Application of wire wound resistance in precision circuits

In the past twenty years, the electronic industry has developed at an amazing speed. Advances in new technology have increased pressure on manufacturers of discrete components to develop devices with desirable performance while reducing the size of their devices.

In these devices, chip resistors are currently in high demand and are the building blocks of many circuits. Their space utilization ratio is better than that of discrete package resistors, which reduces the workload of pre-assembly preparation. With the popularization of applications, chip resistors are playing more and more important roles. The main parameters include ESD protection, thermoelectric emf, thermal coefficient of resistance, self-heating, long-term stability, power coefficient and noise, etc.

The application of wire wound resistors in precision circuits is discussed in the following technical comparisons. Please note, however, that wire wound resistors do not have a wafer type, so they are not used in applications where precision wafer resistors are required due to weight and size limitations.

Although upgrading each component or subsystem improves overall performance, overall performance is determined by the short board in the component chain. Each component in the system has inherent strengths and weaknesses that relate to overall performance, particularly short-term and long-term stability, frequency response, and noise. The discrete resistance industry has made advances in the technology of wire wound resistors, thick film resistors, thin film resistors, and foil resistors, and each resistance has many factors to weigh in terms of unit performance costs.

Precision wire wound resistance

Wire wound resistance is generally divided into "power wire wound resistance" and "precision wire wound resistance". The power wire winding resistance will change greatly in the process of use, and it is not suitable for the use of high precision requirements. Therefore, this resistance is not considered in this discussion.

Wire wound resistance is generally made by winding the insulation resistance wire on a spool of a specific diameter. The required resistance and initial properties can be achieved with different wire diameters, lengths, and alloy materials. Precision wire wound resistance ESD higher stability, noise than thin film or thick film resistance. Wire wound resistance also has the characteristics of low TCR and high stability.

The initial error of wire wound resistance can be as low as 0.005%. Which can achieve 3 PPM / ° C typical values. However, to reduce the resistance value, wire wound resistor in 15 PPM / ° C to 25 PPM / ° C. Thermal noise is reduced, which is on the limit temperature range can be plus or minus 2 PPM / ° C.

In wire-wound resistance machining, the inner surface of the resistance wire shrinks and the outer surface stretches. This process produces permanent deformation - the resistance wire must be annealed as opposed to elastic or reversible deformation. Permanent mechanical changes will cause arbitrary changes in resistance wire and electrical parameters. Therefore, the electrical performance parameters of resistance elements are very uncertain.

Because of the coil structure, the wire wound resistance becomes an inductor, and the coil capacitance will be generated near the winding number. In order to improve the response speed in use, special process can be adopted to reduce the inductance. However, this will increase the cost, and the effect of reducing inductance is limited. Due to the presence of inductance and capacitance in the design, the wire wound resistance has poor high-frequency characteristics, especially at frequencies above 50 kHz.

Two wire-wound resistors with the same rated resistance are difficult to guarantee accurate consistency within a specific temperature range, and it is more difficult to have different resistance values or different sizes. This difficulty increases as the difference in resistance increases. Ω resistance compared with the 100-1 - k k Ω resistance as an example, this inconsistency is due to the diameter, length, and may have caused by resistance wire using different alloy. Also, the resistance cores and the number of revolutions per inch vary - the mechanical characteristics have different effects on the electrical characteristics. Because different resistance values have different thermodynamic characteristics, their working stability is not the same, and the designed resistance ratio will change greatly in the life cycle of the equipment. TCR characteristics and ratios are important for high precision circuits.

Thin film resistor

The film resistor consists of metal deposits on the ceramic substrate of A thickness of 50 A to 250 A. Film resistance unit area resistance is higher than wire-wound resistance or Bulk Metal foil resistance, and cheaper. When high resistance values are required and accuracy is medium, thin film resistance is more economical and saves space.

They have the best temperature sensitive deposit thickness, but the resistance values produced by the best film thickness severely limit the range of possible resistance values. Therefore, the range of resistance values can be varied by the thickness of various sedimentary layers. The stability of film resistance is affected by temperature rise. The aging process of film resistance stability varies with the film thickness required to achieve different resistance values and is therefore variable throughout the resistance range. This chemical/mechanical aging also includes high temperature oxidation of resistance alloys. In addition, changes in the optimum film thickness can seriously affect TCR. Because thinner deposits are more easily oxidized, the degradation rate of high resistance films is very high.

Thick film resistor

As noted earlier, it is not possible to use a wafer type for wire-wound resistance due to size, volume, and weight. Although the precision is lower than wire-wound resistance, thick film resistance is widely used due to its higher resistance density and lower cost. Like thin film and foil resistors, thick film resistors have fast frequency response, but the noise is the highest in the resistance technology currently used. Although the precision is lower than other technologies, the reason why we discuss thick film resistance technology here is that it is widely used in almost every kind of circuit, including high precision circuit in the precision requirements of the high parts.