No load
- Input
- 12V 1.8k 1.3k
- Expected output
- 5.03V
Open output: 12 × 1,300 ÷ 3,100.
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loaded voltage divider
The formula everyone learns, Vin × R2 ÷ (R1 + R2), holds with the output open. In a real circuit something is always connected there, and it changes the result. The tool opens in calculation mode with 12 V, 1.8 kΩ, 1.3 kΩ and a 10 kΩ load, showing the unloaded output, the loaded output and the drop the load causes.
Open output: 12 × 1,300 ÷ 3,100.
Drop of under 1%.
R2 ∥ RL = 1,150 Ω; 7% drop.
19% drop.
Larger values, same ratio, 43% drop.
Pair recomputed with the load.
Vout = Vin × R2 ÷ (R1 + R2), with R1 between the input and the output and R2 between the output and ground. For 5 V with R1 = 1 kΩ and R2 = 2 kΩ, Vout = 5 × 2000 ÷ 3000 = 3.33 V.
Vout = Vin × (R2 ∥ RL) ÷ (R1 + R2 ∥ RL), with R2 ∥ RL = R2 × RL ÷ (R2 + RL). It is the ordinary divider with R2 replaced by the parallel of R2 and the load.
Because the load opens another path to ground in parallel with R2, reducing the resistance of the bottom leg. Less resistance at the bottom means less voltage in the middle.
Compare RL with the divider's output impedance, R1 ∥ R2. If RL is more than a hundred times larger, the error stays below 1%. If it is ten times, the error exceeds 9%.
Redo the resistors with the load in the math, which the "Find the resistors" tab does when you fill in the load field, or isolate the load from the divider with an op-amp buffer.
With load: Vout = Vin × (R2 ∥ RL) ÷ (R1 + R2 ∥ RL), where R2 ∥ RL = R2 × RL ÷ (R2 + RL)
The math runs in your browser; no value is sent to a server. A resistive divider is for signals and references, not for powering circuits that draw current.