Voltage Divider Formulas
Find Vout: Vout = Vin × R2 / (R1 + R2). With load RL: effective R2 = R2 × RL / (R2 + RL), then apply the same formula. Find R2: R2 = R1 × Vout / (Vin - Vout). Then select the nearest E12 or E24 standard resistor value.
Frequently Asked Questions
A voltage divider uses two resistors in series to produce a lower output voltage from a higher input. The output voltage is taken from the junction between the two resistors: Vout = Vin × R2 / (R1 + R2). It is one of the most fundamental circuits in electronics.
Vout = Vin × R2 / (R1 + R2). For example, Vin = 12V, R1 = 10kΩ, R2 = 10kΩ: Vout = 12 × 10000 / (10000 + 10000) = 6V. If R2 = 5kΩ: Vout = 12 × 5000 / 15000 = 4V.
Rearrange the formula: R2 = R1 × Vout / (Vin - Vout). For Vin = 12V, target Vout = 3.3V, R1 = 10kΩ: R2 = 10000 × 3.3 / (12 - 3.3) = 3793Ω. Select the nearest standard E12 or E24 value.
Voltage dividers are not suitable for powering loads that draw significant current, because the output voltage will sag as the load draws current (the load acts in parallel with R2, lowering effective R2). For any but the lightest loads (load resistance >> R1+R2), use a voltage regulator instead.
When a load is connected to the divider output, it appears in parallel with R2. This reduces the effective R2, pulling down Vout. The rule of thumb is that the load resistance should be at least 10× greater than R2 for less than 10% voltage sag. This calculator shows the effect of a connected load.
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