Application And Selection Guide Of Low-ohm Resistors In Current Detection

Low Ohm resistors are precision resistors with resistance values ranging from milliohms to tens of milliohms, and are commonly used as current detection (shunt) components. It achieves precise sampling of current by generating a tiny voltage drop in the current loop and is widely used in scenarios such as power management, BMS, motor drive, and DC-DC converters.

1.working principle

（1）The basis of Ohm's Law

• Voltage drop V = I × R.
• When current I flows through a low-ohm resistor R, the voltage drop V generated across its terminals can be sampled by an ADC, operational amplifier or a dedicated detection chip and converted into a current value.

（2）Signal amplification and processing

Due to the small size of V (usually within tens of millivolts), the downstream circuit needs to be combined with a high common-mode rejection operational amplifier or a high-resolution ADC for amplification and filtering to ensure measurement accuracy.

2.Key selection parameters

（1）Resistance value design

• The small voltage drop is balanced with the signal strength:The smaller the resistance value is, the lower the energy loss and system voltage drop will be, but the signal amplitude will also decrease accordingly.
• Typical goal:The voltage drop under the maximum current is controlled within 20 mV to 100 mV.
• Calculation method:R = V_target/I_max.

（2） Power and Thermal Management

• Power consumption calculation：P = I_max² × R.
• Safety margin：When selecting the model, leave a margin of 2 to 3 times to prevent sudden overcurrent.
• Heat dissipation strategy：Select large packages (such as 2512, 3920) or metal case resistors;Copper sheets and vias are configured on the PCB to increase the heat dissipation area.

（3）Accuracy and temperature drift

• Tolerance：±1%, ±0.5% or higher accuracy grades are recommended.
• Temperature coefficient (TCR)：TCR ≤ 50 ppm/℃ can ensure stable measurement over a wide temperature range. High-precision optional ≤ 25 ppm/℃.

（4） Packaging and Layout

• SMD vs Through hole：SMD (1206, 2512) is convenient for automated production;Through-holes/studs are suitable for ultra-high current and high-power applications.
• Four-terminal Kelvin structure：Separate the current terminal from the measurement terminal to eliminate the resistance errors of the leads and pads.
• PCB Routing Principles：The wiring of the current loop should be wide and short；The measurement signal routing should avoid high-speed or high-current circuits；Place the detection amplifier or ADC near the shunt resistor to reduce noise interference.

3.Typical application scenarios

• Battery Management System (BMS):Sample the discharge current and charging current in the lithium battery charge and discharge circuit to achieve SOC/SOH estimation and overcurrent protection.
• Switching power supply and DC-DC converter:The sampling load current is used to assist in voltage stabilization, soft start and overload protection functions.
• Motor drive control:It is used to detect the current of motor windings or power bridge arms, adjust the PWM duty cycle in real time and achieve locked rotor protection.
• Server and data center power supply monitoring:Centralized current monitoring is carried out on the rack power bus, supporting energy consumption statistics and fault alarm.

4.Selection Example

5.conclusion

Low-ohm resistors are indispensable core components for current detection. By rationally selecting resistance values, power, accuracy, as well as appropriate packaging and PCB layout, high-precision and low-loss real-time current monitoring can be achieved in various power electronics scenarios. Mastering the above key points will help you quickly implement a reliable current detection solution.