Substantial Difference. Difference in Cost. Difference in Digital Models. Difference in Color Wheel Reading. Difference in Application. Metal Film Precision Resistors. Wire-wound Precision Resistors. Metal Foil Precision Resistors. Temperature Excursion. Load Life. Shelf Life. Precision Thick Film Resistor.
Precision Thin Film Resistors. Precision Metal Film Resistor. Precision Wire-wound Resistor. Precision Foil Resistors. Precision resistors refer to resistors that meet the higher standards of resistance accuracy, temperature coefficient, load life, and long-term stability under the same resistance technology and installation method. The long-term stability of precision resistors is a very important indicator, and high precision without stability is meaningless.
Precision resistors are often associated with high-precision resistors. In fact, accuracy is affected by many factors, which are collectively referred to as " stress ". Stress comes from many aspects, such as changes in ambient temperature, self-heating generated by the resistor itself after being powered, pressure or tension from the PCB, moisture from the external environment, and even corrosive gases, as well as welding, pulse, overload, static electricity, radiation, etc.
All these "stresses" will change the resistance value of the resistor, which will also affect the resistance accuracy of the resistor. Besides, though the resistance is within the nominal accuracy range during incoming inspection, it will exceed the range after the resistor is soldered to the PCB. Also, such as humidity, static electricity, etc. Figure 1. It should be emphasized that stability should be considered in the first place, and unilaterally pursuit of high precision is undesirable.
So what is a precision resistor? The answer is a resistor with both stability and accuracy. Within a certain temperature range, the tolerance of precision resistors is much smaller than that of ordinary resistors. The temperature stability of the materials used to make precision resistors is higher than that of ordinary resistors. Explanation of tolerance: For example, if a resistor is of 0. Because precision resistors use materials with higher temperature stability, they are much more expensive than ordinary resistors.
Ordinary resistors are labeled with 3 digits, the first two are valid numbers, the latter is the power. The Three and Four Digit System. The color wheel reading of ordinary resistors and precision resistors are almost the same.
The first two digits of ordinary resistors are numbers. For precision resistors, the first three digits are numbers, the fourth ring is multiplying power and the fifth wheel is the tolerance , so the precision resistor is also called the five-color ring resistor.
Figure 2. A 5-Band Resistor. In a circuit, resistors are generally used for current limiting and voltage dividing. The precision of the resistance need not be considered in the current limiting part, while for the voltage dividing part, there are two situations:.
Here, the voltage dividing resistor should be of high precision. Figure 3. Simple Resistive Voltage Divider. In short, precision resistors can replace ordinary resistors, but ordinary resistors cannot replace precision resistors. If you replace precision resistors with ordinary resistors, it will cause differences in the results of experiments or tests.
We often consider the tolerance value when using the precision resistors, but the premise is that we have selected the precision resistor of a certain technical material.
Therefore, there are three types of precision resistors: metal film precision resistors, wire wound precision resistors, and metal foil precision resistors.
This kind of precision resistor has high precision, but the resistance temperature coefficient and the distribution parameter index are slightly lower. It is usually cylindrical and mainly used in communication equipment, medical electronic equipment, test and measuring equipment, automotive electronics, industrial products, etc. Figure 4. Construction of A Metal Film Resistor. This type of precision resistor is made of constantan, manganin, or nichrome alloy wire wound on a ceramic skeleton.
The surface is often coated with protective paint or glass glaze, and there are mainly two types: fixed type and adjustable type. The packaging materials of such resistors include insulating varnish, silicone resin, coating, ceramics, aluminum case, etc. Most precision wire-wound resistors are packaged in silicone resin because this package does not have a high-temperature sintering process, so the electrical properties of the winding material will not be affected.
The wire-wound precision resistor is generally cylindrical, flat cylindrical, and rectangular, which is mainly used in precision instruments, electronic equipment, and other AC and DC circuits. Figure 5. Basic Construction of a Wire Wound Resistor. A metal foil with known and controllable characteristics is laid on a special ceramic substrate, and then the ultra-precise photoetching process is used for the resistor circuit.
This process combines the important characteristics of low TCR, long-term stability, no inductive reactance, no ESD induction, low capacitance, fast thermal stability, and low noise in one resistor technology. The precision, temperature coefficient and distribution parameters of the resistance value of this kind of precision resistor are all very high.
Besides, this kind of precision resistor is often in the shape of a square or a sheet , which is widely used in aerospace, marine inertial navigation, power distribution control, precision measurement, and other systems. They're also used as current and voltage standard of comparison, constant current and constant voltage circuit components, and various shunt and voltage dividing proportional networks.
Figure 6. It is worth noting that the precision resistor mainly uses thin film and foil technology. Among them, the electrical performance of the foil resistor is more excellent, which can achieve almost zero temperature drift and 0. Therefore, compared to other chip resistors its advantages are very obvious, and its considered to be the closest to the ideal resistance. Accuracy is an important indicator for incoming inspection. The answer is negative. Although there are many different processes and materials for precision resistors, almost all precision resistors need to be adjusted to achieve "high precision".
For example, laser trimming is performed on the surface of the precision thin-film resistors, and precision foil resistors are adjusted by the cutting-off method.
The process of adjusting resistance is actually a measurement process, but the adjusted product is not a finished product, and it must go through some subsequent processes such as packaging.
This process may affect the resistance of the resistor. Also, the accuracy of the measuring instrument and the correct measurement method is also very important, especially for precision resistors with more than one ten-thousandth accuracy, as well as milliohm resistors. Figure 7. The specifications of most precision resistor manufacturers do not clearly define the temperature drift index in different temperature ranges.
