How a voltage divider works
A voltage divider is two resistors in series between a voltage and ground. The same current flows through both, so each one drops a share of the input voltage in proportion to its resistance. Taking the output from the point between them gives you a fixed fraction of the input.
The formula
Vout = Vin × R2 ÷ (R1 + R2)
R1 is the resistor connected to the input and R2 is the one connected to ground. With 12 V in, R1 = 10 kΩ and R2 = 4.7 kΩ, the output is 12 × 4.7 ÷ 14.7 = 3.84 V.
Solving for R2
If you know the voltage you want, pick R1 and rearrange: R2 = R1 × Vout ÷ (Vin − Vout). The answer is rarely a value you can buy, so the calculator also shows the nearest E12 and E24 values and the output each one would give. Pressing Use loads that value so you can check the current and power.
Choosing the resistor size
Only the ratio of the resistors sets the output voltage, so 1 kΩ and 2 kΩ give the same result as 100 kΩ and 200 kΩ. The difference is the current. Small values waste more power and can heat the resistors; large values save power but make the output weaker and more easily disturbed. The output behaves like a source with an internal resistance of R1 and R2 in parallel, shown above as the output impedance. Keep whatever you connect to the output at least ten times higher than that, or the voltage will sag.
Common uses
- Scaling a battery or supply voltage down so a microcontroller ADC can measure it.
- Level shifting a 5 V logic signal down to 3.3 V for a slow input.
- Setting a reference or bias voltage, for example the feedback pins of adjustable regulators.
- Reading resistive sensors such as thermistors and light-dependent resistors.
When not to use one
A divider is not a power supply. The output drops as soon as a load draws meaningful current, and the resistors burn power all the time. To power a circuit at a lower voltage, use a linear regulator or a buck converter.
