Wire wound resistors, also called wire wound resistors, are the most commonly used resistance products in power supplies. The basic manufacturing process is to wind a resistance wire on an alumina ceramic rod-shaped substrate or other insulating substrate. The resistance wire is made of alloy materials such as nickel chromium or manganese copper. The two ends of the resistance wire are connected to the fixed legs, and the resistance wire is surrounded by different packages. Material encapsulation.
Wire wound resistors have the characteristics of high power, high temperature operation and low noise.
Material:
1. Insulating substrate: Wire wound resistors usually use alumina ceramic substrate as the insulator, low-power wire wound resistors generally use solid ceramic rods, and high-power insulation substrates generally use hollow insulating rods. The quality difference of the rods affects the heat dissipation performance of the resistor. And electrical performance will have a very big impact. In contrast, foreign manufacturers generally use slightly gray ceramic tubes. The gray ceramic tubes are lighter in weight, and the white ceramic tubes have greater strength and higher density.
Low-power solid porcelain rods have strong shock resistance to vibration, and the quality is relatively stable during the sintering process. The hollow ceramic tube matrix used for high power is easily broken during transportation, and the poor quality ceramic tube is prone to tube cracking during the resistance sintering process. The high-power ceramic pipeline of the finished product sometimes causes internal cracks in the transportation process that cannot be judged by the naked eye. If the machine works continuously at high temperature, it is very likely that the phenomenon of pipe cracks will occur. Therefore, in the selection process of ceramic tubes, high-quality ceramic tubes are selected to ensure consistency, good high-temperature characteristics and stability.
2. Resistance wire: The difference of the resistance wire directly determines the temperature drift of the resistance and the size of the resistance, and also affects the short-term overload capacity of the resistance and the long-term stability of the resistance. Nickel-chromium alloy is the most commonly used wire, but the quality and craftsmanship of each wire supplier are very different, and the composition of the trace elements in the alloy is also different. The high-quality wire has little change in electrical properties during the high-temperature sintering process, and the quality is stable. The resistance range obtained by winding different grades of wires with the same size of the substrate varies greatly. In contrast, foreign manufacturers can achieve hundreds of K ohms or even WK ohms, while the resistance of our domestic manufacturers is below 100K. In response to this situation,
3. Packaging materials: generally there are eight types of insulating paint, silicone resin, glaze, paint, mixed materials, plastic packaging, ceramics, and aluminum shells.
Insulating varnish is the most economical packaging material, and the process is simple. It only needs to be coated on the base body with the resistance wire wound and dried at low temperature. The insulation performance is average, and it has little effect on the heat dissipation of the resistor, so the wire wound resistor coated with insulating paint generally has a low operating temperature and is not suitable for use in high temperature and high reliability requirements.
Silicone resin encapsulation is divided into ordinary silicone resin and high-temperature silicone resin. Most precision wire wound resistors are encapsulated by silicone resin, because it is basically the same as the production process of varnished resistors. There is no high-temperature sintering process, so it will not be caused by high temperature. Sintering affects the electrical properties of the winding material. The maximum operating temperature of wirewound resistors encapsulated by high-temperature silicone resin is generally 175°C, and the maximum can reach 275°C.
The glazed package form needs to be sintered at a high temperature, and the maximum working temperature can reach 345°C. After the resistance winding is completed, glaze powder is coated on the surface after sintering, and then sintered and molded in a sintering furnace. Due to the complexity of the process, the sintering temperature is as high as 750-800℃, and the sintering process will affect the resistance of the winding, so the highest accuracy of the glaze resistor is generally only 5%. This type of resistance is suitable for continuous full-load high-temperature work. The color of the glaze after firing is generally green and brown. The foreign ones are generally brown, and the domestic ones are generally green.
Insulating coating is also a kind of packaging material, the process is simple, the heat dissipation performance of the resistor is good after the packaging is completed, but the disadvantage is that it is prone to peeling after long-term high-temperature work, so it is not suitable for high-vibration equipment. Half of the insulation coating is wire wound resistors with relatively large power.
The wirewound resistors packaged by mixed materials have good performance, and the cost is between the lacquered resistors and silicone resin resistors. The surface of the package is uneven, but it does not affect the performance. Generally, the wirewound resistor power of mixed material packaging is less than 10W. This type of packaging material is relatively rare in China. At present, this material is also in the trial production stage.
Plastic encapsulation is generally aimed at patch wire wound resistors and low resistance wire wound resistors. It encapsulates the wound substrate in a molded plastic shell. Its maximum operating temperature can reach 175°C, and high temperature work will not cause peeling. , But the heat dissipation performance is average. The power of plastic package wire wound resistors is generally below 10W.
Ceramic packaged wire-wound resistors are customarily called cement resistors in China. This type of resistor has a pre-formed ceramic shell. The wound shaped matrix is put into the shell, and then an insulating material is poured through one side. This type of resistor has good heat dissipation performance and is economical, but it is large in size and high in weight, and is not suitable for high-reliability applications.
Wirewound resistors encapsulated in aluminum shells are very common today, and aluminum shells have super heat dissipation capabilities, so they are generally suitable for high-power applications. Some aluminum shell heat dissipation resistors also combine water cooling to dissipate heat.
Some high-power wire-wound resistors are wound on a ceramic substrate with flat wires. In order to facilitate heat dissipation, they are not packaged. These resistors are generally used as brake resistors and high-current resistors. The resistance is generally low, and the power is generally above 500W.
The above makes a simple analysis of the materials of each part of the wire wound resistor. The perfect wire wound resistor requires a high-quality matrix, high-quality wire and high-quality packaging materials as the basis, and undergoes a reliable production process.
With the continuous advancement of technology, the structure of wire-wound resistors is increasingly adopting new materials and new packaging modes.
