Power resistors come in all shapes, sizes, resistances and power ratings. Power ratings of discrete power resistors range from about 1W to 2.5kW. Resistances generally range from 0.01Ω to 100K. They are used in multitudes of applications—that is the subject of the next discussion. This is a brief, but practical overview on the subject.
Popular resistor symbols
The most common resistance alloy is nichrome (nickel-chromium alloy) wire or strip. Nichrome holds its shape even when red-hot so it is useful in space heaters and electric ranges. In electronics applications, the maximum temperature is limited to a much lower temperature for safety and resistance stability. However, temperature stability is not generally an important issue in power applications.
Alumel (nickel, manganese, aluminum and silicon alloy) is another, but less common material that is used for resistor applications. It is also called a quaternary alloy because it contains (4) elements. Alumel is also used in Chromel-Alumel thermocouples.
Lower resistance alloys include Advance (copper-nickel alloy) and Manganin (copper-nickel-manganese alloy). These are used in low resistance, high accuracy shunts. In this case, temperature stability is essential.
Resistor power rating rule of thumb
Since resistors run very hot at the full power rating, it is customary to apply them at 50% of the maximum power rating. This derating adds reliability and safety factor. For UL/CSA/VDE agency approvals, all resistors must be applied at the 50% figure. Under temporary, short-circuit and other fault conditions, it is permissible to run resistors at the maximum power rating.
Power resistors in circuit board applications
Circuit boards fare poorly when overheated and power resistors can scorch or discolor the laminate, and can also cause traces to lose adhesion to the laminate. Excessive temperature can also cause solder joint deterioration over time. As a result, be very conservative in such applications. Ceramic bead spacers are available to raise hot leaded components off the circuit board. Radial types are preferred due to the inherent spacing between the element and circuit board. Large SMD resistors are generally limited to a maximum of 1W or so and are intended, of course, for direct contact with the PCB.
Panel mounted power resistors
Typical panel mounted power resistor run from about 10 to 300W. The lower resistance versions use an edge-wound ribbon fabrication technique. They can be mounted at right angles to the panel (on end) via a single long bolt, or parallel to the panel via a pair of mounting clips. Note that when orienting power resistors vertically, the top terminal runs considerably hotter than the lower terminal. Mounting clips are generally included, but the single long bolt is a specifically purchased kit.
High power resistors
The largest discrete air-cooled resistors are about 1000W. They have no ceramic encapsulation so that they have a much greater surface area that reduces temperature rise. To obtain even higher power ratings, these are often connected into arrays—I have seen banks of such resistors. Due to the high magnetic fields generated and the mechanically floating resistor element, these resistors often make audible sounds or “sing,” depending upon the current waveform.
Heat sinked power resistors
Some power resistors are packaged in an anodized aluminum case with a flat mounting surface. These depend upon the steel panel or aluminum heatsink to dissipate the power. They are available in power ratings ranging from 5 to 2500W. These are very practical when applying power resistors with heat-sinked power semiconductors. One detail to keep in mind is the voltage clearance between the terminal and case is sometimes an issue. My co-workers called the 300W version the “Gold Brick.”
Adjustable power resistors
Many tubular power resistors are available with an adjustable sliding contact that can make contact with any point along the exposed resistor element winding. Note that such resistors also have a maximum current rating—not simply a maximum power rating. It is obvious that if only a small portion of the resistance element is being used that the power must be proportionately reduced. The easiest way of determining this is by calculating the current rating.
Inductive vs. non-inductive power resistors
Standard power resistors are simply wound like a coil—this type of construction adds series inductive reactance that can be an issue even at power frequencies. Non-inductive power resistors are wound in two layers with each layer wound in the opposite direction so that the magnetic fields cancel.
Power resistor failure modes
Although power resistors are rugged, they can and do fail. Generally it is through abuse—simply running it too hot can cause the vitreous ceramic coating to soften and deform or the resistance element opens. Another common failure mode can be caused by repeated thermal cycling that eventually causes the ceramic to craze and/or flake off. Connection points are also subject to failure as both solder and pressure terminals tend to degrade at high temperatures. Sliding contacts are also subject to failure—especially with high resistance value resistors. To reduce terminal temperature rise, oversize copper conductors that act as heatsinks are recommended. To reduce resistor temperature rise, a little forced air from a fan can be very beneficial.
The fusible resistor is intended to open under short-circuit conditions. They look similar to common axial lead resistors and are available in the power range of 0.25 to 4W. Generally, they are mounted so that they are easily serviceable, but sometimes they are intended for end-of-life purposes. In this case, it is expected that the equipment be discarded rather than serviced when it fails (end-of-life).
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Power resistor applications