how the schematics work together is limited. I have been trying to figure out why a resistive photo cell needs to use a voltage divider when it is already a resistor. But after watching and reading the info on your site I think I have it. A resistor by itself just limits the current, whereas a voltage divider lowers the voltage. This is more useful as the arduino main thing is reading voltage levels.

Is this right or am I still missing so information?

Thanks,

Jason

Hey Jason,

You seem to have figured it out yourself =)

The Arduino reads voltage level, so we need a voltage divider to convert the changing resistance of the LDR into changing a voltage level.

Keep up the good work!

Oyvind

hey Øyvind

whats up ?

i am mahfuj and i am from Bangladesh. i want to design a 7 segment binary decoder for my class project what are the elements needed for the project.

please help me

Can I ask u one doublt….it is very well told that the voltage divider circuit cannot be used for connecting any load. Then what is the real use of dividing voltage since the current is same throughout the circuit.

Please forgive me if the question is a blunder.

Can u please include a bit of stuff relating to the transformers. Am seeing 3 pin, 4pin, etc transformers, making a lot of confusions, some of which having 6 to 8 pins soldered to the board.

Hey Anurag, good question!

You don’t use a voltage divider as a power source. Because once you introduce more resistors – like a load – then the voltage will change.

But you willl see voltage dividers in many circuits. For example, do you see the voltage divider in this ligh-sensor circuit: https://www.build-electronic-circuits.com/ldr-circuit-diagram

For transformers – look up the datasheet to sort out the pins.

Cheers!

Oyvind

Hi Øyvind,

I have a power source (12V Battery), but the rating can vary 5 AH / 7 AH / 9 AH. The device to be used needs to provided with 12 V (DC) and would like to limit the current to 3 amps or 4 amps for experimentation to understand the outcome at various power levels.

Can you suggest me how to limit the current using resistors.

Hey,

The battery rating you refer to only tells you the capacity of the battery. 5AH means the battery *could* give 5A for 1 hour. Or 1A for 5 hours.

To control the current with a resistance is easy – just use ohm’s law.

V = I * R

or

R = V / I

So if you have 12V and want 1A you get:

R = 12 / 1 = 12 Ohm

Just connect the voltage source to the 12 ohm resistor, and you’ll have 1A running through it.

If you’re connected some device, the device itself will control the current as long as you provide the correct voltage.

Best,

Oyvind

Hello sir, i have a question.

I added a 100ohm resister between the middle input pin and ground pin of a volume potentiometer because i wanted to reduce the sudden volume increase caused by low quality pot. It works well. But

I want to ask. Does it mean it will drain the batteries of my radio fast, because isn’t it kind of short circuiting the input volume to ground?.

Plz replt. Thanks.

I will calculate if there are 3 series of connected resistors or more

Combine two of them into one. The total resistance of two resistors in series is the sum of the two.

This is good man .completely useful

Great article on voltage divider, I’m curious when it comes to using resisters and choosing the correct ones. I.e Say I want to go from 5v down 2.5v I could use 220ohm for R1 & R2, or I could

use 10k ohm for R1 & R2, how do you know which way to go in terms of lower or higher ohms?

Thanks! Often, it doesn’t matter as long as it’s not too small (which will give a lot of current) or too high (which will give very little current).

But note that the voltage divider will not work as a power supply. I just added another section above called “Can You Use Voltage Divider as a Power Supply?” that describes why (in case that’s what you were thinking about).

Dear Oyvind,

I am a mechanical engineer and very new to electronics. I have already made pulse charger to charge batteries but now I need to automate it.

Say I have the power source and a mosfet bank in between and the battery. The mosfets are driven by a frequency Generator say at 200Hz and the current flowing is controlled by the PW of the Freq Gen.

Now I do this manually and I need to use a Arduino +

ACS712 module 30A Hall Current Sensor in series prior to mosfet bank and thru the voltage signal of current sensor I want to alter the PW out of the Arduino in order to achieve the set point current, say 8 Amps. I want to fit and forget so that the Arduino UNO runs auto pumping constant set point curent by the way of altering the pulse width.

Can you help me in this.

I am struggling with the Arduino coding for this.

Please Help

Hey Samantha,

Have a look for the AnalogWrite command. That’s what decides the pulse width. Usually it’s a value between 0 (0% pw) and 255 (100% pw).

Best,

Oyvind

Thank you for the great instructional material! Regarding the voltage divider, everything above makes sense to me except the calculation for trying to get 4.5V from a 9V battery using 2, 500 ohm resistors. If R load is 500 ohms, then the parallel resistance between R2 and R load is 250 ohms. So, to get the output voltage from the voltage divider we now have R1/(R1+R2)(9V)=500/(500+250)(9V)=6V, not the 3V as written. Do I have something wrong here?

Hey Woody, you switched R1 and R2. R2 should be on top: 9V * R2/(R1+R2)

Ok, got it. Thank you!