Low resistance sampling resistors (usually referring to milliohms or even lower) have very strict requirements for materials, requiring low resistivity, low temperature coefficient, good long-term stability, high power density tolerance, and good machinability. Here are several commonly used core materials and their characteristics:
Manganese copper alloy
Composition: Based on copper, mainly adding elements such as manganese and nickel (such as CuMn12Ni, CuMn6Sn, etc.).
Advantages:
Extremely low temperature coefficient of resistance: This is the core advantage of manganese copper. Through precise alloy proportioning and heat treatment processes, the TCR can be controlled within ± 10 ppm/° C or even ± 5 ppm/° C, which is crucial for accurate measurement.
Good long-term stability: The resistance value changes little over time.
Moderate resistivity: approximately 0.45 0.49 µ Ω· m, meeting low resistance requirements.
Lower copper thermoelectric potential: The thermoelectric potential generated when connected to copper wires is smaller, reducing measurement errors.
Application: It is the most classic and widely used high-precision low resistance sampling resistor material, especially in instruments, testing equipment, and precision power supplies that require high precision and stability.
Nickel chromium alloy
Composition: Based on nickel, with the addition of chromium (such as NiCr60/15, NiCr30/20, NiCr20/80, etc.).
Advantages:
Higher resistivity: typically around 1.0 1.1 µ Ω· m, significantly higher than manganese copper. This means that shorter and thicker resistors can be used at the same resistance value, which helps to improve power density, reduce parasitic inductance, and is particularly suitable for extremely low resistance values (such as below milliohms) and/or high current applications.
Good high temperature resistance and oxidation resistance: the working temperature can be higher.
Challenge:
The temperature coefficient is relatively high: TCR is usually in the range of ± 50 ppm/° C to ± 100 ppm/° C. Although it can be improved through alloy optimization (such as Evanohm, TCR can be as low as ± 20 ppm/° C), it is generally not as good as high-quality manganese copper.
Copper has a high thermoelectric potential: special attention should be paid to the temperature difference at the connection point.
Application: Widely used in scenarios that require high power density, such as current detection in high-power switching power supplies (SMPS), motor drivers, inverters, and battery management systems (BMS).
Iron chromium aluminum alloy
Composition: Based on iron, with the addition of chromium and aluminum (such as FeCrAl).
Advantages:
High resistivity: about 1.3 1.5 µ Ω· m, similar to nickel chromium alloy, also suitable for manufacturing extremely low resistance resistors.
Excellent high temperature resistance and oxidation resistance: The highest working temperature is usually higher than that of nickel chromium alloys.
Relatively low cost.
Challenge:
High temperature coefficient and non-linearity: TCR is usually within ± 100 ppm/° C or higher, and the curve may be non-linear, which is unfavorable for high-precision measurements.
Relatively poor processability: may be harder and more brittle than manganese copper and nickel chromium.
Copper has a high thermoelectric potential.
Application: Mainly used for applications that require relatively low precision but need to withstand high temperatures, high power, and harsh environments, such as current detection parts for certain industrial heating equipment and high-power load resistors.
Copper alloys (constantan, brass, etc.)
Kangtong:
Composition: Copper nickel alloy (usually Cu55Ni45).
Characteristic: The resistivity is about 0.49 µ Ω· m, close to manganese copper. The TCR is very low (close to the level of manganese copper), and the thermoelectric potential of copper is extremely low. Sometimes used as a substitute or supplement for manganese copper. The work hardening tendency may be more pronounced than that of manganese copper.
Brass:
Composition: Copper zinc alloy.
Characteristic: Low resistivity (about 0.06 0.08 µ Ω· m), which means that a longer length or smaller cross-sectional area is required to achieve the same resistance value. The TCR is relatively high (about+2000 ppm/° C). The main advantages are low cost, easy processing, and good mechanical strength.
Application: Used in low-cost, low precision, and high current situations, such as the sampling section on fuses and certain high current splitters. Usually requires the use of temperature compensation circuits.
Pure copper
Characteristics: The lowest resistivity (~0.017 µ Ω· m) and high TCR (+3930 ppm/° C).
Application: It is not suitable for directly making precision sampling resistors. But commonly used for:
High current parallel path: In a four terminal resistor, thick copper blocks/strips are commonly used as low resistance paths between high current terminals, and the sampling point (voltage terminal) is connected to a precision resistor material (such as manganese copper sheet) in the middle.
Extremely low-cost splitter: In situations where precision requirements are extremely low, a small section of copper wire or copper foil may be used directly, but the temperature drift is very large.
Select logical summary:
Highest accuracy and stability: Manganese copper alloy is the preferred choice, followed by optimized nickel chromium alloys (such as Evanohm) or constantan.
Extremely low resistance (<1m Ω) and high power density: Nickel chromium alloys and iron chromium aluminum alloys have structural advantages (shorter and thicker resistors) due to their high electrical resistivity.
Cost sensitive and low precision requirements/high current: brass or specially designed pure copper structures may be options.
Outstanding demand for high temperature resistance: Nickel chromium alloy and iron chromium aluminum alloy are superior.
Important Notice:
Structural design is equally important: the performance of low resistance sampling resistors depends not only on the material, but also on their structural design (such as four terminal Kelvin connection, low thermal resistance heat dissipation design, and reduced parasitic inductance), which is crucial for accuracy, power capability, and high-frequency performance.
Surface treatment and connection: The connection process between resistance materials and copper terminals (welding, crimping, etc.) has a significant impact on stability, thermoelectric potential, and current carrying capacity.
Temperature drift compensation: For materials with high TCR (such as brass, pure copper), sometimes external components such as NTC thermistors are used for temperature compensation, but this increases complexity.
Therefore, selecting low resistance sampling resistor materials is a process that comprehensively considers accuracy, resistance range, current size, power density, temperature drift requirements, working environment, cost, and overall structural design. Manganese copper and nickel chromium alloys are the mainstream choices in the current market.