In electronic circuit testing and troubleshooting, current measurement is a fundamental and critical operation. Whilst multimeters can directly measure current, shunt resistors prove a more reliable choice when dealing with high currents (e.g. exceeding 10A) or demanding high-precision measurements, owing to their low cost and high stability. This article systematically explains shunt resistor selection, connection methods, practical measurement steps, error control, and safety precautions, starting from the working principle. It aims to help electronic engineers and enthusiasts master this practical technique.

I. Core Principle of Shunt Resistor Current Measurement: Application of Ohm's Law
The design of shunt resistors fundamentally relies on Ohm's Law (V=IR) and the current-splitting characteristics of parallel circuits. Within a circuit, when a low-value resistor (shunt resistor) is connected in parallel with the load, the majority of the current flows through the lower-resistance shunt resistor (i.e., ‘current shunting’), while only a small portion passes through the load. By measuring the voltage drop across the shunt resistor and combining this with its known resistance value, the total circuit current can be calculated inversely.
The specific principle can be broken down into three steps:
Shunting Effect: Based on the parallel circuit principle that ‘voltages across branches are equal, while currents are inversely proportional to resistances,’ the shunt resistor (Rshunt) has a significantly lower resistance than the load resistor (Rload). Consequently, over 99% of the total current (I_total) flows through the shunt resistor, forming the shunt current (Ishunt), while the load current (Iload) becomes negligible (I_total≈Ishunt).
Voltage Acquisition: A voltmeter or data acquisition module (e.g., ADC) measures the voltage drop (Vshunt) across the shunt resistor. Due to the extremely low resistance value, the voltage drop is typically in the millivolt range (e.g., a 100A current through a 1mΩ resistor yields only 0.1V). Consequently, high-precision measurement equipment with low offset voltage must be selected.
Current calculation: Applying a modified form of Ohm's law, I_total≈Vshunt/Rshunt, substituting the measured voltage value and the known shunt resistor value yields the actual circuit current.
For instance, if a shunt resistor has a value of 5mΩ and a measured voltage drop of 0.25V across its terminals, the circuit current is: I = 0.25V / 0.005Ω = 50A. This calculation directly reflects the circuit's actual operating current.

II. Shunt Resistor Selection: Key Parameters and Matching Principles
Selection is the first step in ensuring measurement accuracy. Incorrect selection may lead to measurement errors exceeding tolerances, or even burn out the resistor or circuit. Shunt resistor selection must focus on the following five core parameters:
1. Nominal Resistance (Rshunt): Low Resistance is Paramount
The shunt resistor's resistance must be substantially lower than the load resistance (typically less than 1/100th of the load resistance) to ensure the majority of current flows through it. Practical selection ranges between 1mΩ and 100mΩ, calculated based on the maximum measurement current and permissible maximum voltage drop:
For instance, if the maximum measurement current is 100A and the voltage drop must not exceed 0.1V (to minimise power dissipation), the shunt resistor value should satisfy: Rshunt ≤ 0.1V/100A = 1mΩ. In this case, a 1mΩ shunt resistor should be selected.
2. Rated Power (Pmax): Preventing Overheating and Burnout
The power dissipation formula for shunt resistors is P = I²R. Due to high currents and low resistance values, power loss is concentrated in the ‘current squared’ term. Therefore, resistors with sufficiently high rated power must be selected. Typically, a power margin of 20% to 50% should be reserved to prevent damage from short-term current surges.
Taking a 100A, 1mΩ shunt resistor as an example, the actual power dissipation is P = (100A)² × 0.001Ω = 10W. Therefore, a product with a rated power ≥15W (50% margin) must be selected.
3. Accuracy Class: Directly impacts measurement error
Shunt resistor accuracy is typically denoted as ‘±%’, with common grades being ±1%, ±0.5%, and ±0.1%. Higher accuracy reduces measurement error but increases cost.
Industrial applications (e.g., motor current monitoring) may utilise ±1% accuracy;
Laboratory-grade precision measurements (e.g., power supply testing) require ±0.1% to ±0.5% accuracy;
Note: Accuracy must match the precision of measurement equipment (e.g., voltmeters). If a voltmeter has only ±2% accuracy, using a ±0.1% shunt resistor will not improve overall measurement precision.
4. Temperature Coefficient (TCR): Controlling Errors from Thermal Drift
Current flowing through a shunt resistor generates heat, causing its resistance value to vary with temperature (known as ‘temperature drift’). The temperature coefficient (TCR, measured in ppm/℃) describes this rate of change. A lower TCR minimises the impact of temperature drift on measurement accuracy. Typically, products with TCR ≤ 50 ppm/℃ should be selected; for high-precision applications, ≤ 10 ppm/℃ is required.
For instance, with a TCR of 20 ppm/°C, a 30°C ambient temperature change yields a resistance variation rate of 20 × 30 = 600 ppm (equivalent to 0.06%). For a 100A current measurement, this error amounts to merely 0.06A, rendering it negligible. Whereas with TCR 200ppm/℃, the error increases to 0.6A, compromising measurement accuracy.
5. Packaging Type: Aligning with Installation and Thermal Requirements
Shunt resistor packaging must be selected based on current rating and installation context:
Low current (≤10A): Surface-mount packages (e.g., 2512, 1206) are suitable due to their compact size and PCB integration compatibility;
High current (≥20A): Through-hole packages (e.g., TO-220, bolt-type) are required. Bolt-type packages can enhance heat dissipation via metal heat sinks to prevent overheating.

