What is a low resistance current sensing resistor?

Low resistance current sensing resistors are the core components used in electronic circuits for precise measurement of current. They achieve high-precision current monitoring through low resistance (milliohm level, m Ω) design while minimizing power loss. The following analysis will focus on technical principles, core parameters, application scenarios, and key selection points:
1、 Technical principles and structural advantages
Working principle
Based on Ohm's law V=I × R, a small voltage drop V is generated when current I flows through a resistor, and the current value can be inferred by measuring V. Low resistance design (such as 0.5m Ω) ensures:
Ultra low power consumption: P=I2 × R, for example, the power consumption is only 0.05W at 10A current.
High precision measurement: Moderate voltage drop (such as 10A × 0.5m Ω=5mV), convenient for ADC sampling and strong anti-interference ability.
Structural innovation
Four terminal (Kelvin) connection: independent current path and voltage sampling path, eliminating pin resistance error (error can be reduced by more than 90%).
Alloy material: Low temperature drift alloys such as Mn Cu and Cu Ni are used, with a TCR (temperature coefficient) as low as ± 20ppm/℃.
Thin film process: For example, RALEC's RCS series achieves 0.1% ultra-high precision through laser impedance tuning.
2、 Analysis of Core Technical Parameters
Key points for selecting typical parameter values/ranges
Resistance (R) 0.1m Ω~10m Ω (commonly used) calculated based on ADC range R=Vmax/Imax
Accuracy tolerance: 0.1%, 0.5%, 1%. Precision instruments can choose 0.1%, and industrial control can choose 1%
Temperature coefficient (TCR) ± 20ppm/℃~± 100ppm/℃ Low temperature float design (such as ± 20ppm/℃) is suitable for scenarios with high long-term stability requirements
Rated power (P) 0.1W~5W (depending on the packaging) needs to meet P>IRMS2 × R and be reduced by 50% for use
Package sizes 0201, 0402, 0603, 2512. Miniaturization (such as 0201) is suitable for wearable devices, while larger sizes (such as 2512) are suitable for high current scenarios
3、 Deepening of typical application scenarios
Electric vehicle BMS (Battery Management System)
Function: Real time monitoring of battery pack charging and discharging current, prevention of overcurrent/overcharge, and balancing of individual battery voltages.
Selection case:
Resistance: R=50mV/300A ≈ 0.167m Ω (select nominal value of 0.15m Ω)
Power: P=3002 × 0.00015=13.5W, choose a 2512 packaged resistor with a rated power of 20W.
Accuracy: 0.5% accuracy model, ensuring SOC (remaining charge) estimation error<1%.
Server power module
Function: Closed loop current control, improving DC-DC conversion efficiency (up to 98% or more).
Technical points:
Use four terminal resistors to reduce sampling errors.
Paired with differential amplifiers (such as INA181) to achieve high common mode rejection ratio (CMRR>120dB).
Industrial servo motor drive
Function: Overcurrent protection and vector control to improve motor response speed.
Selection focus:
Surge resistance: It needs to pass the IEC 61000-4-5 surge test (such as 4kV/2kA).
Thermal stability: TCR ≤ ± 50ppm/℃ within the temperature range of -40 ℃~+125 ℃.
4、 Advanced Selection Guide
Resistance and power balance
The lower the resistance, the lower the power consumption, but the smaller the voltage drop, requiring a higher precision ADC.
Example: If the ADC range is 100mV and the maximum current is 50A, then R=100mV/50A=2m Ω, power consumption P=502 × 0.002=5W, and a resistor with a rated power of 10W needs to be selected.
Packaging and heat dissipation design
High current scenario: Priority should be given to metal substrate resistors (such as Vishay WSN series), with a thermal resistance as low as 40 ℃/W.
Compact design: 0201 packaging resistor needs to be matched with PCB copper plating (recommended ≥ 10mm ² copper foil) to enhance heat dissipation.
Brand and Series Recommendations
RALEC: RCS series (anti surge film resistor, supports 0.1% precision customization).
Vishay: WSLs series (high-power metal strip resistors with TCR as low as ± 20ppm/℃).
ROHM: MCR series (four terminal alloy resistor, certified by AEC-Q200).
5、 Key points of reliability verification
Temperature cycle test: After 1000 cycles from -55 ℃ to+155 ℃, the resistance change is less than 0.1%.
Sulfurization test: Exposure to H2S gas environment for 96 hours, resistance drift<1% (sulfide resistant model).
Mechanical stress test: After bending the PCB 1000 times, the resistance change is less than 0.05%.
6、 Industry Trends
Material innovation: Nanocrystalline alloy resistors (TCR<± 10ppm/℃) are gradually being commercialized.
Integration: Resistance array (such as 4-channel 0402 package) reduces PCB footprint.
Intelligence: Combining temperature sensors to achieve self calibration function (such as TDK's intelligent sampling resistor).
Low resistance current sensing resistors are the "nerve endings" of current monitoring systems, and their selection requires a comprehensive consideration of resistance, accuracy, power, packaging, and reliability requirements. In high reliability scenarios such as electric vehicles and 5G communication, it is recommended to prioritize products that have passed AEC-Q200 or equivalent certification, and pay attention to the four terminal connection and thermal design details.