Thermal dynamic model of precision wire-wound resistors

Wire-wound type resistors are often used as reference elements in high precision electronic instruments of industrial frequencies (50-60 Hz). One of the main factors in the measurement uncertainty of these instruments is the "self-heating" affected drift of the built-in wire-wound resistors. The materials, geometry, construction and the thermal environment of the resistors determine the dynamics of the load-affected drift. This paper investigates a thermal model that can be used for online estimation of the load affected thermal dynamic drift of commercially available encapsulated precision wire-wound resistors. Using the suggested model, the achievable estimation error of the relative change in resistance is below 10%. The model and the model-based online error estimation allowed us to reduce the measurement uncertainty of industrial high precision calibrators and to extend the power range of standard resistors in laboratories.