Okay, let's explore in detail the impact of different materials on the key parameters of low resistance sampling resistors. The choice of material is essentially a trade-off between resistance range, rated power, accuracy/temperature drift, inductance, cost, and applicable scenarios.
Core requirements: Low resistance (usually in the milliohm range, m Ω), used for current sampling, requiring accuracy and stability.
The following are several common materials and their typical parameter ranges that can be achieved:
Metal foil resistors
Material: Precision alloy foil (such as nickel chromium alloy modified type) is bonded to a ceramic substrate and formed by precision photolithography etching.
Resistance range: very wide, particularly good at extremely low resistance values, with a typical range of 0.1 m Ω to 10 m Ω (or even lower, such as 0.05 m Ω), and can also achieve higher resistance values. It is the preferred choice for manufacturing ultra-low resistance precision resistors.
Rated power: medium to high. Under standard packaging dimensions (such as 2512), the power can reach 1W 3W or even higher. Its structure is conducive to heat dissipation, and some models can obtain higher power by connecting an external heat sink.
Tolerance: Extremely high. Typical values are ± 0.1% and ± 0.5%. It is one of the most accurate types of sampling resistors.
Temperature coefficient (TCR): extremely low. It is the best sampling resistor, with typical values of ± 5 ppm/° C, ± 10 ppm/° C, and ± 15 ppm/° C. The temperature drift is extremely small, ensuring that the sampling accuracy is not affected by temperature changes.
Parasitic inductance (ESL): extremely low. The planar structure etched by photolithography gives it a natural low inductance characteristic (<1 nH to several nH), which is very suitable for current sampling in high-frequency switching circuits such as switching power supplies and motor drives.
Long term stability: very good.
Cost: High. The manufacturing process is complex and the material cost is high.
Summary: Pursuing the highest precision, lowest temperature drift, lowest inductance, stable and reliable low resistance sampling is the first choice, especially suitable for precision measurement, high-end power supply, servo drive, etc. The disadvantage is that it has the highest cost.
Manganin/Isotan/Evanohm Alloy Resistors
Material: Copper manganese nickel based precision resistance alloy (such as Manganin, Isotan 75, Evanohm). Usually made into sheets, rods, or used as thick/thin film materials.
Resistance range: Wide, especially suitable for low resistance values. Typical range is 0.5 m Ω to 100 m Ω (or even higher). It is the mainstream choice for low resistance sampling resistors.
Rated power: medium to high. It depends on the specific form (patch, plug-in, power resistor). Standard surface mount packaging (such as 2512) typically has a power of 0.5W 2W. Large sized plugs or power resistors with heat sinks can reach 5W, 10W, or even tens of watts.
Tolerance: High. Typical values are ± 0.5% and ± 1%. It can achieve higher accuracy (± 0.1%), but the cost increases.
Temperature coefficient (TCR): low. Manganese copper alloy itself has an extremely low TCR. Typical values are ± 20 ppm/° C and ± 50 ppm/° C. It is slightly inferior to metal foil, but far superior to ordinary thick/thin films.
Parasitic inductance (ESL): low to medium. It depends on the structure and packaging. Surface mount inductors are usually designed with low inductance, while plug-in wound inductors are relatively high. Low sensitivity design versions (such as four terminal Kelvin connections) are the mainstream choice.
Long term stability: good.
Cost: Medium to high. The cost of alloy materials is relatively high, and the manufacturing process (especially precision alloy sheet resistors) is more complex than thick films.
Summary: A very good balance has been achieved between accuracy, temperature drift, power, cost, and inductance. It is the main force in industrial grade, automotive grade, and high-end applications in consumer electronics, such as power supplies, inverters, battery management, and motor control. High cost-effectiveness and reliable performance.
Metal Plate/Thick Film Resistors
Material:
Metal plate: usually refers to copper alloy (such as brass) resistor components that are stamped and formed, sometimes with nickel plating or alloy on the surface to improve performance.
Thick film: printed resistor paste (containing metal oxides or alloy powders such as silver, palladium, ruthenium, etc.) on a ceramic substrate and sintered. Copper containing pastes are commonly used for low resistance.
Resistance range:
Metal plate: relatively low, typically 1 m Ω to 20 m Ω.
Thick film: wider, but limited at the low resistance end, typically ranging from 10 m Ω to 100 m Ω (lower resistance accuracy and temperature drift will significantly deteriorate).
Rated power:
Metal plate: usually higher (as a power resistor element), several watts to tens of watts, depending on heat dissipation design.
Thick film: Under standard surface mount packaging, it is lower, such as about 0.5W 1W in 2512 packaging. Power type thick film resistors (larger size or with heat dissipation) can be higher.
Accuracy (tolerance):
Metal plate: relatively low, typically ± 1%, ± 5%.
Thick film: medium low, typical ± 1%, ± 5%. ± 0.5% can also be achieved, but it is difficult and has a large temperature drift at low resistance values.
Temperature coefficient (TCR):
Metal plate: relatively high, typically ranging from ± 100 ppm/° C to ± 300 ppm/° C or higher. This is its main weakness.
Thick film: relatively high, especially at low resistance values. Typical range: ± 100 ppm/° C to ± 300 ppm/° C or even worse.
Parasitic inductance (ESL):
Metal plate: structurally determined, usually lower (if it is a flat design).
