分圧計算機

How to use this calculator

1. Enter the input voltage Vin.
2. Enter the two resistor values R1 (top) and R2 (to ground).
3. Click Calculate to get the output voltage, divider current and power dissipated.

The voltage divider formula

Vout is measured across R2, the resistor connected to ground:

The current through the (unloaded) divider is I = V_in / (R1 + R2), and the total power dissipated is P = V_in × I.

Important note about loading

This formula assumes no load (or a load with very high resistance) on the output. If you connect a load whose resistance is comparable to R2, it will pull the output voltage down. For driving real loads, keep R1 and R2 much smaller than the load resistance, or use an op-amp buffer.

Worked example

With Vin = 12 V, R1 = 10 kΩ and R2 = 10 kΩ: Vout = 12 × 10000 / (10000 + 10000) = 6 V. Change R2 to 4.7 kΩ and Vout becomes 12 × 4700 / 14700 ≈ 3.84 V.

よくある質問

How do I calculate the output of a voltage divider?

Vout = Vin × R2 / (R1 + R2), where R2 is the resistor across which the output is measured.

Does a voltage divider work as a power supply?

No. It is only suitable for high-impedance loads. Any real load comparable to R2 pulls the output down; use a regulator or buffer for power.

How do I pick R1 and R2 for a target voltage?

Set the ratio R2 / (R1 + R2) equal to Vout / Vin, then choose values large enough to limit current but small relative to the load.