I. The core principle of shunt resistor current measurement: the application of Ohm's law
The design of shunt resistors is essentially based on Ohm's law (V=IR) and the current-sharing characteristics of parallel circuits. In a circuit, when a low-resistance resistor (shunt resistor) is connected in parallel with the load, most of the current will flow through the smaller-resistance shunt resistor (i.e., "shunting"), while the load only carries a small amount of current. By measuring the voltage drop across the shunt resistor and combining this with the known resistance value, the total current in the circuit can be calculated in reverse.
The specific principle can be broken down into three steps:
Shunting effect: According to the characteristics of parallel circuits that "the voltage of each branch is equal and the current is inversely proportional to the resistance", the resistance of the shunt resistor (Rshunt) is much smaller than the load resistance (Rload). Therefore, more than 99% of the total current (I_total) will flow through the shunt resistor, forming the shunt current (Ishunt), while the load current (Iload) can be ignored (I_total≈Ishunt).
Voltage acquisition: The voltage drop (Vshunt) across the shunt resistor is measured using a voltmeter or data acquisition module (such as an ADC). Since the resistance value is extremely low, the voltage drop is usually in the millivolt range (e.g., when a 100A current flows through a 1mΩ resistor, the voltage drop is only 0.1V). Therefore, a high-precision measurement device with low offset voltage is required.
Current calculation: According to the modified Ohm's law formula I_total≈Vshunt/Rshunt, by substituting the measured voltage value and the known shunt resistance value, the actual current in the circuit can be calculated.
For example, if the shunt resistor has a resistance of 5mΩ and the voltage across it is 0.25V, then the circuit current is: I = 0.25V / 0.005Ω = 50A. This calculation result can directly reflect the actual operating current of the circuit.
II. Selection of Shunt Resistors: Key Parameters and Matching Principles
Selecting the right component is the first step in ensuring measurement accuracy. Incorrect selection can lead to excessive measurement errors or even burn out the resistor or circuit. When selecting a shunt resistor, pay close attention to the following five core parameters:
1. Nominal resistance (Rshunt): Low resistance is the core requirement.
The shunt resistor value needs to be much smaller than the load resistance (usually less than 1/100 of the load resistance) to ensure that most of the current flows through the shunt resistor. In actual selection, the resistance value is usually between 1mΩ and 100mΩ, and the specific value needs to be calculated based on the maximum measuring current and the maximum allowable voltage drop.
For example, if the maximum measuring current is 100A and the voltage drop is not expected to exceed 0.1V (to avoid excessive power loss), the shunt resistor value should meet the following condition: Rshunt ≤ 0.1V/100A = 1mΩ. In this case, a 1mΩ shunt resistor should be selected.
2. Rated power (Pmax): To prevent the resistor from overheating and burning out.
The power loss calculation formula for a shunt resistor is P=I²R. Due to the large current and low resistance, the power loss is concentrated in the "current square" term. Therefore, a resistor with a sufficiently large rated power must be selected. Typically, a power margin of 20% to 50% should be reserved to prevent damage caused by short-term current surges.
Taking a 100A, 1mΩ shunt resistor as an example, the actual power loss is P=(100A)²×0.001Ω=10W. Therefore, a product with a rated power ≥15W (with a 50% margin) should be selected.
3. Accuracy class: Directly affects measurement error.
The accuracy class of shunt resistors is usually expressed in "±%", with common classes being ±1%, ±0.5%, and ±0.1%. Higher accuracy means smaller measurement error, but also higher cost.
For industrial applications (such as motor current monitoring), an accuracy of ±1% is available.
For high-precision measurements in the laboratory (such as power supply testing), an accuracy of ±0.1% to ±0.5% should be selected.
Note: The accuracy must match the accuracy of the measuring device (such as a voltmeter). If the voltmeter accuracy is only ±2%, using a shunt resistor with ±0.1% accuracy will not improve the overall measurement accuracy.
4. Temperature coefficient (TCR): Controls the error caused by temperature drift.
When current flows through a shunt resistor, it generates heat, causing the resistance value to change with temperature (i.e., "temperature drift"). The temperature coefficient (TCR, measured in ppm/℃) describes this rate of change. The smaller the TCR, the less impact temperature drift has on measurement accuracy. Typically, products with a TCR ≤ 50 ppm/℃ should be selected, and for high-precision applications, ≤ 10 ppm/℃ is required.
For example, if the TCR is 20ppm/℃ and the ambient temperature changes by 30℃, the resistance change rate is 20×30=600ppm (i.e. 0.06%), and the measurement error for a 100A current is only 0.06A, which can be ignored; if the TCR is 200ppm/℃, the error will increase to 0.6A, affecting the measurement accuracy.
5. Packaging: Adaptable to installation and heat dissipation requirements.
The package of the shunt resistor needs to be selected based on the current magnitude and the installation scenario:
Low current (≤10A): Surface mount packages (such as 2512, 1206) can be used, which are small in size and suitable for PCB integration;
High current (≥20A): Requires through-hole package (such as TO-220, screw type). Screw type package can enhance heat dissipation through metal heat sink to avoid overheating.
