Precision level classification
Resistor accuracy is typically classified by its permissible deviation (the maximum range of error between the actual resistance value and the nominal resistance value). Common grades include ±0.1% (0.1 grade), ±0.25% (0.25 grade), ±0.5% (0.5 grade), ±1% (1 grade), and ±2% (2 grade). Precision resistors have even higher accuracy, such as ±0.01% (one ten-thousandth of a percent accuracy). These resistors often use thin-film materials to meet the requirements of precision manufacturing processes.
Precision resistor characteristics
Low temperature coefficient: The temperature coefficient of resistance (TCR) for precision resistors typically ranges from 1 ppm/℃ to 75 ppm/℃; for example, the temperature coefficient of resistance for metal foil can be as low as ±0.3 ppm/℃.
High stability: Aging rate (long-term stability) is usually expressed in ppm/year or %/year. High-quality precision resistors can control their aging rate to a low level.
Materials and Processing: Common materials include three categories: metal film, wire winding, and metal foil. Among them, metal foil resistors offer the best overall performance, with an accuracy of 0.001%-0.01%, making them suitable for high-frequency circuits.
Application scenarios
Precision resistors are widely used in high-precision industrial applications such as medical equipment, testing instruments, aerospace, and automotive electronics. Their packaging forms include cylindrical, chip, and frame structures, and manufacturing processes involve technologies such as vacuum sputtering and photolithography to ensure stability.
In fact, for those who aren't in metrology, such a clear distinction isn't necessary; generally speaking, high precision, high accuracy, and low error are all synonymous. However, the term "precision" can be broken down into three elements:
1. Temperature Coefficient: Temperature change is a major enemy of resistance. The temperature coefficient is usually expressed in ppm/℃, which means that a change in temperature of 1 degree corresponds to a change in resistance of a few parts per million. 100ppm/℃ is 0.01%/℃.
2. Aging: This refers to long-term stability, usually expressed in ppm/year, but sometimes in %/year. No matter how accurate a resistor is at the factory, if it ages significantly, it will change rapidly and lose its high accuracy.
3. Initial adjustment error: This is not very important. As long as the deviation remains unchanged, it is fine and can be corrected during measurement.
For resistors with relatively low precision, we can use a general statement about their precision, without specifying a particular value. For example, a resistor with a precision of 0.1% represents a comprehensive error. In reality, it means that under normal temperature conditions (e.g., 10℃-35℃) for one year, including all possible errors, the resistance should still be within 0.1%.
However, for applications with high requirements, the specific characteristics of the resistor will be tested to ensure proper selection. This article will describe common high-precision resistors based on two indicators: temperature coefficient and aging performance.
Classification of precision resistors
There are three common types of precision resistors: metal film resistors, wire-wound resistors, and block resistors.
1. Metal film resistors are the most common type, but better ones are precision metal film resistors, characterized by a small temperature coefficient and relatively stable resistance. However, because the film is relatively thin, they are more fragile, and problems are prone to occur at the spiral cutting and crimping parts.
2. Wire-wound resistors are also commonly used, and at one time were even the primary resistors in high-precision equipment. There are currently three types of resistance wire materials used:
● Constantan: It is relatively old and heat-resistant, but its temperature coefficient is not very good. It has a higher thermoelectric potential than copper.
● Manganese copper: There is precision manganese copper, which, although hot, has a very small temperature coefficient and a low thermoelectric potential compared to copper. It is a widely used wire-wound resistor material.
● Evanohm: Translated as Evanohm, it is a nickel-chromium-aluminum-copper alloy, also known as nickel-chromium resistance alloy. It has the smallest temperature coefficient, is relatively hard, and has poor weldability.
3. Block resistors, also known as metal foil resistors, are manufactured by foreign companies like Vishay. They involve bonding an alloy resistor layer to a ceramic substrate and then using photolithography without contact lenses. They utilize nickel-chromium resistor alloys and further temperature compensation on the ceramic substrate, resulting in a very small temperature coefficient, often less than 1 ppm/℃. The domestically produced model is RJ711, which has slightly lower performance.
Precision Resistor Grade Selection and Application Examples
When selecting precision resistors, it is necessary to comprehensively consider the circuit requirements, cost budget, and the resistor's accuracy class. For example, in a high-precision measuring device, a high-precision 250Ω precision resistor should be selected to ensure the accuracy of the measurement results. However, in some ordinary electronic projects, to reduce costs, medium-precision or low-precision resistors can be chosen.
