Introduction to Different Power Resistor Types
In demanding design parameters, specialized components can be necessary. When the power to dissipate is at least 5 watts, that means power resistors.
All resistors dissipate energy and generate some heat. Power resistors differ in the larger amounts of energy they can burn off as heat, through a challenging application of Ohm’s law. That requires designs that will let the components maintain predictable and stable performance under heavy loads while
Power resistors make regular appearances in such circuits as power supplies, power amplifiers, and power conversion. They’re important in power infrastructure—in transmission and distribution networks—but those aren’t the only applications. They also appear in dummy loads, braking systems for trains using electric motors, automotive applications, snubber resistors to manage current fluctuations, current sensing, bleeders to discharge capacitors, current balancing in semiconductors, and more.
These uses dictate many of the common characteristics of power resistors. They are often larger than other types of resistors to have the physical capacity to handle higher loads, although they are available in many shapes and sizes.
The construction materials not only must provide the necessary resistance but be good at transmitting heat away from the component and to some recipient: forced air, a metal heat sink with or without thermal conductive grease or gel, or an exchanger that can transfer heat to a circulating liquid coolant.
There are seven different types of power resistor construction: composition, ceramic composition, film, foil, wirewound, and element or shunt.
Carbon Composition: Carbon composition construction involves finely ground carbon and an insulating binder. The ratio of the two determines the resistance. The materials are mixed and compressed into a cylinder with terminals on its ends and an applied insulator coating on the outside. These are good for high energy and surge applications that require smaller sized components. The resistors can absorb high levels of energy repeatedly without degradation to the resistive value.
Ceramic composition: Ceramic composition resistors use ceramic materials rather than carbon. They are common in a wide array of applications, including rail charging stations, switchgear, motor controls, defibrillators, accelerators, circuit breakers, and high voltage power supplies. The components are chemically inert, thermally stable, and are non-inductive.
Film: There are two types of film resistors: thick film and thin film. Thick film uses a paste of a glass and conductive material paste on a substrate. The construction allows high resistance values, and they can use a serpentine printing design to eliminate inductance, important in steady frequency applications.
Thin film resistors use vacuum deposition method to deposit the conductive material onto a ceramic base. The thicknesses of these are three orders of magnitude smaller than thick film. Thin film resistors also allow high resistance values but with better thermal stability than thick film. Some common applications are precision test equipment and constant current source circuits.
Foil: Metal foil resistors are constructed with photoetched resistive material on a ceramic base with high heat conductivity. The etching process determines the component’s value with high precision. They have good long-term stability, low noise, low capacitance, and no inductance. A low temperature coefficient of resistance means little change in resistive value as internal and external temperatures vary. A common application is current sense circuitry.
Wirewound: These resistors are formed, as the name suggests, by conductive wire wrapped around an insulating core, typically ceramic for high power applications. Resistance varies with the wire alloy materials and the thickness of the wire. They frequently appear in high current or braking applications. For high power applications they often come in finned aluminum cases that act like heat sinks. Because of their construction, wirewound resistors can present significant parasitic inductance, especially at high frequencies, requiring special internal wiring patterns as well as circuit design to mitigate the effects. They do have high value tolerance, long-term stability, and a low temperature coefficient, so retain their values as temperatures change.
Metal Element and shunt: Called both element and shunt resistors, this type is a solid piece of resistive material that can provide high stability in very low values, a characteristic difficult to achieve in other types of construction. Element/shunt resistors can also handle high current, sometimes up to 1,200 amps. They are commonly used to measure current output of a circuit.
Power resistors are a critical component in circuit design. Choosing the right type will influence cost, performance, size, possible cooling methods, best applications, and other design considerations.
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