Calculator Inputs
Use the layout below: 3 columns on large screens, 2 on smaller, 1 on mobile.
Formula Used
The four-wire method drives a known current through the device and senses the voltage directly at the device terminals, minimizing lead and contact voltage drops.
- R4w = VDUT / I
- VDUT = Vmeas − Isense·Rsense,total (optional loading model)
- P = I·VDUT
- R2w ≈ R4w + Rlead,total + Rcontact,total (comparison estimate)
- R(Tref) ≈ R(T) / (1 + α·(T − Tref)), with α = ppm·10−6/°C
- Uncertainty (independent): σR ≈ |R|·√[(σV/V)2 + (σI/I)2]
How to Use This Calculator
- Connect two force leads to drive current through the device.
- Connect two sense leads directly at the device terminals.
- Enter the sensed voltage and the sourced current.
- Optionally enter uncertainties to estimate resistance uncertainty.
- Add lead and contact resistance to compare with two-wire readings.
- Enter temperature coefficient and temperatures to correct to a reference.
- Press Calculate to see results above the form.
Example Data Table
| Vmeas (mV) | I (mA) | R4w (mΩ) | Lead+Contact (mΩ) | R2w (mΩ) | P (mW) |
|---|---|---|---|---|---|
| 12.500 | 100.0 | 125.000 | 60.0 | 185.000 | 1.250 |
| 1.800 | 50.0 | 36.000 | 25.0 | 61.000 | 0.090 |
| 0.240 | 10.0 | 24.000 | 15.0 | 39.000 | 0.002 |
Four-Wire Resistance Measurement Notes
Why two leads fail at low ohms
In a two-lead test, the reading includes the device plus lead, probe, and contact resistance. Typical leads can add 0.05–0.20 Ω round trip, and contacts can add tens of milliohms, which overwhelms shunts, coils, and battery tabs.
Kelvin connection principle
Four-wire (Kelvin) measurement separates current and voltage sensing. One pair forces a known current through the device under test. A second, high-impedance pair senses voltage directly at the component terminals, so almost no current flows in the sense leads. This makes sense-lead drops negligible and greatly improves low-ohm accuracy.
Core formula
The calculator applies Ohm’s law: R = V / I, where V is the sensed voltage and I is the source current. It also shows an optional two-wire estimate: R2w ≈ (V/I) + Rleads, useful for seeing how much cabling can bias results.
Choosing test current
Select a current that produces a measurable voltage without heating the part. A practical target is 10–100 mV across the unknown, since many meters resolve millivolts reliably. Example: 0.100 Ω at 1.0 A gives 100 mV. For delicate parts, 10–200 mA reduces self-heating while keeping a stable signal.
Lead and contact effects you can quantify
Measure lead resistance by shorting the force leads and taking a two-wire reading. If the leads total 0.12 Ω, a true 0.08 Ω part may look like 0.20 Ω in two-wire mode, a large systematic error. Repeat the test after repositioning to confirm consistency.
Thermal EMF and noise control
At micro-ohm levels, tiny thermoelectric offsets from dissimilar metals can rival the signal. Keep connections clean and stable, and average repeated samples to reduce random noise. If your setup supports it, reverse the source current and average the computed resistance from both polarities.
Practical setup checklist
Place the sense probes inside the force connections, close to the device body. Use thicker force leads, keep loops small, and avoid shared junctions between force and sense. Let temperature stabilize before recording a final value.
Interpreting results and uncertainty
Match reported precision to instrument limits. With a 100 mA source (±0.1%) and a 1 mV resolution voltmeter, milliohm reporting is realistic for values that generate tens of millivolts. If resistance changes with current, suspect self-heating or non-ohmic contacts. Logging current and time helps track drift.
FAQs
1) What is a four-wire resistance measurement?
It uses two leads to force current and two separate leads to sense voltage at the component terminals. This removes most lead and contact resistance error in low-ohm measurements.
2) When should I prefer four-wire over two-wire?
Use four-wire when the expected resistance is below about 1 Ω, or when lead/contact resistance is comparable to the device value. It is also preferred for high-accuracy shunt calibration.
3) How do I choose a safe test current?
Choose current so the device voltage is around 10–100 mV while power stays low. Check P = I²R. If temperature rises or readings drift, reduce current.
4) Why does my reading change when I move the clips?
Contact resistance and pressure change the drop at the connection points. In four-wire mode, this mainly affects the force path, but poor sense placement can still add error.
5) Can I measure very high resistances with this method?
Yes, but four-wire offers little advantage at high resistance because lead drops are negligible. The main limits become leakage, noise, and the stability of the current source.
6) What if the calculator shows a negative or unstable value?
Negative values usually indicate offset voltage, wiring polarity issues, or very small signals dominated by noise. Verify polarity, increase current slightly, and consider current reversal averaging.
7) Does temperature matter for low-resistance parts?
Yes. Temperature coefficients and self-heating can shift milliohm readings. Minimize power, allow stabilization, and note measurement temperature when comparing results.