Two resistors in series between a voltage and ground make a voltage divider. The point between them sits at a fixed fraction of the input voltage. Laptop boards use dividers everywhere: adapter detection on the charger, enable thresholds, battery voltage sensing and the feedback network of every regulator.

How to use it

  • Find Vout: enter Vin, R1 and R2.
  • Find a resistor: enter Vin, the target Vout and one resistor, and leave the other empty. The calculator shows the exact value and the nearest standard E96 value with the voltage it would give.

R1 is the resistor from the input to the middle point; R2 is the resistor from the middle point to ground.

The formula

Vout = Vin × R2 ÷ (R1 + R2)

The current through both resistors is Vin ÷ (R1 + R2), and the result assumes nothing else draws current from the middle point. A chip input with very high impedance barely changes the result; a low-value load in parallel with R2 pulls Vout down.

Worked example: adapter detection

Many charger ICs detect the adapter by comparing a divided-down version of its voltage with an internal threshold. With a 19.5 V adapter, R1 = 100 kΩ and R2 = 20 kΩ, the detection pin sits at 19.5 × 20 ÷ 120 = 3.25 V. If R2 has drifted to 15 kΩ after corrosion, the pin drops to 2.54 V — possibly below the threshold, so a good adapter is rejected and the laptop does not charge. Measuring the detection pin and comparing it with this calculation is a quick way to catch that fault, as described in The Laptop Charging IC: How It Works.

Where it matters on a laptop board

  • Charger adapter detection and battery-voltage sensing.
  • Enable and power-good thresholds set by resistor pairs.
  • Regulator feedback — a special case with its own calculator: Regulator Feedback Resistor Calculator.
  • Thermistor circuits, where one of the two resistors changes with temperature.

The concept is explained in the glossary under voltage divider.

FAQ

Why does the measured voltage differ from the calculation?

Something is loading the middle point, one resistor has drifted, or your meter's input resistance is affecting very high-value dividers. Measure each resistor out of circuit if the difference matters.

What resistor values should I choose?

Higher values waste less current; lower values are less sensitive to leakage and noise. Copy the designer's values from the schematic when repairing.

Does the divider current matter?

Yes, on always-on circuits. A divider across the battery drains it continuously, so designers use high values there.