What is the role of low resistance resistors in circuits?

Low resistance resistors (usually in the milliohm range, such as 10m Ω, 50m Ω, 100m Ω, etc.) play several key roles in circuits, and their core value lies in their ability to achieve specific circuit functions while introducing extremely small external voltage drops and power consumption. The following are its main functions:

1. Current detection and measurement:
Principle: This is the core application of low resistance resistors. According to Ohm's law (V=IR), when a current (I) flows through a resistor (R), a voltage drop (V) is generated across the resistor. If the resistance value (R) is very small, then the voltage drop (V) will also be very small (in millivolts), thereby minimizing interference and power loss to the main power path to the greatest extent possible (P=I ² R).
Application: Connect low resistance resistors (often referred to as "current sensing resistors" or "splitters") in series along the path that requires current monitoring (such as power input, motor drive, battery charging and discharging circuits). By accurately measuring the small voltage drop across the resistor and using a high-precision differential amplifier (current detection amplifier) for amplification and processing, the current value flowing through can be accurately calculated.
Requirement: This type of resistor requires extremely low resistance tolerance, extremely low temperature coefficient, and sufficient power rating to ensure measurement accuracy and long-term stability.

2. Overcurrent protection/fuse function:
Principle: When an abnormally high current occurs in a circuit, even a low resistance resistor, according to Joule's law (P=I ² R), will consume a sharp increase in power, causing the temperature of the resistor to rise rapidly.
Application: Select low resistance resistors (sometimes referred to as "fuse resistors" or "safety resistors") made of specific materials and processes. When their power exceeds the rated value to a certain extent, the resistors will melt or burn out, physically disconnecting the circuit and providing protection similar to a fuse. Its advantages are that the resistance value is more precise than traditional fuses, the response speed may be faster (depending on the design), and the loss is extremely low under normal operating current.

3. Current equalization:
Principle: In scenarios where multiple devices or power modules need to work in parallel (such as multiple power MOSFETs in parallel, multiple power modules in parallel redundancy, multiple LED beads in parallel), direct parallel connection may result in severe uneven current distribution due to small differences in device parameters, causing some devices to overload.
Application: Connect a low resistance resistor (called a "current sharing resistor") in series in each parallel branch. This resistor introduces a small, controllable 'negative feedback' in the branch. If the current of a branch attempts to increase, the voltage drop across the resistance of that branch will also increase accordingly, thereby relatively suppressing the growth of the current in that branch and promoting a more uniform distribution of the current among the parallel branches. Its resistance needs to be small enough to reduce losses, but also large enough to achieve effective current sharing.

4. Bypass/damping/impedance matching in high-frequency circuits:
Principle: In high-frequency (RF, high-speed digital) circuits, low resistance resistors are sometimes used for:
Reduce Q value/broaden bandwidth: Connecting a small resistor in series with an LC resonant circuit can lower the quality factor (Q value) of the circuit, sacrificing some selectivity to obtain a wider frequency band.
Suppressing parasitic oscillations/providing damping: Connecting small resistors in series or parallel at amplifier outputs, long transmission line terminations, etc. can absorb reflected energy, increase damping, and prevent unwanted oscillations or ringing in the circuit.
Impedance matching: In situations where impedance needs to be fine tuned to achieve minimum reflection, small resistors can be used as part of the matching network.
Application: In these applications, the resistance value of the resistor is usually accurately calculated, and its parasitic inductance (ESL) and parasitic capacitance (ESC) also need to be very small to avoid introducing additional phase shifts or effects at high frequencies.

Summarize key points:

Core advantage: The core value of low resistance resistors lies in their "minimally invasive" nature. It can perform important circuit functions such as current measurement, protection, current sharing, and high-frequency control, while minimizing the additional voltage drop and power loss introduced by the main circuit.
High precision and stability requirements: Especially in current detection applications, there are high demands on the accuracy, temperature stability (TCR), long-term stability, and power processing capability of the resistor itself.
Not just 'wires': although the resistance is very low and close to wires, it plays an active role in the circuit, and its precise resistance is the key to achieving the above functions, which cannot be replaced by ordinary wires.

In summary, low resistance resistors are indispensable passive components in modern electronic circuits. They cleverly utilize their extremely small resistance characteristics to achieve key functions such as current sensing, protection, balancing, and high-frequency optimization without affecting the performance of the main circuit. They are important elements in high-performance and high reliability circuit design.