Regular Power vs. Ceramic Power Resistors
Having to dissipate more than five watts of power in your circuit topology? You need a power resistor. They are important in many applications: power infrastructure, 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.
But that’s overly simplistic, like saying you need a vehicle and assuming a passenger sedan, sports car, or SUV are equivalent.
There are two major categories of power resistors. One is the widely used “regular” variety, including both carbon- and metal-based. The other, ceramic, has some additional attractive characteristics for demanding needs.
The first type includes carbon composition, which are made of finely ground carbon and an insulating binder formed into cylinders for smaller components in high-energy and surge applications. Thick or thin metallic film resistors use a conductive paste on a substrate for high resistance values. Foil resistors have photoetched resistive material on a non-conductive substrate for long-term stability, low noise, and low capacitance. In wirewound construction, a conductive wire is wrapped around an insulating core with a natural application in high-current use with a low temperature coefficient for stability in higher temperatures. Element or shunt resistors provide solid blocks of resistive material with high stability at low resistive values.
Although not true for all of the carbon resistors, there is a potential issue with many, particularly wirewound and film. Spiraling a conductor around a base core can also turn the component into an inductor, affecting circuit design with parasitic inductive effects. There may additional be parasitic capacitance effects.
In many circuit designs, such considerations aren’t significant issues. In others, however, what would normally be insignificant characteristics with little effect can wreak subtle havoc and change the performance of a design or incorporate problems that might be hard to diagnose, identify, and isolate after the fact.
When that is, or as important, might be, the case, ceramic power capacitors can provide great answers. Outside of avoiding such design problems, some types of ceramic capacitors add other features that can improve the ability for designs to deliver on performance goals.
For example, ceramic composition resistors are chemically inert, thermally stable, and are non-inductive, as they use a mix of ceramic and binder rather than carbon. Axials are non-inductive bulk resistors that handle high peak power or high-energy pulses in a small size and are intended as replacements for carbon composition resistors. Disk and washer resistors are flat and offer greater surface area for better heat dissipation. Encapsulated ceramic resistors are not only non-inductive but come in ruggedized packages to offer mechanical shock and vibration resistance and sealed protection against contaminants. Machined aluminum case and packaging materials are suitable for harsh industrial and aerospace applications. Metallic load bank resistors are typically used in high power load testing of emergency power systems, including generators, uninterruptible power supplies, turbines, battery systems and dynamic braking power dissipation for generators and large motors. Slab ceramic resistors have non-inductive bulk construction to provide higher power and energy dissipation in more compact form factors. The component design allows energy and power to pass uniformly through the entire resistor body. Bulk tubular resistors are available in a wide range of standard sizes, ceramic materials terminations, and mounting hardware. They allow energy and power to pass uniformly through the entire resistor body and not concentrate through limited pathway constructions like wire or film. Then there are also water-cooled resistors that allow either direct and indirect circulating fluid and conduction to dissipate more heat at a faster rate than air cooling. However, water quality can affect the component lifespan, which typically runs from one to five years.
Electric and physical performance characteristics as well as size, value ranges, and price and availability all become part of the many trade-offs. As with any type of component, certain variations may lend themselves to particular uses. Choosing the right type of resistor will influence cost, performance, size, possible cooling methods, best applications, and other design considerations.
Sometimes the right choice will be obvious. Other times, perhaps not. In those cases, contact Ohmite and communicate with an expert who at no charge can help you specify the best type of resistor for your application and requirements.