What is a low resistance sampling resistor?

Low resistance current detection resistor is an electronic component designed for precise measurement of current in circuits. Its core function is to generate a small voltage drop through low resistance (usually in the milliohm range, m Ω), thereby achieving high-precision monitoring of current. The following is a comprehensive analysis of its technical details and application scenarios:
1、 Core principles and structure
Working principle
Based on Ohm's law V=I × R, when a current I flows through a resistor, a voltage drop V occurs at both ends. By measuring V, the current value can be inferred. Low resistance design (such as 1m Ω) ensures:
Low power consumption: P=I2 × R, the lower the resistance, the lower the power consumption (for example, at 10A current, a 1m Ω resistor consumes only 0.1W).
High precision measurement: Moderate voltage drop (such as 10A × 1m Ω=10mV), convenient for ADC sampling and strong anti-interference ability.
Structural characteristics
Four terminal (Kelvin) connection: independent current path and voltage sampling path, eliminating pin resistance error (see figure below).
Low temperature coefficient (TCR): usually ≤ ± 50ppm/℃, reducing the impact of temperature drift on measurement accuracy.
High power bearing capacity: using metal alloy materials (such as manganese copper, constantan) or special processes to support transient high current impact.
2、 Key technical parameters
Typical parameter values/range description
Resistance range of 0.5m Ω~10m Ω (commonly used) covers most current detection needs
High precision models (such as 0.1%) with accuracy levels of 0.5%, 1%, and 5% are used for precision instruments
Temperature coefficient (TCR) ± 20ppm/℃~± 100ppm/℃ Low temperature float design ensures long-term stability
Rated power of 1W~5W (depending on the packaging) needs to be calculated based on the maximum current: P=Imax2 × R
Package sizes 2512, 2010, 1206, and other miniaturized packages (such as 0603) are suitable for compact designs
3、 Typical application scenarios
Battery Management System (BMS)
Monitor the charging and discharging current of the battery pack to prevent overcurrent/overcharging.
Example: In electric vehicles, sampling resistors are used to detect the motor drive current, achieving precise control.
Power module and DC-DC converter
Closed loop current control improves conversion efficiency.
Example: In the server power supply, a low resistance resistor provides real-time feedback on the output current.
Motor drive and inverter
Protect the motor from damage caused by current overload.
Example: In industrial servo motors, sampling resistors are used in conjunction with MCU to achieve overcurrent protection.
Consumer Electronics
Fast charging adapter, current monitoring in laptops.
4、 Key selection points
Resistance selection
Determine R=VADC_max/Imax based on the maximum current Imax and ADC range.
Example: If the maximum input of the ADC is 50mV and Imax=20A, then select R=50mV/20A=2.5m Ω.
Power verification
Calculate actual power consumption: P=IRMS2 × R, which should be lower than the rated power of the resistor (usually reduced by 50% for use).
Packaging and heat dissipation
Priority should be given to large-sized packages (such as 2512) or metal substrate resistors in high current scenarios to enhance heat dissipation capabilities.
Brand and Series Recommendations
RALEC: RCS series (anti surge film resistor, supports low resistance customization).
Vishay: WSLs series (high-power metal strip resistors with TCR as low as ± 20ppm/℃).
ROHM: MCR series (four terminal alloy resistor with an accuracy of 0.1%).
5、 Design considerations
Four terminal connection: Kelvin connection must be used to avoid errors caused by pin resistance.
Thermal design: Resistors will generate heat under high current, and PCB copper plating or forced air cooling should be considered for heat dissipation.
EMC compatibility: In noise sensitive environments, filtering circuits need to be added to the sampling line.
6、 Example: Sampling resistor in electric vehicle BMS
Requirement: Monitor the discharge current of the battery pack (maximum 200A) with an accuracy requirement of 1%.
Selection:
Resistance: R=50mV/200A=0.25m Ω (select nominal value of 0.25m Ω).
Power: P=2002 × 0.00025=10W, choose a 2512 packaged resistor with a rated power of 15W.
Accuracy: Select a 0.5% accuracy model to ensure that measurement errors are within an acceptable range.
Summary: Low resistance current detection resistors are the core components of current monitoring, which can achieve efficient and reliable current monitoring through precise design. They are widely used in fields such as new energy, industrial control, and consumer electronics. When selecting, it is necessary to comprehensively consider resistance, power, accuracy, and packaging, and pay attention to the four terminal connection and heat dissipation design.