Low resistance sampling resistors play a key role as current "translators" and "safety sentinels" in electronic systems. Its core function is to efficiently and low loss convert difficult to directly measure current signals into easily measurable voltage signals, thereby achieving precise monitoring, control, and protection of current.
The following is a detailed explanation of its core functions and design logic:
Current voltage conversion (core sensing function):
Principle: Based on Ohm's law (V=I × R). When current I flows through a resistor with a known resistance value R, a small voltage drop V is generated across the resistor.
Function: A low resistance sampling resistor is a precision resistor with a very small resistance value R. It is connected in series in the measured current path, linearly and proportionally converting the current 'I' into a tiny voltage signal 'V'.
Meaning: This tiny voltage signal 'V' can be easily read and processed by subsequent circuits such as amplifiers, analog-to-digital converters, or comparators to determine the magnitude of the current flowing. This is the foundation of all subsequent functions.
Realize high efficiency and low power consumption:
The key to low resistance: the power loss of the resistor itself is P=I ² × R. In applications that carry high currents (such as power conversion, motor drive, battery charging and discharging), even if the resistance is small, the I ² term will still be large. If the resistance of the sampling resistor is too high, its own power consumption will be very large, resulting in:
Severe fever: wasting energy, reducing system efficiency, and even requiring a large heat dissipation device.
Temperature drift: Heating causes changes in resistance values, affecting measurement accuracy.
System efficiency decreases: precious energy is wasted on the sampling resistor.
Function: By using extremely low resistance values (usually in the milliohm range, such as 0.5m Ω, 1m Ω, 2m Ω, 5m Ω, 10m Ω, etc.), the power loss (P_loss) of the resistor itself when carrying high currents can be minimized. This is a direct manifestation of its low power consumption and high efficiency characteristics, which are crucial for energy-saving systems.
Accurate current measurement and monitoring:
Foundation: Built on the function of current voltage conversion.
Requirement: The low resistance sampling resistor itself needs to have high precision (low tolerance, such as 1%, 0.5%, 0.1%) and low temperature drift coefficient (TCR, such as ± 50ppm/℃, ± 25ppm/℃). In this way, within a wide temperature range, the resistance value changes very little, ensuring that the converted voltage 'V' can truly reflect the magnitude of the current 'I'.
Application: Used in situations where precise knowledge of current magnitude is required, such as:
Measure the charging and discharging current of the battery to calculate the remaining capacity.
Measure the output current of the power supply for load adjustment or efficiency evaluation.
Measure the phase current of the motor to achieve precise vector control.
Measure the LED driving current to ensure constant brightness.
Overcurrent and short circuit protection (safety guard):
Principle: By real-time monitoring of the voltage drop 'V' across the sampling resistor, it is possible to immediately determine whether the current 'I' flowing through exceeds the preset safety threshold.
Function:
Quick response: When overcurrent or short circuit (rapid increase in current) is detected, the voltage signal provided by the sampling resistor will quickly rise. This signal can be directly input to the comparator or protection chip, triggering the protection circuit within microseconds (such as turning off the switch tube, melting the fuse).
Core component: It is the front-end sensor that builds an overcurrent protection loop, providing critical fault detection capabilities for the system and preventing expensive power devices (such as MOSFETs, IGBTs) or batteries from burning out due to overcurrent.
Applications: Core protection mechanisms for switch mode power supplies, motor drivers, battery management systems, power inverters, etc.
Current closed-loop control (source of stability):
Principle: In systems that require precise control of current (such as constant current sources, motor drivers, precision power supplies), the current feedback voltage signal 'V' provided by the sampling resistor is the key input of the control loop.
Function: The controller (such as MCU, PWM IC) compares the detected actual current value (V/R) with the set target current value, and adjusts the driving signal of the power device (such as PWM duty cycle) in real time according to the error, so that the actual current accurately tracks the set value.
Requirement: The accuracy, stability, and low inductance characteristics of low resistance sampling resistors have a decisive impact on the stability, response speed, and accuracy of the control loop. Low inductance is particularly important to avoid interference voltage spikes in high-speed switch circuits and ensure the authenticity of feedback signals.
Applications: Motor FOC/SVPWM control, LED constant current drive, precision welding power supply, battery constant current charging, etc.
Key design considerations (complementary to functionality):
Resistance selection: It is necessary to find the optimal balance between measurability and low power consumption. The resistance value is too small, and the generated voltage signal 'V' is too weak, which is easily affected by noise interference and requires a higher gain/precision amplifier; The resistance is too high, resulting in excessive power consumption. The design should be determined based on the maximum current, acceptable voltage drop loss (V_drop=I × R), and the minimum measurable voltage of the subsequent circuit.
Power rating: It must be able to withstand the maximum power consumption generated by the measured current at its resistance value (I ² × R) without overheating or damage, usually requiring sufficient heat dissipation design (PCB copper foil area).
Low inductance design: For high-frequency switching circuits (such as DCDC, motor drive), the parasitic inductance of the sampling resistor itself must be very low to avoid generating additional induced voltage (V=L × di/dt) during rapid current changes, which can interfere with the true resistance voltage drop signal. Metal foil resistors and four terminal Kelvin connections are commonly used solutions.
Temperature coefficient: Low TCR ensures accurate current measurement over a wide temperature range.
Four terminal Kelvin connection: For precision measurement or high current applications, using a four terminal structure (two current terminals, two voltage sensing terminals) can eliminate the influence of lead resistance and contact resistance on measurement accuracy.
The core value of low resistance sampling resistors lies in building a bridge: it allows the system to accurately and real-time convert elusive currents into easily processed voltage signals with minimal energy cost (low power consumption). This makes it an indispensable foundational component for achieving the three major goals of efficient energy utilization, precise current control, and critical circuit protection. It is an unknown but crucial "behind the scenes hero" in modern power electronics, battery management, motor drive, power conversion and other fields.