Introduction to the Family of Ceramic Resistors
Ceramic resistors offer some great characteristics and capabilities for demanding applications, particularly in high-power circuits. They can handle from 0.5 watts to 1,000 watts in a single component rather than needing an array of devices on the upper end, providing advantages in size, simplicity of topology, and, with enough reduction in complexity and number of components, improved total cost of design.
Another advantage that ceramic resistors provide over wirewound, film, or carbon composition is less susceptibility to parasitic inductance and capacitance effects, reducing or eliminating the need to incorporate compensatory measures in circuit designs.
There is a third general category of advantage, as well, in the form of construction. Unlike wirewound, film, or carbon composition resistors, there is no wire or film that can potentially malfunction, increasing reliability and, as a result and improving mean time between failures and reducing support requirements.
Ceramic power resistors are compatible with a broad array of end applications, including rail charging stations, switchgear, motor controls, defibrillators, accelerators, circuit breakers, or high voltage power supplies. However, they can create electrical noise, and so aren’t the best suited for such applications as sensitive radio receivers and transmitters (including in wireless devices) or other equipment inherently susceptible to interference.
There are nine basic types of ceramic resistors:
Axial: These non-inductive bulk resistors handle high peak power or high-energy pulses in a small size and are intended as replacements for carbon composition resistors. There are three material types: SP, for high operating temperatures; AS, for high energy and voltage pulse applications; and BA, for high energy and voltage pulse applications where the required resistance value is above the resistance values available in Type SP and Type AS resistors.
Custom assemblies: Custom assemblies provide the flexibility of high energy dissipation, while saving space, time, and money. These can take the forms of motor drive discharge resistors; high load testing systems; braking and crowbar resistors; encapsulated assemblies; linear accelerator slabs; or electrostatic precipitators.
Disk and washer: Disk and washer resistors offer greater surface area for heat dissipation. In free air, parts can safely dissipate 2.5 watts per square inch of surface area at 40° C (104° F).
Encapsulated: These ceramic resistors offer the characteristics other types provide, including non-inductive high energy, voltage, and power performance. In addition, they offer ruggedized packages to offer mechanical shock and vibration resistance, sealed for protection against contaminants. Machined aluminum case and packaging materials are suitable for harsh industrial and aerospace applications and can contain multiple series-parallel connected or multiple individual resistors within a single package.
Metallic load bank: 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. Special allows allow for longer element life and consistent operating performance. Resistant stainless steel support rods with high-temperature ceramic insulators support elements across their entire length. Load elements come assembled in discrete trays designed for modularity and spiral element design allows for greater power density and heat dissipation.
OC series: Ohmite’s OC series are fixed ceramic resistors intended for circuitry associated with surges, high peak power, or high energy. They offer enhanced performance in high voltage power supplies, R-C snubber circuits, and inrush limiters. OC resistors can often replace carbon composition resistors, which can be difficult to source.
Slab: 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 and not concentrate through skin effects or limited pathway constructions like wire or film.
Tubular: These non-inductive, high voltage, high power, bulk tubular resistors are available in a wide range of standard sizes, ceramic materials terminations, and mounting hardware. The bulk construction is inherently non-inductive and allows energy and power to pass uniformly through the entire resistor body and not concentrate through skin effects or limited pathway constructions like wire or film. There are three different material types. SP is best used to withstand high operating temperatures; AS withstands high energy and high voltage applications; and A can reach higher resistance values.
Water-cooled: Direct and indirect water cooling allows resistors to dissipate more heat at a faster rate than air cooling. Water quality will affect component lifespan, which can run from one to five years. While the ceramic resistor part is relative rugged, acrylic and polycarbonate parts with epoxy seals require care during assembly or transport. Also, operating pressures should run approximately half of tested limits.
Ceramic resistors can be a critical component in circuit designs with the right requirements. Choosing the best type influences cost, performance, size, possible cooling methods, best applications, and other design considerations.
Contact Ohmite and communicate with an expert who can help you specify the best type of power resistor for your application and requirements.