How a potential divider works
A voltage divider (or potential divider) turns a larger voltage into a smaller one using a series combination of two resistors: Vout = Vin × [R2 / (R1 + R2)].
Because the same current passes through both resistors in an unloaded circuit, the voltage drop across each resistor is directly proportional to its resistance value.
When an external load (RL) is connected across the output, the effective resistance of the bottom branch becomes R2 in parallel with RL: R2_eff = (R2 × RL) / (R2 + RL). This loading effect reduces Vout below its unloaded value, which is critical when interfacing sensors to microcontrollers.
Worked example: 12V to 5V sensor interface
To step down a 12V automotive battery signal to 5V for a 5V-tolerant microcontroller input:
Using R1 = 14 kΩ and R2 = 10 kΩ: Vout = 12 × [10 / (14 + 10)] = 12 × (10 / 24) = 5.0 Volts.
Quiescent current is 12 V ÷ 24 kΩ = 0.5 mA, with power dissipation in R1 of only 3.5 mW.
Inputs for the voltage divider calculator
Input voltage (Vin)
The source DC or AC voltage applied across the top of R1 and ground.
Resistors R1 and R2
R1 is the upper resistor connected to Vin; R2 is the lower resistor connected to ground across which Vout is measured.
CALCULATOR QUESTIONS
Potential divider output calculator questions
What is the voltage divider formula?
Vout = Vin × (R2 / (R1 + R2)), where R1 is the top resistor and R2 is the bottom resistor connected to ground.
How does a load resistor affect Vout?
A load resistor connects in parallel with R2, lowering the effective resistance of the bottom branch and decreasing the measured output voltage.