Taking audio signal processing circuits as an example, the transmission and processing of audio signals require high precision resistors. In this case, we should choose high-precision 250Ω resistors to ensure stable transmission and processing quality of audio signals.
In conclusion, understanding the classification of 250Ω precision resistor grades is of great importance to electronic engineers. In practical applications, we should select the appropriate resistor grade based on specific needs and budget to ensure circuit stability and performance. Furthermore, with technological advancements, the manufacturing technology of precision resistors will continue to improve, providing us with more high-precision, high-performance resistor products to meet the ever-evolving needs of electronic engineering.
Resistors are fundamental components in electronic circuits, used to control the magnitude and direction of current. Based on their accuracy and stability, resistors can be broadly classified into two categories: ordinary resistors and precision resistors.
I. Basic Definitions of Ordinary Resistors and Precision Resistors
Ordinary resistors are among the most commonly used components in electronic circuits, primarily used to limit current. They are typically made of materials such as carbon film, metal oxides, or conductive polymers, and have a fixed resistance value and allowable tolerance range. Ordinary resistors exhibit good performance in general circuit applications and are relatively inexpensive.
Precision resistors are resistive devices with high accuracy and stability. They are typically made of special materials, such as metal thin films, metal foils, or resistance alloys, which have low temperature coefficients and better linear characteristics. Precision resistors have very small resistance errors, typically reaching 0.1%, 0.01%, or even lower, making them widely used in applications requiring precise control of resistance values.
The main difference between ordinary resistors and precision resistors
1. Different levels of precision
Ordinary resistors have relatively low precision, and their resistance value error is usually relatively large. In general circuit applications, this error is acceptable because the main function of ordinary resistors is to limit current, not to precisely control the resistance value. However, in applications requiring precise control of the resistance value, the precision of ordinary resistors is insufficient.
In contrast, precision resistors offer extremely high accuracy, with resistance errors controllable within a very small range. This gives precision resistors significant advantages in applications such as precision measurement, sensor circuits, medical equipment, and scientific experiments.
2. Different levels of stability
Besides accuracy, ordinary resistors and precision resistors also differ significantly in stability. Ordinary resistors are relatively less stable, and their resistance values are easily affected by environmental conditions such as temperature and humidity. While this variation may be negligible in some applications, it can lead to serious problems in situations where high stability is required.
Precision resistors exhibit excellent stability, with minimal fluctuations in resistance value over time and under varying environmental conditions. This allows them to maintain a consistent resistance value over extended periods, thereby improving circuit reliability and performance.
3. Different manufacturing processes and costs
The manufacturing process of ordinary resistors is relatively simple and the cost is also low. This makes ordinary resistors highly economical when mass-produced. However, due to limitations in the manufacturing process, it is difficult for ordinary resistors to achieve very high precision and stability.
In contrast, the manufacturing process of precision resistors is more complex and refined. To obtain high-precision and highly stable resistance values, precision resistors require special materials, precise processing techniques, and rigorous testing procedures. These factors contribute to the relatively high cost of precision resistors. Nevertheless, the use of precision resistors remains essential in many applications requiring precise control of resistance values.
Precision resistors are resistors that meet stringent standards in terms of resistance value error, thermal stability, and distributed parameters. For resistors with a resistance value greater than 1Ω, if the error compared to the marked resistance value is within ±0.5%, it can be called a precision resistor. If the precision is even higher, reaching 0.01%, or what electronic engineers refer to as one ten-thousandth of a second precision, such resistors are usually made of thin-film material to meet the requirements of precise manufacturing processes. In contrast, ordinary series resistors with a resistance value of 1Ω or higher typically have a precision of ±5% or higher, while the 5% precision resistors most commonly found in electronic products do not fall into the category of precision resistors.
Resistors with a resistance of less than 1Ω typically have a precision within ±1%, and are therefore classified as precision resistors. This is because when the resistance value is relatively small, even a 1% error has a relatively small impact on practical applications. To achieve higher precision, such as within ±0.5%, the requirements for manufacturing processes and technology will be correspondingly increased.
Comprehensive characteristics of precision resistors
Precision resistors not only focus on the accuracy of their resistance value, but also require stringent standards in many aspects such as thermal stability and distributed parameters. Such resistors play a crucial role in the electronics field, ensuring the stability and reliability of circuits.