The measurement is of high cost, and accurate measurement may be destructive to the product itself. In addition, the stress caused by the temperature drift measurement will change the resistance value. The control of temperature excursion is mainly based on the resistance material itself and the manufacturing process. For example, temperature drift of precision foil resistors can achieve near zero by using special low-temperature drift alloys and stress compensation methods.
Load life is related to three factors, power , temperature, and time. Derating can help slow down the change in resistance. Figure 8. Temperature Derating Curve. The active period of the change in resistance value is often the first several hundred hours in the using process, and the resistance tends to stabilize as the use time increases. This is because the resistance element itself stabilizes, or the stress between the resistance element and the substrate is gradually released over time.
The index of load life can only be monitored by sampling test because this kind of test requires at least 1, hours and even 10, hours for testing aerospace applications. Beyond that, the test is destructive. Power training of precision resistors before use can effectively accelerate the aging of the resistor and stabilize it, but it will change the resistance of the resistor. Shelf life is used to examine the stability of the resistance value under standard or specified storage conditions.
As with the life of the load, the longer the resistor is stored, the more stable the resistance will be. This is why some precision instrument manufacturers do not use precision resistors immediately but store them for months or even years before using them. Of course, storage will change the resistance value, but this change is getting smaller and smaller.
So for the old batch of resistors, as long as the accuracy is within the nominal value and the solder pins are not oxidized, its stability is better than the new batch of resistors.
Figure 9. Basic Number of the Shelf-life for Different Technologies. We should pay special attention to humidity control in the storage of resistors.
Humidity will have a great impact on the resistance of any resistor. For example, an electrolyte will be formed in the resistance layer of all kinds of film resistors once moisture enters, which will seriously affect the resistance.
In addition to moisture, there are various corrosive substances in the air, such as sulfur and salt spray. The standard resistor used as a metering application will be completely sealed after being filled with oil, thereby isolating the impact of external environmental stress on the resistance value and reducing the annual change rate of the resistance.
In the application of high voltage, high resistance, and high precision, precision thick film resistor is the mainstream technology. The disadvantages of thick film resistors are that it is difficult to achieve high precision and low-temperature drift in low resistance parts, and the noise label is not good.
The long-term stability is generally worse than other precision resistors. Figure A Precision Thick Film Resistor. Size Specifications: , , , , , etc. Low manufacturing cost and high-cost performance;. In the shape of chips, suitable for SMT placement;. The outer glass protects the structure and has good moisture resistance. Large temperature coefficient of resistance value;. Poor electric shock performance and low power;.
Poor resistance value stability and loud noise. In the glass phase of the thick-film resistor, the particles are connected to each other. Part of the resistance path will be blocked by thermal stress during the use of the resistor, the resistance will increase, and its stability will become poor. The particle structure will result in a high noise factor. The higher the resistance value, the lower the metal ratio; the greater the noise is and the more unstable the resistance will be.
Among various precision resistors, thick film resistors are the noisiest. Experimental results for resistance noise spectrum. Widely used in general precision circuits, or local circuit parts requiring less precise in high precision circuits. The technical development of precision thin-film resistors represents the precision resistor technology that can be used in a large number of commercial applications and is also the most popular precision resistor technology at present.
However, the power is not large, the indicators of low resistance values are not good. Thin Film Precision Chip Resistor. Size Specifications: , , , , , High resistance accuracy and low-temperature coefficient;. Less loss and low noise;. Low parasitic effect, excellent high-frequency performance;. The thin film will oxidize in high temperatures, chemical corrosion, and other occasions, which will continuously change the resistance value.
The film of the high resistance film resistor is very thin and is more susceptible to oxidation and is easily affected by ESD. ESD Safety.
Thin-film resistors have the largest application and use in the precision resistor market. It has a wider range of resistance values and is cheaper than wire wound resistors and foil resistors. Thin-film resistors are a more economical choice for applications with medium-precision, high-resistance resistors, and space efficiency considerations.
Precision metal film resistors are similar to precision thin-film resistors. Wafer metal film precision resistors are gradually being replaced by chip thin film precision resistors, but pin metal film precision resistors are still mainstream low-cost precision resistor technology. Like precision thin-film resistors, the resistance adjustment will cause a hot spot effect, which will affect the stability and reliability of the resistor.
A Precision Metal Film Resistor. The best precision wire-wound resistors have a resistance value close to 50M, which is suitable for ultra-precision high-resistance applications. Due to the development of other resistance technologies, precision wire-wound resistors tend to be eliminated, because they are expensive and having an inductance.
Figure 1 5. Precision Wire-wound Resistors. Size Specifications: lead type and patch type. Rated Power: 0. High resistance accuracy and small temperature coefficient;. High power, high voltage surges resistance, good short-term overload performance;.
Low working noise, which is lower than thin-film resistors and thick film resistors;. Stable and reliable, good static stability, no change in resistance value over the years. Large volume, not suitable for small electronic equipment;. The wire-wound structure has large parasitic capacitance and inductance, and cannot be used in high-frequency circuits. Occasions that require high precision and high stability like resistance boxes , measuring instruments multimeters , telecommunications instruments, etc..
A Resistance Box. Although the precision foil resistor was invented as early as , it is still the most precise resistor technology so far. By attaching a nickel-chromium alloy to a ceramic substrate for balancing stresses, a temperature drift close to zero is obtained. The disadvantage of the foil resistor is that the resistance cannot be very high. You don't need the liquid helium, and it superconducts at room temperature!
Gand Mara sale. There are no particular advantages or disadvantages of series or parallel circuits. Some circuits require one while other circuits require the other.
Frequency filters often require one or the other depending on whether you need a pass or rejection filter. Policy statements need to be specific and precise. That is a very difficult question. Need to know in what context these resistors are used for. If they are in series, you need to add the resistance values.
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