Although both precision resistors and ordinary resistors can be used to limit current, their precision and application scenarios are different, and they cannot be substituted for each other.
The difference between precision resistors and ordinary resistors
1. Different levels of precision
Precision resistors typically have higher precision than ordinary resistors, with an error generally below 0.01%, while the error of ordinary resistors is around 1%. The precision of a precision resistor mainly depends on factors such as the manufacturing process and raw materials.
2. Different application scenarios
Precision resistors are typically used in applications where sensitivity to current, voltage, resistance, and temperature is critical. For example, they are used in testing instruments, automatic control systems, and medical devices. Ordinary resistors, on the other hand, are widely used in various electronic circuits.
3. Different prices
Precision resistors are more expensive to manufacture and therefore more costly. In contrast, ordinary resistors are relatively inexpensive.
Applications of precision resistors and ordinary resistors in circuits
Both precision resistors and ordinary resistors can be used in electronic circuits, but they need to be selected and used according to their characteristics in specific applications.
1. Application of precision resistors in circuits
Precision resistors are mainly used in precision instruments such as automatic control systems, measurement systems, and medical devices to limit current, divide voltage, and connect turns to achieve precise control and measurement.
2. Application of ordinary resistors in circuits
Ordinary resistors are widely used in various electronic circuits, playing an important role in current limiting, voltage division, and voltage regulation. For example, components such as transformers, capacitors, and inductors in electronic circuits all need to be connected to the outside through ordinary resistors.
[Resistance error and thermal stability]
The key to precision resistors lies in their resistance error and thermal stability (temperature coefficient), characteristics that surpass those of ordinary resistors, ensuring their superior performance. Precision resistors exhibit significantly smaller errors over a specific temperature range compared to ordinary resistors. This is primarily due to the higher temperature stability of the materials used in precision resistors. For example, the error range of an ordinary resistor might be 1% to 5%, while the error of a precision resistor can be controlled to 0.1% or even less.
[Influence of distribution parameters]
The distributed capacitance and inductance parameters of precision resistors are strictly controlled to ensure their stability under various circuit conditions. This means that, under the same conditions, precision resistors can maintain their resistance value more accurately, thus meeting the requirements of high-precision applications.
02. Differences between precision resistors and ordinary resistors: [Material and performance differences]
Precision resistors use materials with higher temperature stability, resulting in significantly lower errors than ordinary resistors. They maintain their resistance value more accurately under the same conditions. The materials used in manufacturing precision resistors have superior temperature stability compared to ordinary resistors. This means that, under the same conditions, precision resistors can maintain their resistance value more precisely, thus meeting the demands of high-precision applications.
The difference between precision resistors and ordinary resistors
1. Differences in material properties
Within a certain temperature range, the tolerance of precision resistors is much smaller than that of ordinary resistors.
2. Substantive differences
The materials used to manufacture precision resistors must have higher temperature stability than those used for ordinary resistors.
For example, the temperature tolerance of ordinary resistors is 1%-5%, while the tolerance of precision resistors is only 0.1% or lower.
Explanation of tolerance
For example,
If the accuracy of a resistor is 0.1%, it actually means that under normal temperature (such as 10℃-35℃), all tolerances of the resistor should be guaranteed not to exceed 0.1% within one year. This is essentially a component tolerance.
3. Cost differences
Because precision resistors use materials with high temperature stability, they are much more expensive than ordinary resistors.
4. Differences in digital models
Ordinary resistors are labeled with three digits: the first two are significant figures, and the last digit indicates the power rating.
For example: 152 means 15 * 102 = 1500 ohms.
Precision resistors are marked with four digits; the first three are significant figures, and the last digit indicates the power rating.
For example, 1502 means 150 * 102 = 15000 ohms.
5. Differences in color ring readings
The color ring readings of ordinary resistors and precision resistors are almost identical.
The first two digits of a standard resistor are numbers.
For precision resistors, the first three digits are the numbers, the fourth ring is the exponentiation, and the fifth ring is the tolerance.
Therefore, precision resistors are also known as five-color-ring resistors.
6. Differences in application
Compared to ordinary resistors, precision resistors are more expensive, so they are mainly used in high-precision circuits for signal conversion, bridge circuits, sampling circuits, precision voltage divider circuits, etc.
In circuits, resistors are generally used for current limiting and voltage division.