Classification of precision resistors
Precision resistors can be categorized by material into metal film precision resistors, wire-wound precision resistors, and metal foil precision resistors. Metal film precision resistors excel in precision, but are slightly inferior in terms of resistance temperature coefficient and distributed parameters. Wire-wound precision resistors perform well in resistance accuracy and temperature coefficient, but their distributed parameters are somewhat lacking. Metal foil precision resistors are considered all-rounders, achieving high standards in accuracy, resistance temperature coefficient, and distributed parameters. Their accuracy can reach 10⁻⁶, their temperature coefficient is as low as ±0.3×10⁻⁶/℃, and their distributed capacitance and distributed inductance are also lower than industry standards.
However, with the improvement in performance, the price of these three types of precision resistors also increases accordingly. Therefore, a trade-off must be made based on actual needs and budget when making a selection. For example, in DC or low-frequency AC circuits, wire-wound precision resistors or metal film precision resistors are sufficient to meet the requirements, and there is no need to invest in expensive metal foil precision resistors.
In terms of appearance, metal film precision resistors are typically cylindrical; wire-wound precision resistors come in various shapes, including cylindrical, flat cylindrical, and rectangular frame shapes; while metal foil precision resistors are mostly square or sheet-shaped. For wire-wound precision resistors, when the number of turns is large, a non-inductive winding method is used to reduce distributed inductance. Furthermore, rectangular frame-shaped wire-wound precision resistors are commonly used in the manufacture of instruments and meters; the resistance wire of this shape can be customized according to requirements.
If a rectangular wire-wound precision resistor is damaged, it can be replaced with a new alloy resistance wire of the same material, diameter, and length as the original. If only the surface insulation layer is damaged, simply remove the original resistance wire, re-impregnate it, and let it dry. These simple repair steps can extend the resistor's lifespan.
Mainstream precision resistor technologies and their advantages and disadvantages
(I) Precision Thick Film Resistors
With continuous improvements in thick-film resistor pastes, the most advanced thick-film resistor technology can now achieve a temperature drift of ±5ppm/°C, and this can be further reduced to ±2ppm/°C by combining multiple mutually compensating thick-film resistor chips. Its highest accuracy can reach ±0.01%. Thick-film resistors have become the mainstream technology for applications requiring high voltage, high resistance, and high precision. However, they struggle to achieve high precision and low temperature drift in the low resistance range, and their noise levels and long-term stability are relatively poor.
(II) Precision Thin Film Resistors
Precision thin-film resistor technology has matured and become a widely used precision resistor technology in the market. Through multilayer film deposition, precise resistance adjustment, and rigorous screening, optimal thin-film resistors can achieve a temperature drift of ±2ppm/°C, an accuracy of ±0.01%, and excellent long-term stability. However, it has limited power capacity, poor performance in the low-resistance range, and is sensitive to electrostatic discharge, resulting in a poor power coefficient. In addition, maintaining consistency in small-batch supply is a significant challenge.
(III) Precision Metal Film Resistors
Precision metal film resistors are similar in performance to precision thin-film resistors. Although wafer-level precision metal film resistors are being replaced by surface-mount precision thin-film resistors, through-hole precision metal film resistors still maintain their mainstream position due to their low cost advantage. Similar to precision thin-film resistors, the hotspot effect generated during resistance adjustment can affect their stability and reliability.
(iv) Precision wire-wound resistor
As a long-established precision resistor technology, high-precision wire-wound resistors can achieve a temperature drift of ±1ppm/°C and an accuracy of ±0.001%, a performance advantage that thin-film and thick-film resistors cannot match. Their resistance values can reach nearly 50MΩ, making them ideal for ultra-precision, high-resistance applications. However, due to the development of other more advanced resistor technologies and their high price and inherent inductance, precision wire-wound resistors are gradually being phased out of the market.
(V) Precision Foil Resistors
Although precision foil resistors have held a prominent position in the field of resistor technology since their invention in 1962 due to their outstanding precision, they still maintain their status as the most precise resistors. This technology achieves extremely low temperature drift, approaching zero, by cleverly bonding a nickel-chromium alloy onto a ceramic substrate and applying stress balance. With the aid of laser-etched resistor patterns and a refined resistance adjustment process, their accuracy can reach ±0.001%. Impressively, high-quality foil resistors exhibit a resistance drift of only ±2ppm after six years of storage, demonstrating excellent anti-static properties, a non-inductive and non-capacitive design, low noise, and a low voltage coefficient. However, precision foil resistors also have certain limitations; their resistance range is relatively limited. The largest surface-mount resistors can only reach a resistance of 150KΩ, and the largest through-hole resistors can only reach a resistance of 2MΩ.
