Current Sense Shunt Calculator
Design precision current measurement shunts: calculate voltage drop, power dissipation, and shunt resistance for ammeter and battery management circuits.
Shunt Measurement Formulas
Voltage Drop: V = I × R
Power Dissipation: P = I² × R = V² / R
Resistance Change (temp): ΔR% = TC × ΔT (ppm/°C)
Required Shunt: R = Vtarget / Imax
Amplifier Gain: G = Vout / Vshunt (V/V)
Precision Shunt Selection Guide
| Application | Typical R | Accuracy | Temp.Co. |
|---|---|---|---|
| Battery Management (BMS) | 1–10 mΩ | 0.2–0.5% | 100 ppm/°C |
| Power Supply Current Monitoring | 10–100 mΩ | 0.5–1% | 100–200 ppm/°C |
| LED Driver Feedback | 100 mΩ–1 Ω | 1–2% | 200 ppm/°C |
| Motor Control (High Current) | 0.1–1 mΩ | 0.1–0.2% | 50 ppm/°C |
Frequently Asked Questions
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A current sense shunt is a precision low-resistance resistor placed in series with a circuit to measure current. The voltage drop across the shunt (V = I × R) is measured and converted to current. Typical shunt resistances: 1 mΩ to 100 mΩ for high-current applications, up to 1 Ω for low-current.
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Precision shunt resistors have tolerance ±0.1% to ±1%, temperature coefficient < 100 ppm/°C. Accuracy depends on resistor tolerance, ADC resolution, and amplifier gain. Typical current measurement accuracy: ±1–5% with good design.
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Voltage drop (V) = Current (I) × Shunt Resistance (R). Example: 100 A through a 1 mΩ shunt = 0.1 V drop. Minimize shunt resistance to reduce power loss, but ensure ADC can measure the small voltage difference (typically 10–200 mV full-scale).
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Power dissipation (P) = I² × R = V² / R. Example: 100 A through 1 mΩ = 10 W dissipation. Thermal design (heatsink, PCB layout) is critical for high-current shunts. High dissipation increases temperature, changing resistance and introducing error.
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Shunt resistance changes with temperature at a rate of ppm/°C (parts per million per degree). Example: 100 ppm/°C means 1°C rise = 0.01% resistance change. High-precision shunts have 50–100 ppm/°C; standard shunts 500+ ppm/°C. Self-heating from power dissipation causes additional drift.
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Shunts are: (1) Passive (no power), (2) Accurate (precision resistors), (3) Wide temperature range, (4) Low cost. Hall sensors are non-invasive but less accurate and more expensive. Shunts are preferred for high-precision battery management, power supplies, and metering.
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A precision current amplifier (like INA128, LT6375) amplifies the tiny voltage across a shunt. Gain example: 100 V/V amplifies a 100 mV shunt drop to 10 V for an ADC. Amplifier characteristics: low offset voltage (<1 mV), high CMRR (>80 dB), low noise (<1 µV/√Hz).
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Balance power dissipation vs. measurement resolution. Higher R = larger voltage drop (easier to measure), but more power loss. Typical design: target shunt voltage drop 50–200 mV at max expected current. Example: 100 A max, target 100 mV → R = 1 mΩ.
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Power dissipation heats the shunt, raising its temperature and resistance. If uncompensated, this causes measurement drift. Thermal management (good heatsinking, airflow) and temperature compensation (measuring shunt temperature with a thermistor) minimize drift.
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For rough measurements, yes. But precision is lost: film/carbon resistors have 5–10% tolerance and 500+ ppm/°C drift. For accurate current measurement, use precision shunt resistors (Vishay, Yageo, Susumu) with calibrated resistance and published temperature coefficient.
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Four-terminal shunts have separate sense leads to avoid PCB trace resistance errors. Current flows through two terminals; voltage is sensed across two others. Eliminates measurement error from lead resistance. Required for high-accuracy, high-current applications.
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(1) Use precision shunt with matched temperature coefficient. (2) Low-noise amplifier with high CMRR. (3) Temperature compensation. (4) Shielded/twisted signal leads. (5) Good PCB layout (separate analog/power grounds). (6) Regular calibration.