The accuracy of the resistor does not need to be considered in the current-limiting section, but for the voltage divider section, there are two cases:
(1) If the accuracy of the voltage does not have a significant impact on the subsequent circuit, then choose a regular voltage divider resistor.
(2) If the accuracy of the voltage has a significant impact on the subsequent circuitry, then the voltage divider resistors should be of high precision.
In short, a precision resistor can replace a regular resistor, but a regular resistor cannot replace a precision resistor.
Using ordinary resistors instead of precision resistors will cause differences in experimental or test results.
III. Classification of Precision Resistors
When using precision resistors, we often consider tolerance values, but this is on the premise that we have selected precision resistors with certain technologies and materials.
Therefore, there are three types of precision resistors:
Metal film precision resistors, wire-wound precision resistors, and metal foil precision resistors.
1. Metal film precision resistors
These precision resistors offer high accuracy, but their temperature coefficient of resistance and distributed parameters are slightly lower. They are typically cylindrical and are mainly used in communication equipment, medical electronic equipment, testing and measuring equipment, automotive electronics, and industrial products.
2. Wire-wound precision resistors
Wire-wound precision resistors are made by winding constantan, manganese, or nickel-chromium alloy wire onto a ceramic frame. The surface is often coated with a protective varnish or glass enamel, and there are two main types: fixed and adjustable.
Packaging materials for these types of resistors include insulating varnish, silicone resin, coatings, ceramics, and aluminum casings. Most precision wire-wound resistors are packaged in silicone resin because this packaging does not involve a high-temperature sintering process, thus preserving the electrical properties of the wire winding material.
Because they are wound on a ceramic core, these resistors feature high temperature resistance, good thermal stability, low temperature coefficient, high power rating, high resistance accuracy, and a high temperature coefficient index, with an operating temperature up to 300℃. Wire-wound precision resistors are generally cylindrical, flat cylindrical, or rectangular, and are mainly used in precision instruments, electronic equipment, and other AC/DC circuits.
3. Precision resistors made of metal foil
Metal foil with known and controllable properties is laid on a special ceramic substrate, and then a resistive circuit is fabricated using an ultra-precise photolithography process.
This technology combines important characteristics such as low TCR, long-term stability, non-inductive reactance, no ESD, low capacitance, fast thermal stability, and low noise into a single resistor. This precision resistor exhibits extremely high accuracy, temperature coefficient, and resistance distribution parameters.
Furthermore, these precision resistors are often square or plate-shaped and are widely used in aerospace, marine inertial navigation, power distribution control, and precision measurement systems. They are also used as current and voltage comparison standards, constant current and constant voltage circuit elements, and various shunt and voltage divider networks.
It's worth noting that precision resistors primarily utilize thin-film and metal foil technologies. Among these, metal foil resistors offer superior electrical performance, achieving near-zero temperature drift and an accuracy of 0.001%. Therefore, compared to other surface-mount resistors, their advantages are significant, and they are considered the closest to ideal resistors. For this reason, they are only used in higher-end products or applications with very stringent resistance requirements.
IV. Main Parameters of Precision Resistors
1. Accuracy
Accuracy is an important indicator for incoming material inspection.
Do all precision resistor manufacturers perform 100% accuracy testing before their products leave the factory?
The answer is no. Although precision resistors use many different processes and materials, almost all precision resistors require adjustment to achieve "high precision".
For example, laser trimming is performed on the surface of precision thin-film resistors, while precision aluminum foil resistors are adjusted by cutting (trimming).
The process of adjusting the resistance is actually a measurement process, but the adjusted product is not a finished product; it must go through some subsequent processes, such as packaging.
This process may affect the resistance of the resistor.
In addition, the accuracy of the measuring instrument and the correct measurement method are also very important, especially for precision resistors with an accuracy exceeding one ten-thousandth, and milliohm resistors. Commonly used laser trimming...
2. Temperature deviation
In resistor specifications, we often see only one temperature drift indicator, such as ±5ppm/℃.
However, in reality, this indicator may not cover the operating temperature range required by the product. In other words, the temperature deviation of the same resistor will be different in different temperature ranges.
Most precision resistor manufacturers' datasheets do not explicitly specify the temperature drift index for different temperature ranges.
A common phenomenon is that some manufacturers only select the optimal "curve" within the entire operating temperature range as the temperature drift index in their instruction manuals.
Another fact is,
Temperature drift index is difficult to measure 100% before a product leaves the factory.
Measurement is costly, and accurate measurements can damage the product itself.
