Low Ohmic Current Sensing Resistors are high-precision resistor devices specifically designed for current detection and signal sampling. Their core function is to introduce a controllable small voltage drop in the circuit through a very small resistance value (usually in the milliohm range, m Ω), thereby converting the current signal into a voltage signal for precise measurement or feedback control by backend circuits such as amplifiers and ADCs. The following is a detailed analysis of its technical principles, core features, and typical applications:
1、 Basic Definition and Working Principle
Basic concepts
Resistance range: milliohms (m Ω) to several ohms (Ω), typical values such as 1m Ω~100m Ω, and some scenarios can reach several ohms.
Design objective: To convert the current flowing through a resistor into a voltage signal using Ohm's law (V=I × R), achieving high-precision current monitoring or closed-loop control.
Workflow
Current flows through the sampling resistor: a small voltage drop is generated across the resistor (for example, when 100A current passes through a 1m Ω resistor, the voltage drop is 0.1V).
Signal extraction: The backend circuit (such as differential amplifier, analog-to-digital converter ADC) collects the voltage and calculates the actual current value.
Key requirements: Accurate resistance, small temperature drift, anti-interference, ensuring minimal sampling error.
2、 Core Features and Technical Indicators
Extremely low resistance and high precision
Resistance accuracy: usually ± 0.1%~± 1%, high-precision models can reach ± 0.05% (such as Wangquan RN series).
Low temperature drift coefficient (TCR): required to be<± 50ppm/℃, ensuring that the effect of temperature changes on resistance can be ignored (for example, when the temperature rises by 50 ℃, the resistance change is only ± 0.25%).
Low Thermal Electromotive Force (EMF) and Low Contact Resistance
Terminal material: Copper alloy or gold plating process is used to reduce thermoelectric effects (such as the potential introduction of μ V level errors in EMF at the copper tin interface).
Kelvin connection: Separate the current path and voltage detection path to eliminate the interference of lead resistance on measurement.
High power carrying capacity and heat dissipation capability
Power formula: P=I ² × R, which requires structural design (such as metal substrate, wide electrode) to improve heat dissipation efficiency.
Typical power: Surface mount resistors range from 1W to 5W, and metal shell resistors can reach over 10W (as supported by Wangquan automotive standard resistors) 3W@1m Ω).
Environmental adaptability and reliability
Sulfur resistant/moisture resistant: Special packaging (such as ceramic substrate+epoxy coating), suitable for harsh environments such as automobiles and industries.
Vehicle certification: Passed AECQ200 standard, meeting mechanical reliability requirements such as vibration and impact.
3、 Typical application scenarios
New energy vehicles
Battery Management System (BMS): Real time monitoring of battery charging and discharging current, prevention of overcurrent, and estimation of State of Charge (SOC).
Motor drive: detect motor phase current and optimize control algorithms (such as FOC vector control).
On board charger (OBC): Charging current sampling to ensure charging safety and efficiency.
Industrial power and energy systems
DCDC converter: Output current feedback to achieve closed-loop voltage regulation and overcurrent protection.
Photovoltaic inverter: detects the input current on the DC side to improve the maximum power point tracking (MPPT) efficiency.
Consumer Electronics and Smart Devices
Fast charging adapter: current detection in USB PD protocol, triggering overcurrent protection.
Smart home appliances: current monitoring and fault diagnosis for air conditioning compressors and servo motors.
4、 Selection and Design Points
Resistance selection
Sensitivity and power balance: The larger the resistance value, the more significant the voltage drop (high sensitivity), but the higher the power consumption (I ² R).
Example: When detecting 100A current, a 1m Ω resistor drops 0.1V (with a power consumption of 10W), and a 0.5m Ω resistor drops 0.05V (with a power consumption of 5W), but a higher precision amplifier is required.
Packaging and heat dissipation design
Surface mount packaging (such as 2512, 3720): suitable for space limited scenarios, requiring large-area copper plating on the PCB for heat dissipation.
Metal shell resistor (such as TO220): directly installed with heat sink, suitable for high-power scenarios.
PCB layout: Keep away from heat sensitive components to reduce the impact of temperature rise on sampling accuracy.
Signal chain optimization
Differential amplifier: requires a high common mode rejection ratio (CMRR>100dB) to suppress common mode noise (such as the dedicated sampling IC for the Wangquan LRE series).
Filter circuit: Add RC low-pass filter to suppress high-frequency switching noise (such as PWM interference in motor drive).
5、 Example of RALEC Vehicle Gauge Sampling Resistance
RN series
Resistance range: 2m Ω~19m Ω, accuracy ± 0.5%, TCR ± 50ppm/℃, power 1W~3W (surface mount packaging).
Application: BMS current detection for new energy vehicles, onboard charging module.
LRE series (alloy resistors)
Features: 0805 small package, as low as 2m Ω, four wire connection, sulfur resistant design.
Scenario: Motor drive, DCDC converter current sampling.
6、 Common Problems and Solutions
Excessive sampling error
Check contact resistance: Use a four wire connection to optimize PCB routing symmetry.
Reduce the impact of temperature drift: choose a lower TCR resistance (such as ± 15ppm/℃).
Resistance overheating and damage
Power margin design: Actual power ≤ 70% of nominal value, and improved heat dissipation (such as adding heat dissipation holes and thermal conductive adhesive).
Noise interference
Shielding and filtering: The sampling line adopts differential routing and parallel 0.1 μ F ceramic capacitors to filter out high-frequency noise.
Low resistance sampling resistor is a "precision current sensor" in power electronic systems, which has become the core component of current detection due to its milliohm resistance, high precision, and anti-interference characteristics. Reasonable selection and design can significantly improve system efficiency, safety, and reliability in fields such as automotive electronics, new energy, and industrial control. When selecting, it is necessary to comprehensively weigh resistance, accuracy, temperature drift, power, and environmental adaptability, and optimize overall performance in combination with signal chain design.