Calculator Inputs
Select a mode. Enter known values. Results update automatically.
Live Circuit Diagram
Calculation Steps
Resistor Pair Suggestions
Pairs use the selected E-series and search limits.
| Rank | R1 | R2 | Actual output | Error | Divider current | Output impedance | Action |
|---|---|---|---|---|---|---|---|
| Choose pair finder mode, then start the search. | |||||||
Tolerance, Range, and Power Analysis
Charts and Visual Comparisons
Output Versus Load Resistance
Output Versus R1
Frequency Response
Calculation History and Comparison
| Saved | Mode | Input | R1 | R2 | Output | Error | Power | Action |
|---|---|---|---|---|---|---|---|---|
| No saved calculations. | ||||||||
Formula Used
The load appears in parallel with the lower resistor. Source resistance joins the upper path.
How to Use This Calculator
- Select the calculator mode matching your design task.
- Enter voltage and resistance values with correct units.
- Enable loading when another circuit connects to output.
- Open tolerance controls for worst-case design checks.
- Review power, current, impedance, and safety warnings.
- Use pair finder for nearby standard resistor values.
- Save useful calculations or export their result data.
Example Data Table
| Application | Vin | R1 | R2 | Ideal output | Practical note |
|---|---|---|---|---|---|
| Five-volt midpoint | 5 V | 10 kΩ | 10 kΩ | 2.5 V | Good reference for high-impedance loads. |
| Twelve-volt sensing | 12 V | 27 kΩ | 10 kΩ | 3.243 V | Suitable near a 3.3 V ADC limit. |
| Automotive battery sensing | 16.8 V | 47 kΩ | 10 kΩ | 2.947 V | Add filtering and input protection. |
| Logic threshold | 5 V | 18 kΩ | 10 kΩ | 1.786 V | Verify the receiving input threshold. |
| Comparator reference | 12 V | 14 kΩ | 10 kΩ | 5 V | Consider resistor tolerance and noise. |
| Low-power monitor | 24 V | 390 kΩ | 100 kΩ | 4.898 V | High impedance may require buffering. |
Designing a Reliable Voltage Divider
A voltage divider creates a lower voltage from a higher source. Two resistors form a simple series path. The output is taken across the lower resistor. This approach works best with light, predictable loads. It does not regulate voltage like a power supply.
Understanding Loading Effects
A connected load changes the lower resistance. The load sits parallel with the lower resistor. Their combined value is always smaller. Therefore, the real output usually falls below the ideal output. Large load resistance produces less error. Small load resistance can cause serious error. A buffer amplifier can isolate sensitive divider networks.
Selecting Divider Current
Divider current affects power and output stiffness. Lower resistance increases current and reduces loading sensitivity. However, it wastes more energy and creates heat. Higher resistance saves power but raises output impedance. High impedance also increases noise sensitivity. ADC sampling circuits may need stronger drive. Choose a current well above expected load current. Ten times greater is a common starting point.
Checking Resistor Tolerance
Real resistors never equal their printed values exactly. Their tolerances shift the divider ratio. Worst-case analysis uses the largest upper resistance and smallest lower resistance for minimum output. The opposite combination produces maximum output. Input supply tolerance adds another variation source. Critical thresholds need adequate safety margin.
Protecting ADC Inputs
Microcontroller inputs have absolute voltage limits. Design below those limits during every condition. Include the highest source voltage and resistor tolerances. Add clamping or protection when transients are possible. The divider output resistance also affects sampling accuracy. A small capacitor can improve charge delivery. It also filters noise and creates settling delay.
Choosing Standard Values
Calculated resistor values may not exist commercially. Preferred E-series values provide practical choices. E24 parts suit many general designs. E96 parts offer finer ratio control. Pair searches should compare voltage error, current, and impedance. The lowest numerical error is not always best. Power use and source loading also matter.
Power and Heat Considerations
Each resistor converts electrical energy into heat. Calculate power using current squared times resistance. Select a rating above the calculated value. A two-times margin improves reliability. High-voltage circuits may also exceed resistor voltage ratings. Several series resistors can share voltage stress safely.
Common Design Mistakes
Do not use a divider for motors or heavy loads. Their changing current destroys output accuracy. Never ignore source resistance or meter loading. Avoid placing excessive voltage on ADC pins. Confirm grounding between connected circuits. Check tolerance across temperature when precision matters.
Measurement and Verification
Measure the assembled divider before connecting sensitive equipment. Confirm supply polarity and shared ground connections first. Use a meter with sufficiently high input resistance. Compare measured voltage with both loaded and unloaded predictions. Test minimum and maximum supply conditions when possible. Record resistor values, temperatures, and connected loads. Unexpected differences may reveal wiring faults, damaged parts, leakage, or hidden protection circuits. Careful verification prevents costly downstream failures.