Calculator
Example Data Table
| Method | Source Voltage | Series Resistance | Meter Reading | Internal Resistance | Loading Error |
|---|---|---|---|---|---|
| Series resistor | 10 V | 100 kΩ | 9.09 V | 999 kΩ | 9.1% |
| Voltage drop | 12 V | 500 kΩ | 8 V | 1 MΩ | 33.33% |
| Sensitivity | 10 V range | 20,000 Ω/V | Full scale | 200 kΩ | Depends on circuit |
Formula Used
Internal resistance: Rv = Vm × Rs / (Vs − Vm)
Series resistance: Rs = Rv × (Vs − Vm) / Vm
Measured voltage: Vm = Vs × Rv / (Rs + Rv)
Source voltage: Vs = Vm × (Rs + Rv) / Rv
Voltage drop method: Rv = R × V2 / (V1 − V2)
Meter sensitivity: Rv = Range × Ω per volt
Loading error: Error % = ((Vs − Vm) / Vs) × 100
How to Use This Calculator
Choose the calculation type first. Enter the known electrical values in matching fields. Use volts for voltage and ohms for resistance. Press the calculate button. The result appears above the form. Review current, loading error, power, and sensitivity. Use CSV or PDF buttons to save the report.
About Voltmeter Internal Resistance
Why Voltmeter Resistance Matters
A voltmeter should draw very little current. Its resistance should be high. Real meters are not ideal. They still pull some current from the circuit. That current can lower the measured voltage. The effect is called loading. It is small in low resistance circuits. It can be large in sensor circuits. It can also change bias points in electronic tests. Knowing internal resistance helps you choose the right meter range. It also helps you judge whether a reading is trustworthy.
Practical Measurement Notes
A common test uses a known source and a series resistor. The voltmeter reads only part of the source voltage. The missing part appears across the series resistor. Ohm's law then gives the current. The same current flows through the voltmeter. Dividing the meter voltage by that current gives internal resistance. This method is simple. It still needs clean connections and a stable source. Use a resistor with a known tolerance. Use a source that does not sag during the test.
Understanding the Results
A higher calculated resistance means lower meter loading. The loading error shows how much the meter has reduced the circuit voltage. A small error is usually acceptable. A large error means the meter changed the circuit too much. The correction factor estimates the original source voltage from the loaded reading. The sensitivity value, in ohms per volt, is useful for analog meters. It relates full scale voltage to meter resistance.
Good Use Cases
This calculator helps students, technicians, and hobbyists. It is useful when checking old analog meters. It is also useful when comparing digital meters. You can test a meter range before using it on delicate circuits. Enter realistic values. Then review current, power, resistance ratio, and loading error. Do not exceed the rating of any resistor or meter. If readings drift, repeat the test after the circuit warms. Use the average of repeated trials. The result should support safe measurement choices. For best accuracy, keep lead resistance low. Let the meter settle before recording. Select a series resistor that gives a clear voltage drop. Avoid very small drops because rounding errors grow. Record units beside every value. This prevents mistakes during later review and reporting in practice.
FAQs
What is voltmeter internal resistance?
It is the resistance inside the voltmeter input path. A high value draws less current from the tested circuit. That gives a more accurate voltage reading.
Why should a voltmeter have high resistance?
High resistance reduces loading. It stops the meter from stealing much current. This keeps the circuit voltage closer to its true value.
Which formula is most common?
The common series method is Rv = Vm × Rs / (Vs − Vm). It uses source voltage, known series resistance, and measured meter voltage.
Can this calculator check analog meters?
Yes. Enter the meter range and ohms per volt. The calculator finds input resistance for that range and estimates full scale current.
What is loading error?
Loading error is the voltage drop caused by the meter. It compares source voltage with measured voltage. Lower loading error is better.
What units should I use?
Use volts for voltage and ohms for resistance. The calculator displays large resistance values in kΩ or MΩ for easier reading.
Why is source voltage greater than measured voltage?
The series resistor drops part of the source voltage. The voltmeter receives the remaining voltage. That difference helps calculate current and resistance.
Can I export the result?
Yes. After calculation, use the CSV button for spreadsheet data. Use the PDF button for a simple printable report.