For example, for chip resistors, it is generally recommended to solder them onto the PCB before temperature drift measurement.
In addition, the stress caused by temperature drift measurement will change the resistance value.
Temperature deviation control is mainly based on the resistive material itself and the manufacturing process.
For example, by using special low-temperature drift alloys and stress compensation methods,
The temperature drift of precision aluminum foil resistors can be close to zero.
3. Load life
Load lifespan is related to three factors: power, temperature, and time. Derating can help slow down changes in resistance.
The period of most active resistance value changes is usually in the first few hundred hours of use. As the usage time increases, the resistance value tends to stabilize.
This is because the resistive element itself tends to stabilize, or the stress between the resistive element and the substrate is gradually released over time.
Load life metrics can only be monitored through sampling tests, as such tests require at least 1,000 hours, and even 10,000 hours when testing aerospace applications.
Beyond this time, the test becomes destructive.
Performing power training on a precision resistor before use can effectively accelerate its aging process and stabilize it, but it will change the resistor's resistance value.
4. Shelf life
Shelf life is used to test the stability of resistance values under standard or specified storage conditions.
Similar to the lifespan of a load, the longer a resistor is stored, the more stable it becomes.
This is why some precision instrument manufacturers do not use precision resistors immediately, but instead store them for months or even years before use.
Of course, storage will change the resistance value, but this change becomes smaller and smaller.
Therefore, for older batches of resistors, as long as the accuracy is within the nominal value and the solder joints are not oxidized, their stability is better than that of newer batches of resistors.
We should pay special attention to humidity control during resistor storage.
Humidity can have a significant impact on the resistance of any resistor.
For example, once moisture enters, an electrolyte will form in the resistive layer of various thin-film resistors, which will seriously affect their resistance.
In addition to moisture, the air also contains various corrosive substances, such as sulfur and salt spray.
As a standard resistor used in metering applications, it is completely sealed after being filled with oil, thereby isolating the resistance value from the influence of external environmental pressure and reducing the annual rate of change of resistance.
V. Advantages and disadvantages of mainstream precision resistor technology
1. Precision thick film resistors
Through continuous improvements to thick-film resistor paste, the most precise thick-film resistor technology can achieve a temperature drift of ±5ppm/℃. Furthermore, by using multiple mutually compensating thick-film resistor chips, a temperature drift of ±2ppm/℃ can ultimately be achieved. Its highest accuracy can reach ±0.01%.
In applications requiring high voltage, high resistance, and high precision, precision thick-film resistors are the mainstream technology.
The disadvantages of thick-film resistors are that it is difficult to achieve low-temperature drift in high-precision, low-resistance components, and their noise rating is not ideal. Their long-term stability is generally worse than other precision resistors.
(1) Range of main performance parameters
Size specifications: 0201, 0402, 0603, 0805, 1206, etc.
Resistance range: 10Ω - 1MΩ
Resistance accuracy: 0.1%, 0.25%, 0.5%
Rated power: 1/20, 1/16, 1/10, 1/8, 1/4 (W)
Temperature coefficient of resistance (TCR): ± 50ppm/℃
ESD (Electrostatic Discharge) withstand capability: ESD2KV
(2) Advantages
Low manufacturing cost and high cost-performance ratio.
It is chip-shaped and suitable for SMT mounting.
The outer glass protective structure provides excellent moisture resistance.
(3) Disadvantages
The temperature coefficient of resistance is large.
It has poor electric shock performance and low power.
It has poor resistance stability and high noise.
In the glass phase of a thick-film resistor, the particles are interconnected.
During the use of a resistor, some of the resistor path can be blocked by thermal stress, which increases the resistance and reduces its stability.
The particulate structure leads to a high noise figure. Higher resistance values and lower metal ratios result in greater noise and less stable resistance. Among various precision resistors, thick-film resistors are the noisiest.
(4) Application
Widely used in general precision circuits, or in less precise local circuit components within high-precision circuits.
2. Precision thin-film resistors
The technological development of precision thin-film resistors represents a precision resistor technology that can be widely used commercially and is currently the most popular precision resistor technology. Through long-term multilayer thin-film deposition, high-precision resistance adjustment, and subsequent screening, the optimal precision thin-film resistor can achieve a temperature drift of ±2ppm/℃ and an accuracy of ±0.01%, as well as good long-term stability. However, its power rating is low, and its low-resistance performance is not ideal. In addition, it is not anti-static, has a poor power factor, is difficult to supply in small batches, and has poor consistency between different batches.
