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# Voltage divider

The output voltage is a fixed fraction of the input voltage. The ratio is determined by two resistors. Created by Willy McAllister.

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• What do the loose ends around the lower resistor mean? I don't really know how this is supposed to work, does it mean the circuit continues off-screen? • Conceptually, in this oversimplified case, why not just use 1 resistor? Or basically I don't know why you would want two resistors if you aren't actually using the middle wire. If you are using the middle wire, what is an example of a circuit literally requiring that complexity? • Hello Horus,

Yes, at first glance it seems like a pointless exercise. However the voltage divider is a useful tool from two perspectives:

A voltage divider is a physical assemblage of resistors that allows you to lower a voltage. For example, let's assume we have a source that provided 5 VDC connected to another device that required 3.3 VDC. A properly designed voltage divider would allow us to connect the devices together. This scenario is common. The 5VDC may be an Ardunio microcontroller and the 3.3 VDC source a new sensor you would like to connect tot he Arduino...

On a big picture level a voltage divider is a mathematical model. At first it will allows us to talk about DC voltages and the relationships in series circuits. Later is will help us understand much more interesting AC circuits items like power factor and filters.

Regards,

APD
• Why we only take R2 for V out? • Assigning the output voltage across R2 is the most common way to use a voltage divider. (Where R2 is the resistor connected to the reference node, ground). There's nothing keeping you from using the voltage across R1, but I have not seen that done too often.

There isn't anything special about the voltage divider circuit, other than it happens to be a very common resistor configuration. So we spend a little time looking at its properties and developing an equation for the output voltage. It's just a quick mental shortcut you will use frequently.
• In the first circuit, you mention over and over that the two resistors R1 and R2 are in series. But how are they in series when the middle wire is extending from the middle? • The formula is very mathematically nice but why does it work out this way? What is the intuition behind the voltage being proportional to the proportion of resistance across the two resistors?
(1 vote) • Tough question. Let me give it a try.
Draw a picture of one long resistor 10 cm tall on the page. Connect the top of the resistor to voltage Vin. Connect the bottom to 0v. The value of the long tall resistor is R.

Touch your finger to the resistor at the top. What is the voltage of your fingertip? Ans: Vin .

Touch your finger to the bottom of the resistor. What is the voltage of your fingertip? Ans: 0.

Touch your finger right in the middle of the resistor. What is the voltage of your fingertip? Ans: what does your intuition say?

Touch 1/4 the way down from the top. Touch 1/4 of the way up from the bottom. What are the voltages? Ans: (work out the proportions).

You can figure out any voltage from the physical length of resistance above and below your fingertip.

Can you come up with a formula to give the answer? Hint: Break the resistor into two parts: R1 is the portion above your fingertip, and R2 is the portion below your fingertip. The total resistance R = R1 + R2. Find Vfingertip in terms of Vin, R1, and R2.

Please let me know if this helps.
• Why is the voltage on the node between the resistors assumed to be very small or 0? How would you achieve this with a real circuit? • i have a question, so for a parallel series combination circuit his formula wouldn't work right i.e the voltage divider formula works only for series circuits??
(1 vote) • Can the V out be any one of the 2 wires that come out?
(1 vote) • Hello Ammar,

No, Vout is the potential difference between the two wires on the right.

If this were a real circuit there would likely be a ground on the lower node. Think of a ground as a common point from which all measurements are made. Vin in measured form the top wire (left side) to ground and Vout is measured from the top wire (right side) to ground.

Regards,

APD  