There are many power resistor technologies available but the most common are Wirewound and Thick Film. Other resistor technologies tend to be reserved for specialist applications
Power Resistor Design Considerations
For any resistor technology, the four main design considerations are:
-The resistor power handling capability
-The change in the resistance value over time
-The change in the resistance value with temperature
For a power resistor, the resistor power handling capability and the impact of temperature are of particular concern. The materials and methods used to construct the resistor have a direct impact on its long-term performance and stability.
Total resistance is determined by the resistivity of the resistor material, and the resistor track length, width and thickness. The resistor material for thick film power resistors is the resistor film and for wirewound the resistor wire material itself. Choice of this material is critical as there are obvious limitations associated with resistor length, width and thickness. The long term stability of the resistor material and the impact of temperature are also key considerations.
Temperature effects can degrade the power resistor performance or, in extreme cases, cause resistor failure and require careful consideration. In the thick film example, temperature effects can be minimised by choosing an appropriate substrate material of a suitable thickness. For Wirewound resistors, the wire type and diameter, the core and the construction require careful consideration.
The key advantages of thick film power resistors over Wirewound are smaller size (high resistance density) significantly lower cost (including tooling cost), lower inductance (faster frequency response) and improved thermal management (heat sinking). Thick film technology also facilitates production of resistors with higher resistance values and higher voltage rating than available from Wirewound resistors.
Thick film substrates are robust and able to operate in harsh environments. Tooling costs are relatively low when compared with other technologies with similar performance and the technology has excellent high frequency characteristics.
The major advantage of thick film technology in high power applications is the high thermal conductivity of the ceramic (Alumina) substrate. The characteristics of the substrate material mean it is an excellent match with the aluminium material in heat sinks and it is chemically inert and mechanically stable. The high integration density of the substrate makes it ideal for applications where high power dissipation is required in a small area.
Although Wirewound resistors do offer higher precision (improved tolerance), higher stability and lower values it is often at the expense of much larger footprint and higher inductance than an equivalent thick film device. Any Wirewound resistor is fundamentally a coil of wire (an inductor) and although the choice of material can limit the impact of the high inductance, it remains a major disadvantage. Addressing heat dissipation from any Wirewound resistor is a major issue and often drives the decision to select a thick film power resistor as an alternative.