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Introduction - Basic Potentiometer Theory • Resistors • Building Potentiometers • Modern Variable Resistive Devices • Applications Modes • The Parts of a Potentiometer • Resistive Elements • Cermet • Conductive Plastic • Hybrid Elements • Terminations • Contacts • Electrical Parameters • Total Resistance • Minimum Resistance • End Resistance • Contact Resistance • Contact Resistance Variation • Equivalent Noise Resistance • Output Smoothness • Adjustability • Temperature Coefficient • Resolution • Conformity • Linearity • Power Rating • Summary of Electrical Parameters • Useful Terms
Cermet
Cermet is a recently developed material. It combines very fine particles of ceramic or glass with those of precious metals. It is fired in a kiln to produce a ceramic metal resistive material.
Cermet, also known as thick film, refers to resistive and conductive films greater than .0001 inch thick, resulting from firing a paste, or ink, that has been deposited on a ceramic substrate. Similar materials and techniques are used to manufacture hybrid circuits and fixed resistor networks.
The paste is applied by a silk screening process. A hard rubber squeegee forces the ink through a stainless steel or nylon screen. This process is illustrated by Figure 2.40.
Figure 2.40
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In terms of advantages, cermet elements provide good stability of total resistance over time and temperature changes. They offer good noise performance in both a static and dynamic state. Frequency response is good. For trimming applications, cermet elements usually offer the best performance per dollar per unit space.
In terms of limitations, cermet elements have poorer dynamic noise performance than those of conductive plastic. Their temperature coefficient is better than that of conductive plastic, but inferior to that of wirewound. They are also more abrasive than conductive plastic elements.
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