III. Practical Measurement Procedure: From Connection to Data Calculation
After determining the appropriate type, actual measurement must adhere to the principle of ‘safety first, standardised connection’. The specific steps can be divided into four phases:
1. Circuit power disconnection and pre-treatment
Before measurement, the circuit power supply must be disconnected to prevent electric shock or short circuits from live operations;
Inspect the shunt resistor's appearance: confirm no cracks, pin oxidation, or other damage. Use a multimeter's ohm setting to measure its resistance, verifying it matches the nominal value (within the accuracy tolerance range);
Prepare tools: In addition to the shunt resistor, prepare a high-precision voltmeter (or oscilloscope, data acquisition card), wires (cross-sectional area must match the maximum current; e.g., 16mm² copper wire for 100A), insulating tape, screwdriver, etc.
2. Correctly connect the shunt resistor: Parallel connection is key
The shunt resistor must be connected in parallel with the load, and the connecting wires must meet the following requirements:
Minimise wire resistance: Use short, thick wires to prevent wire resistance from being included in the total shunt resistance value (e.g., if wire resistance is 0.5 mΩ and connected in series with a 1 mΩ shunt resistor, the total resistance becomes 1.5 mΩ, increasing measurement error to 50%);
Secure terminal connections: In high-current applications, bolt-fasten terminals to prevent contact resistance (which elevates voltage readings and overestimates current).
Polarity distinction: For DC circuits, ensure voltmeter polarity aligns with shunt resistor polarity (current input as ‘+’, output as ‘-’) to avoid negative readings.
Taking DC motor current measurement as an example, the connection sequence is: power supply positive terminal → motor positive terminal, motor negative terminal → one end of the shunt resistor, other end of the shunt resistor → power supply negative terminal. Simultaneously, connect the voltmeter in parallel across the shunt resistor (positive terminal connected to the motor negative side, negative terminal connected to the power supply negative side).
3. Voltage Measurement and Interference Shielding
Select an appropriate voltmeter range: As the voltage drop across the shunt resistor is typically in the millivolt range, set the voltmeter to the ‘DC millivolt range’ (e.g., 200mV range) to prevent insufficient reading accuracy due to excessive range;
Shield against electromagnetic interference: Industrial environments contain interference sources such as motors and inverters. Connect the voltmeter using shielded cable or install a metal shield around the shunt resistor to prevent fluctuating readings caused by interference signals.
Take multiple measurements and average: To minimise random error, after the circuit has stabilised (e.g., 30 seconds after motor start), measure the voltage 3–5 consecutive times and use the average as the final reading.
4. Current Calculation and Error Verification
Substitute the measured average voltage into the formula I=Vshunt/Rshunt to calculate the circuit current. For example:
Measured average voltage: 0.18V
Shunt resistor value: 1mΩ (0.001Ω)
Calculated current: I=0.18V/0.001Ω=180A
Verify whether the calculated error falls within the permissible range: If the shunt resistor accuracy is ±0.5% and the voltmeter accuracy is ±0.2%, the total error is ±(0.5% + 0.2%) = ±0.7%. For a 180A current, the error range is 180A × 0.7% = ±1.26A. If this error is acceptable for the application, the measurement result is valid.

IV. Error Control and Safety Precautions
1. Common Error Sources and Solutions
Even with correct selection and connection, errors may persist and require targeted control:
Contact resistance error: Oxidation or loosening of terminals causes contact resistance. Solution: Clean terminal surfaces (using emery paper) and tighten bolts;
Temperature drift error: Prolonged measurement causes shunt resistors to heat up, altering their resistance value. Solutions include selecting resistors with low TCR (Temperature Coefficient of Resistance) or pausing the circuit during measurement intervals to allow resistors to cool before resuming measurement;
Voltmeter offset error: At low ranges, voltmeters inherently possess an offset voltage (e.g., 0.1mV). The solution is to first short-circuit the voltmeter, record the offset voltage, and then subtract this voltage from the measured value (e.g., if the measured value is 0.18V and the offset voltage is 0.0001V, the actual voltage is 0.1799V).
2. Safety Operating Guidelines
Prohibit over-range usage: The maximum current and power of shunt resistors must not exceed rated values. Should circuits potentially experience transient surge currents (e.g., motor start-up currents reaching five times the rated current), select shunt resistors capable of withstanding surge currents (e.g., pulse power rated products).
Avoid hot swapping: Disconnecting or connecting shunt resistors during measurement is prohibited, as this may cause circuit open-circuit, generate high-voltage arcs, burn out resistors, or cause injury.
High-temperature protection: Shunt resistors generate heat during high-current measurement (e.g., a 10W resistor may reach surface temperatures exceeding 80°C). Direct contact must be avoided, and heat shields should be installed around the resistor to prevent burns or ignition of nearby flammable materials.
In summary, shunt resistors represent one of the optimal choices for high-current, high-precision measurement in DC and low-frequency AC circuits. Mastering their selection and usage methods provides reliable data support for circuit testing and maintenance.