A voltage divider is the simplest circuit for taking just part of an input voltage with two resistors. It is often used to bring a sensor voltage down to a range a microcontroller can read, or to make the reference voltage of a comparator. The calculator above works out the output voltage from R1, R2, and the input voltage, and also shows how far the voltage drops when you connect a load to the output.
The formula
As in the figure, connect R1 on top and R2 on the bottom in series, and take the output from the junction of the two.
Vout = Vin × R2 / (R1 + R2)
When a load resistance (R_load) is connected to the output, R2 and the load are in parallel, so the lower resistance gets smaller and the output drops by that much.
R2' = R2 × R_load / (R2 + R_load) Vout = Vin × R2' / (R1 + R2')
Examples
| Vin | R1 | R2 | Load | Vout |
|---|---|---|---|---|
| 5V | 10kΩ | 10kΩ | None | 2.5V |
| 5V | 10kΩ | 10kΩ | 10kΩ | about 1.67V |
| 5V | 1kΩ | 2kΩ | None | about 3.33V |
| 12V | 10kΩ | 3.3kΩ | None | about 2.98V |
In the second row, the same divider drops from 2.5 V to 1.67 V, by a third, once a 10 kΩ load is attached. This happens because a voltage divider is not a circuit that "makes voltage" but one that "divides by a ratio".
Design tips
- Make the load resistance at least 10 times R2. Then the drop shrinks to within about 10%. When the load has a very high input resistance, like an ADC input, there is almost no problem.
- Designing the divider current to be at least 10 times the load current keeps the output stable, but wastes that much power. In battery circuits, several hundred kΩ are sometimes used.
- Do not use it for loads that draw a lot of current. Making the supply of a load such as a motor or LED with a divider is inefficient and the voltage wobbles. A regulator is the right choice there.
- Tolerance multiplies into the output. With two ±5% resistors, the output ratio can be off by up to about ±5%. If you need an accurate reference voltage, use ±1% resistors.
Continue with the design principles of dividers in the guide Designing Voltage Dividers and the Loading Effect, and how to combine resistors in series and parallel in the series & parallel resistor calculator.