(1) Range of main performance parameters
Size specifications: 0402, 0603, 0805, 1206, 2010, 2512
Resistance range: 1Ω - 2MΩ
Resistance accuracy: +0.05%, +0.1%, +0.25%, +0.5%
Rated power: 1/16 1/10 1/8 1/4 1/2 3/4 1 (W)
Temperature coefficient of resistance (TCR): +5 +10 +15 +25 +50 (ppm/℃)
Antistatic capability: ESD 2KV
(2) Advantages
It has high resistance accuracy and low temperature coefficient.
Low loss and low noise.
It has low parasitic effects and good high-frequency performance.
It is chip-shaped and suitable for SMT mounting.
(3) Disadvantages
Thin films oxidize under conditions such as high temperatures and chemical corrosion, which continuously alters their resistance. High-resistance thin-film resistors have very thin films, making them more susceptible to oxidation and ESD.
(4) Application
Thin-film resistors have the widest range of applications and uses in the precision resistor market. They offer a wider range of resistance values and are less expensive than wire-wound and foil resistors. For medium-precision, high-resistance applications, and considering space efficiency, thin-film resistors are a more economical choice.
3. Precision metal film resistors
Precision metal film resistors are similar to precision thin-film resistors. While wafer-level metal film precision resistors are gradually being replaced by chip-based thin-film precision resistors, pin-type metal film precision resistors remain the mainstream low-cost precision resistor technology. Like precision thin-film resistors, adjusting the resistor can cause hotspot effects, thus affecting the resistor's stability and reliability.
4. Precision wire-wound resistors
As one of the earliest precision resistor technologies, high-precision wire-wound resistors can achieve a temperature drift of ±1ppm/°C and an accuracy of ±0.001%, which is unattainable by thin-film and thick-film resistors. The best precision wire-wound resistors have resistance values approaching 50MΩ, suitable for ultra-precision, high-resistance applications. Due to the development of other resistor technologies, precision wire-wound resistors have often been phased out because they are expensive and inductive.
(1) Main performance parameters
Size specifications: Lead wire type and surface mount type
Resistance range: 1mΩ - 275KΩ
Resistance accuracy: ±0.001%, ±0.005%, ±0.1%, ±0.5%.
Rated power: 0.5, 1, 5, 7, 10 (W)
Temperature coefficient of resistance (TCR): ±3, ±10, ±20, ±30 ppm/℃.
Antistatic capability. ESD 25KV
(2) Advantages
It has high resistance accuracy and a small temperature coefficient.
High power, resistant to high voltage surges, and good short-term overload performance.
It is heat-resistant and dissipates heat quickly, and can still work normally at an ambient temperature of 170℃.
It operates with low noise, lower than that of thin-film and thick-film resistors.
It is stable and reliable, with good static stability, and its resistance value has remained unchanged over the years.
(3) Disadvantages
Its large size makes it unsuitable for use in small electronic devices.
The resistance is low, mostly below 100KΩ.
The wire-wound structure has large parasitic capacitance and inductance, and cannot be used in high-frequency circuits.
(4) Application
Applications requiring high precision and stability, such as resistance boxes, measuring instruments (multimeters), and telecommunications instruments.
5. Precision aluminum foil resistors
Although precision foil resistors were invented as early as 1962, they remain the most accurate resistance technology to date. Near-zero temperature drift can be achieved by attaching a nickel-chromium alloy to a ceramic substrate to balance stress. This is accomplished through laser etching and trimming of the resistor pattern.
Accuracy up to ±0.001%. The best aluminum foil resistors exhibit a resistance drift of only ±2ppm after 6 years of storage. They are characterized by anti-static properties, no inductance or capacitance, no hot spots, low noise, and a low voltage coefficient. The disadvantage of foil resistors is that their resistance cannot be very high. The maximum size for chip resistors is 150KΩ, while the maximum size for leaded resistors is 2MΩ.
(1) Range of main performance parameters
Size specifications: 0603, 0805, 1206, 1506, 2010, 2512
Resistance range: 1mΩ-150KΩ
Resistance accuracy: ±0.005%, ±0.01%, ±0.05%, ±0.1%, ±0.25%, ±0.5%.
Rated power: 0.1 ~ 8 (W)
Temperature coefficient of resistance (TCR): ±0.2, ±2, ±5, ±10 (ppm/℃).