III. Actual Measurement Steps: From Connecting to Data Calculation
After mastering the selection, the actual measurement must follow the principle of "safety first, standardized connection". The specific steps can be divided into 4 steps:
1. Circuit power outage and pretreatment
The power supply must be disconnected before measurement to avoid electric shock or short circuit caused by operating the circuit while it is energized;
Inspect the shunt resistor's appearance: confirm there are no cracks, lead oxidation, or other damage; use a multimeter in ohm mode to measure its resistance value and confirm it matches the nominal value (within the accuracy range);
Tools needed: In addition to the shunt resistor, you will need a high-precision voltmeter (or oscilloscope, data acquisition card), wires (the cross-sectional area must match the maximum current, such as 16mm² copper wire for 100A), insulating tape, screwdriver, etc.
2. Correctly connect the shunt resistors: parallel connection is key.
The shunt resistor must be connected in parallel with the load, and the connecting wires must meet the following requirements:
Minimize wire resistance: Use short, thick wires to avoid including wire resistance in the total shunt resistance (if the wire resistance is 0.5mΩ, connecting it in series with a 1mΩ shunt resistor will result in a total resistance of 1.5mΩ, increasing the measurement error by 50%).
Secure the wiring terminals: In high-current scenarios, bolts must be used to secure the wiring terminals to prevent poor contact and additional resistance (contact resistance will cause the voltage measurement to be too high and the calculated current to be too high);
Polarity distinction: When using a DC circuit, ensure that the positive and negative terminals of the voltmeter are consistent with the voltage polarity of the shunt resistor (the current inflow terminal is "+", and the outflow terminal is "-") to avoid negative measurements.
Taking DC motor current measurement as an example, the connection method is as follows: power supply positive terminal → motor positive terminal, motor negative terminal → one end of shunt resistor, the other end of shunt resistor → power supply negative terminal. At the same time, the voltmeter is connected in parallel across the two ends of the shunt resistor (positive terminal connected to the motor negative terminal side, negative terminal connected to the power supply negative terminal side).
3. Voltage Measurement and Interference Shielding
Select the appropriate voltmeter range: Since the voltage drop across the shunt resistor is typically in the millivolt range, the voltmeter should be set to the "DC millivolt range" (e.g., 200mV range) to avoid insufficient reading accuracy due to an excessively large range;
Electromagnetic interference shielding: Industrial environments contain interference sources such as motors and frequency converters. Shielded cables should be used to connect the voltmeter, or a metal shielding cover should be added around the shunt resistor to prevent interference signals from causing voltage reading fluctuations.
Take the average value of multiple measurements: To reduce random errors, after the circuit is working stably (such as 30 seconds after the motor starts), take the voltage value 3 to 5 times in succession and take the average value as the final measurement value.
4. Current Calculation and Error Verification
Substitute the measured average voltage value into the formula I=Vshunt/Rshunt to calculate the circuit current. For example:
Average measured voltage: 0.18V
Shunt resistor value: 1mΩ (0.001Ω)
Calculate the current: I = 0.18V / 0.001Ω = 180A
After calculation, it is necessary to verify whether the error is within the allowable range: if the shunt resistor accuracy is ±0.5% and the voltmeter accuracy is ±0.2%, then the total error is ±(0.5%+0.2%)=±0.7%, and the error range for 180A current is 180A×0.7%=±1.26A. If this error is allowed in actual application, then the measurement result is valid.
IV. Error Control and Safety Precautions
1. Common sources of error and solutions
Even with correct selection and connection, errors may still exist and require targeted control:
Contact resistance error: Oxidation or looseness of the terminals can cause contact resistance. The solution is to clean the terminal surface (sandpaper) and tighten the bolts.
Temperature drift error: Long-term measurement causes the shunt resistor to heat up and change its resistance value. The solution is to select a resistor with a low TCR, or to pause the circuit during the measurement interval and remeasure after the resistor has cooled down.
Voltmeter offset error: The voltmeter itself has an offset voltage (e.g., 0.1mV) at low range. The solution is to first short-circuit the voltmeter, record the offset voltage, and then subtract the voltage from the measured value (e.g., if the measured value is 0.18V, the offset voltage is 0.0001V, and the actual voltage is 0.1799V).
2. Safe Operating Red Lines
Do not exceed the rated range: The maximum current and power of the shunt resistor must not exceed the rated value. If the circuit may experience short-term inrush current (such as a motor starting current that is 5 times the rated current), a shunt resistor that can withstand the inrush current (such as a product with a pulse power rating) must be selected.
Avoid hot-plugging: Do not disconnect or connect the shunt resistor during measurement, otherwise it will cause an open circuit, generate a high-voltage arc, burn out the resistor or cause injury;
High temperature protection: The shunt resistor will heat up when measuring high current (e.g., the surface temperature of a 10W resistor can reach above 80℃ when it is working). Avoid touching it directly with your hands, or set up a heat insulation board around it to prevent burns or ignition of nearby flammable materials.
In summary, shunt resistors are one of the optimal choices for high-precision measurement of large currents in DC and low-frequency AC circuits. Mastering their selection and usage methods can provide reliable data support for circuit testing and maintenance.
