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Current Shunt Resistor

Calculate shunt resistor voltage drop, power dissipation, amplifier output, and ADC resolution for current sensing circuits. Free, instant results.

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Formula

Vsh=I×Rsh,P=I2×RshV_sh = I × R_sh, P = I² × R_sh
R_sh— Shunt resistance (Ω)
I— Measured current (A)

How It Works

This calculator determines current shunt resistor parameters and amplifier requirements for precision current measurement, essential for battery management system designers, motor control engineers, and power electronics developers. A current shunt is a low-resistance precision resistor (1-100 mOhm typical) in series with the current path, producing a voltage drop Vsh = I Rsh per Ohm's law. The key trade-offs are power dissipation (P = I^2 Rsh, which must not overheat the shunt or degrade efficiency) and signal-to-noise ratio (larger Rsh = more voltage = better SNR but more loss). A 5 mOhm shunt at 20A dissipates 2W and produces 100 mV. Dedicated current-sense amplifiers (INA240, INA219, MAX9634) amplify this millivolt signal with gains of 20-200 V/V and CMRR >120 dB per datasheets. High-side sensing (shunt between supply and load) detects fault currents and ground faults but requires high-common-mode amplifiers rated to 80V+ per INA240. Low-side sensing (shunt between load and ground) uses simpler amplifiers but the load ground floats by Vsh.

Worked Example

Problem

Design current sensing for a 48V/30A e-bike motor controller. Requirements: +/-0.5% accuracy, <0.5% efficiency loss, 12-bit ADC with 3.3V reference.

Solution
  1. Power budget: 0.5% of 48V*30A = 7.2W max; target 2W -> Rsh = P/I^2 = 2/900 = 2.22 mOhm
  2. Use standard 2 mOhm shunt (Vishay WSL2512, +/-0.5%, 1W per element, use 2 in parallel)
  3. Full-scale voltage: Vsh = 30A * 2 mOhm = 60 mV
  4. Required gain for 3.0V output: G = 3000/60 = 50 V/V
  5. Select INA240A2 (gain = 50 V/V, CMRR = 132 dB, bandwidth = 400 kHz)
  6. Resolution: 3.3V/4096/50 = 16.1 uV = 8.1 mA/LSB
  7. Power dissipation: 30^2 * 0.002 = 1.8W (within 2W budget, 0.125% efficiency loss)
  8. Kelvin connection required: 4-terminal shunt eliminates PCB trace resistance error
Result: 2 mOhm shunt with INA240A2 provides 8 mA resolution, +/-0.5% accuracy, and only 0.125% efficiency loss at full current.

Practical Tips

  • ✓Use dedicated current-sense amplifier ICs (INA240, INA219, MAX9634) rather than discrete instrumentation amplifiers; these include EMI filters, precision gain, and optimized common-mode rejection for switching environments per Texas Instruments SLVA458
  • ✓For battery management, low-side sensing (shunt between battery negative and load ground) avoids high common-mode voltage but monitor for ground faults separately; high-side sensing detects both load and fault currents
  • ✓Add an RC filter at the amplifier input (10 Ohm + 100 nF differential) to suppress high-frequency switching noise from PWM motor drivers; this limits bandwidth to 160 kHz while rejecting >1 MHz switching harmonics

Common Mistakes

  • ✗Using high-side shunt with ground-referenced amplifier: high-side sensing requires amplifiers rated for common-mode voltage (INA240 to 80V, INA282 to 110V); ground-referenced op-amps saturate when Vcm exceeds supply rails
  • ✗Ignoring Kelvin connections: standard 2-terminal shunts include PCB trace resistance that adds measurement error; use 4-terminal (Kelvin) shunts and route voltage-sense traces directly from the shunt sense pads per Vishay application note AN-28e
  • ✗Undersizing shunt power rating: at high currents, I^2 dominates; a 10 mOhm shunt at 10A dissipates 1W and will drift significantly if rated for only 0.25W; use 2x power derating per IPC-2221

Frequently Asked Questions

Low-side sensing places the shunt between load ground and system ground; simple single-supply amplifiers work (Vcm near 0V), but load ground floats by Vsh (typically 50-100 mV) and fault currents to ground bypass the shunt. High-side sensing places the shunt between supply and load; provides true common ground and detects all currents including faults, but requires high-common-mode amplifiers. INA240 handles 80V common mode; INA282 handles 110V. High-side is preferred for safety-critical applications per ISO 26262.
Target 50-100 mV full-scale voltage for good SNR while limiting power loss to <1% of system power. Rsh = Vsh_target / Imax. For 100 mV at 10A, use 10 mOhm. Then verify power: P = I^2 * R = 100 * 0.01 = 1W. Select shunt rated for 2x calculated power with TCR <50 ppm/C. Common values: 1-10 mOhm for >10A, 10-100 mOhm for 1-10A, 100 mOhm-1 Ohm for <1A per Vishay and Bourns selection guides.
Only for non-precision applications. Standard 1% metal-film resistors have TCR of 100 ppm/C, causing 1% drift over 100C. Precision shunt resistors (Vishay WSL, Bourns CSM, Ohmite LVK) have TCR <50 ppm/C, better thermal coupling, and 4-terminal Kelvin connections for accurate sensing. For +/-0.5% current accuracy, use precision shunts with +/-0.5% tolerance and TCR <20 ppm/C per IEC 62576 requirements.

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