Thick film: One of its main advantages is that it can achieve very low (flat structure design).
Long term stability: Thick film is better, metal plate is average.
Cost: Low (thick film) to medium (metal plate). Thick film is the lowest cost manufacturing technology for surface mount resistors. The cost of metal plates depends on processing and materials.
Summary:
Metal plate: Its advantages lie in high power carrying capacity, low cost, and structural strength. It is commonly used in scenarios that require high current sampling and do not have strict requirements for accuracy and temperature drift, such as some household appliances and simple power supplies. The disadvantages are large temperature drift and low accuracy.
Thick film: Its advantages lie in extremely low cost, mature surface mount technology, good manufacturability, and low inductance design. It is the most common type of low resistance sampling resistor (especially above 10m Ω) in consumer electronics, ordinary power supplies, and low-cost applications. The main disadvantages are large temperature drift and relatively low accuracy at low resistance values.
Copper Alloy Stamps
Material: Stamped directly from copper alloy sheets (such as brass, phosphor bronze, copper manganese alloy).
Resistance range: very low, typically 0.1 m Ω to 5 m Ω.
Rated power: very high. Completely determined by material volume and heat dissipation, it is easy to achieve tens of watts to hundreds of watts or even kilowatts. Commonly used in the main circuit of battery packs and high-power motor controllers.
Tolerance: Low. Typical ± 1%, ± 3%, ± 5%. Greatly affected by material uniformity and processing accuracy.
Temperature coefficient (TCR): high. The TCR of copper itself is very high (about+3900 ppm/° C). Although alloys can improve, the typical TCR is still in the range of ± 100 ppm/° C to ± 500 ppm/° C, which is its biggest disadvantage. Temperature compensation is required.
Parasitic inductance (ESL): low. The structure is simple, usually flat or U-shaped, and the inductance can be very low (especially when measuring with four terminals).
Long term stability: depends on the alloy and load, generally acceptable.
Cost: Low (relative to its power capacity). Low material cost and relatively simple processing.
Summary: Designed specifically for ultra-high current sampling. The core advantages are extremely low resistance value, extremely high power carrying capacity, very low cost, and low inductance. The disadvantage is that the temperature drift is very large and the accuracy is low. It is necessary to cooperate with a temperature compensation circuit to obtain usable accuracy. Widely used for main current detection in electric vehicles, energy storage systems, and industrial high-power equipment.
Summary comparison table:
|Material | Typical Resistance Range (m Ω) | Typical Power (2512 Package Reference) | Typical Accuracy | Typical TCR (ppm/° C) | Parasitic Inductance | Main Advantages | Main Disadvantages | Typical Application Scenarios | Cost|
|Metal foil | 0.05 10 | 1W 3W+| ± 0.1% | ± 5 to ± 15 | Extremely low | Highest accuracy, lowest temperature drift, lowest inductance | Highest cost | Precision measurement, high-end power supply, servo, medical | High|
|Manganese copper alloy | 0.5 100 | 0.5W 2W+| ± 0.5% | ± 20 to ± 50 | Low | Optimal balance of precision/temperature drift/power/cost | More expensive than thick film | Industrial power supply, inverter, motor control, BMS | Medium to high|
|Thick film | 10 100+| 0.5W 1W | ± 1% | ± 100 to ± 300+| Low | Lowest cost, low inductance, easy production | Temperature drift/poor accuracy under low resistance | Consumer electronics, ordinary power supply, low-cost equipment | Low|
|Metal plate | 1 20 | High (dependent on design) | ± 1% ± 5% | ± 100 to ± 300+| Low | High power, low cost, strong structure | Large temperature drift, low precision | Home appliances, simple high current power supply | Low to medium|
|Copper alloy stamping | 0.1 5 | Extremely high (tens to hundreds of W) | ± 1% ± 5% | ± 100 to ± 500+| Low | Extremely low resistance, extremely high power, lowest cost | Extremely high temperature drift, low precision to compensate | Electric vehicles, energy storage, industrial high-power main circuits | Low|
Select key points:
Accuracy and temperature drift requirements: The highest requirement is to choose metal foil; High requirements and balanced selection of manganese copper; Choose thick film/metal plate/copper alloy with low requirements (the latter needs to be compensated).
Power requirement: Select all types for low power; Medium power options include metal foil, manganese copper, and power thick film; High power selection of manganese copper (large size), metal plates, and copper alloy stamping.
Resistance requirement: For ultra-low resistance (<0.5m Ω), metal foil or copper alloy stamping should be selected; Select metal foil, manganese copper, metal plate, and copper alloy for low resistance (0.5m Ω 10m Ω); Slightly higher resistance (>10m Ω) is available for all types, and thick film has high cost-effectiveness.
Frequency/inductance requirements: Low inductance design metal foil, manganese copper, thick film, copper alloy (structural design) are preferred for high-frequency switch circuits.
Cost limitations: cost sensitive options include thick film, metal plate, and copper alloy stamping; Prioritize metal foil and manganese copper for performance.
Application scenarios: Match the most suitable materials according to the comprehensive needs of specific applications (consumer electronics, industrial equipment, automotive electronics, precision instruments, high-power systems).
Understanding the relationship between these material properties and parameters is the foundation for designing reliable and accurate current sampling circuits.