Power Resistors: Types and Applications
Power resistors are manufactured to withstand and dissipate large amounts of power. While they are generally compact in size, all power resistors are designed to dissipate as much energy as possible. They have a power rating of at least 5 W, and they are usually made of materials with a high thermal conductivity to ensure efficient cooling. Moreover, they can be coupled with heat sinks to dissipate tremendous amounts of power. However, some power resistors may need a forced air or liquid cooling mechanism while under maximum load to efficiently remove heat.
Different Types of Power Resistors
There are a wide range of power resistors on the market, each of which are designed to resist and dissipate electrical power in various ways, and in this section, we will outline some of the most commonly used types.
Wirewound Resistors
Wirewound resistors are constructed by winding a metal wire around a solid form that is made from ceramic, fiberglass, or plastic. Metal caps are fitted at the end of the winding and metallic leads are attached to the ends. The end product is typically coated in a non-conductive paint, enamel, or plastic to offer additional protection from the environment. For their design, wirewound resistors can be built to withstand high temperatures and high tolerances, depending on whether they are made from a nickel alloy or Nichrome. Additionally, several winding methods can be used including helical winding, edge-winding, and bifilar winding.
Grid Resistors
Grid resistors are composed of large matrices of metal strips connected between two electrodes. They vary in size, but can be as large as refrigerators, and they provide resistance values under 0.04 Ω that can withstand current over 500 A. In most cases, grid resistors are used as brake resistors and load banks for railroad vehicles, load-testing of generators, and more.
Chip/SMD Resistors
Similar in appearance to integrated circuit chips, chip resistors are often made from an array of materials like pressed carbon, ceramics, or metal foil. SMD resistors, on the other hand, are a smaller form of surface mounted chip resistors that consist of a metal oxide film that has been deposited onto a ceramic substrate, and the thickness and length of this film determines the resistance of the resistor.
Water Resistors
Water resistors feature a tube filled with a saline solution that controls its resistance and an electrode fitted at both ends. Meanwhile, the water in the tube provides a large heat capacity which enables high energy dissipation. Keep in mind that some high power resistors utilize copper sulfate solutions instead of saline.
Liquid Rheostats
Liquid rheostats, or salt water rheostats, are a type of variable resistor in which the resistance is controlled by submerging the electrodes into a saline solution. The resistance can be increased or decreased by adjusting the electrode position within the liquid, but please note that the mixture cannot be boiled in order to stabilize the load.
Applications of Power Resistors
Power resistors are primarily used when large amounts of energy need to be safely converted into heat using electrical energy as a medium. As such, they are utilized as controllable power dissipation devices, protective devices, and devices that simulate real world loads. High power resistors are frequently used in locomotives and trams, and load bank resistors are often used to load-test generators, turbines, and battery UPS systems. Finally, neutral grounding resistors are used in the power grounding of Y-connected generators and in medium-voltage AC distribution systems.
Conclusion
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