Antistatic capability: ESD 25KV
(2) Advantages
Metal foil precision resistors are considered ideal resistors due to their characteristics such as low-temperature drift, long-term stability, no inductance, insensitivity to static electricity, low capacitance, fast thermal stability, and low noise.
(3) Disadvantages
The resistance value cannot be increased. Currently, it can only be KΩ, MΩ, or even TΩ.
It is expensive.
What kind of precision resistors should we choose?
1. For precision sampling resistors with resistance values from 1 milliohm to 1 ohm, generally only precision foil resistors can be used. This is because only resistors made of alloy materials can remain stable under low resistance and high current. Temperature drift can be as low as ±5ppm/°C, and accuracy can reach ±0.1% or even higher.
Temperature drift is the main factor determining the price of this type of resistor.
Because its primary application is current sensing, a four-pin structure facilitates accurate sampling. Using a larger size or reserving more power supply space, and adding an auxiliary heatsink, helps reduce the temperature of the resistor surface and improves the stability of the sampling resistor. A precise current sampling resistor can replace a Hall effect current sensor and offers cost advantages.
2. Achieving resistance values from 1 ohm to 10 ohms presents a challenge for any resistor technology. This range falls into the low-resistance category, requiring thick resistive material and short current paths to achieve low resistance. Thick resistive material hinders bonding with the substrate to balance temperature drift, while short current paths impede precise resistance adjustment.
Precision thin-film resistors and precision metal thin-film resistors struggle to provide excellent temperature drift within this resistance range; ±20ppm/°C is the best they can achieve. Precision wire-wound resistors, whose temperature drift is entirely based on the alloy wire itself, can achieve around ±10ppm/°C, but wire-wound resistors typically only have leads and inductance. Within this resistance range, precision foil resistors can control temperature drift within ±5ppm/°C, achieving an accuracy of over ±0.1%, and can be provided with both chip and leads.
3. 10 ohms to 150K ohms is a typical resistance value range. Within this range, many products can be selected to meet different accuracy requirements.
If the temperature drift requirement is below ±2ppm/°C, or if long-term stability is a concern, precision foil resistors can be used, with selectable lead sizes for surface mount components. If the temperature drift requirement is around ±5ppm/°C, and long-term stability is not critical, thin-film or metal-film resistors can be considered.
Aluminum foil resistors can achieve a maximum accuracy of ±0.001%, while thin-film and metal-film resistors can achieve ±0.01%. It should be noted that different sizes of chip foil resistors have different resistance value ranges. For example, the maximum resistance of a 0805 chip foil resistor is 10K. In addition, many manufacturers produce precision chip film resistors. While the specifications may be the same, their actual performance varies greatly. Temperature drift and exceeding accuracy limits are common issues, and different brands of resistors also differ in their long-term stability and noise levels.
4. The resistance range from 150K to 1MΩ is considered the intermediate resistance range. In this section, if precision surface mount resistors are required, only precision thin-film resistors can be used.
[Cost and Model Number Differences]
Precision resistors are relatively expensive due to the high cost of materials. However, this cost investment is worthwhile because their application in high-performance circuits brings significant benefits. The designation numbers for precision resistors differ from those for ordinary resistors. Precision resistors typically contain four digits, with the first three representing significant figures and the last digit indicating the exponent. For example, 1502 represents 150 multiplied by 10 squared, or 15000 ohms. Ordinary resistors, on the other hand, have only three digits, with the first two being significant figures and the last digit indicating the exponent.
【Differences between color ring readings and applications】
Precision resistors have five color bands. The first three bands represent the numerical value, the fourth band indicates the multiplier, and the fifth band indicates the tolerance. Therefore, precision resistors are also called five-band resistors. Although the color bands of ordinary resistors and precision resistors are read similarly, the color bands of precision resistors contain more information. Precision resistors are mainly used in high-precision signal conversion, bridge circuits, sampling circuits, and precision voltage divider circuits. These applications require extremely high accuracy, thus necessitating the use of more expensive and higher-performance precision resistors. 1. When the voltage accuracy has a relatively small impact on subsequent circuits, ordinary voltage divider resistors can be used; however, if the voltage accuracy has a significant impact on subsequent circuits, then high-precision voltage divider resistors must be selected.
In conclusion, while precision resistors can replace ordinary resistors, ordinary resistors cannot replace precision resistors. Randomly substituting ordinary resistors for precision resistors may lead to deviations in experimental or test results, causing unnecessary trouble for users.
