Electrical surges are a common failure mode for many types of resistors in a variety of harsh environments, including power supply, automotive and industrial applications. Thin film, solid composition, thick film and wirewound resistors all have different strengths and limitations, particularly under surge conditions. The underlying principle in this application is that increased film/wire mass and elimination of areas of current constriction will increase surge capability. Of the four types of resistors, the two that are most often used in surge applications are thick film and wirewound. When specifying any resistor for a specific application, it is important to consider not only the capabilities of each resistor technology, but also their characteristics under surge conditions.
Thin film resistors
Thin film resistors are comprised of an extremely thin layer of resistive material, usually deposited by evaporative or sputtering processes on a ceramic or silicon substrate. While thin film resistive technology offers extreme precision and stability, these resistors have limited surge capabilities due to the low mass of resistive material. For these reasons, thin film resistors are rarely used in surge applications, since thick film and wirewound devices are more effective at withstanding surge conditions.
Solid composition resistor
Another resistor technology used for surge applications is the solid composition-type device. These devices consist of a solid core comprised of carbon particles in a surrounding ceramic or polymeric matrix. The traditional carbon composition version of this product is manufactured by compressing and molding a mixture of carbon particles and organic and inorganic filler materials. Fired at a high temperature, the ceramic variant fuses a conductive phase (carbon or metals such as tantalum) and a ceramic filler into a solid core that forms the resistive element. Because of this ceramic core, these solid composition resistors dissipate heat through the full cross-section of the ceramic, thus producing surge withstand characteristics that may exceed that of either film or wirewound resistors. One of the limitations of these types of resistors is that their resistance tolerances are typically only available down to 5-10% due to difficulties in achieving the resistance value.
Thick film resistors
Thick film resistors are constructed by applying a slurry mixture of metallic and ceramic particles over a ceramic substrate, either a flat or cylindrical surface. For example, IRC's tantalum-based MetalGlaze process involves firing the thick film resistive material to a solid cylindrical ceramic core at 1000°C, thus providing a rugged, stable base for the resistor to withstand surge conditions. Fine-tuning the thick film formulation and tailoring the production processes can be used to calibrate the resistor's surge.