Voltage-divider designer

Assumptions to check

• Linear, ideal resistors at their nominal values.
• A resistive input load when one is entered.
• No source impedance, leakage, noise, or dynamic loading model.

A divider is a source, not just a ratio

A voltage divider is often introduced as a two-resistor ratio. In a working circuit, the bottom leg can be loaded by an ADC input, comparator, bias network, or measurement instrument. That load is parallel with the bottom resistor, changing both the output voltage and the output resistance.

This calculator shows the unloaded and loaded results together. That makes the loading penalty visible instead of hiding it behind a target ratio. It also shows divider current and resistor power so a low-resistance choice can be weighed against standby current and dissipation.

Worked example

A 12 V input with 10 kΩ on top and 20 kΩ on the bottom produces 8 V without a load. The output resistance is 6.667 kΩ. Add a finite load and the effective bottom resistance falls, so the output moves downward. When an input requires a tight source impedance or the load varies, the divider may need a buffer rather than lower resistor values.

Limits and next checks

A resistor ratio does not check input common-mode range, ADC acquisition time, leakage over temperature, or fault voltage. Do not use it to infer that an IC input is safe. The next useful calculation is often a Thevenin equivalent or an input-loading check, followed by preferred values, tolerance corners, and the selected device data sheet.

Common mistakes

• Calculating only the unloaded ratio when a circuit input loads the node.
• Reducing resistance solely to lower output impedance without checking wasted current.
• Ignoring resistor tolerance and the receiving input’s allowable range.

Model limit and handoff

Check receiver input limits, source impedance requirements, leakage, resistor tolerance, and fault conditions in the chosen component data sheets.

FAQs