New Foil Resistor with Very Low Power Coefficient

摘要

The stability of a high-precision resistor depends mainly on its temperature, which depends on: 1. Changes in the ambient temperature 2. Self-heating as a result of load -- the power dissipated by the resistor (Joule effect) The relative change of resistance with temperature is specified as TCR -- temperature coefficient of resistance -- and refers to both effects. In the foil resistor, which is a high-precision component and achieves very low values of TCR, the difference between the two effects must be taken into account in order to achieve high stability with changes in load. Unfortunately, very often both effects are present simultaneously. As a consequence, power changes (self-heating) plus ambient temperature changes (TCR) result in substantial resistance changes. It is known how to control (or correct for) ambient temperature change (TCR), but what is less well understood is how to control the power-inducted resistance change. The heat is generated in the foil, which is cemented to one side of the substrate and evacuated partially through the thickness of the cement and of the substrate. Thus, application of power to a resistor (or a change in the level of power) will cause temperature gradients in the substrate, causing additional strain in the foil, followed by a change of resistance. This doesn't take place when a resistor at no load is submitted to slow changes in ambient temperature. This paper analyzes the above phenomena in more detail and describes a new foil resistor that reduces very substantially the sensitivity to changes of applied power.