Carbon, Cermet and Conductive Plastic potentiometers

Heat dissipation / Wattage

The wattage rating of a potentiometer is a function of the resistive ink, the resistive substrate and the design of physical package

Cermet – Ceramic based materials exhibit a greater ability to absorb and dissipate heat than most other composites; i.e. higher wattage rating. Cermet resistive elements incorporate a ceramic based ink which allows a higher level of heat absorption and dissipation than most other composite materials used in potentiometers. Cermet resistive elements also have a wider temperature operating range compared to other substrates.

Hot Molded Carbon – Carbon based materials also exhibit a greater ability to absorb and dissipate heat than most other composites. Carbon resistive elements incorporate a carbon based ink film which, depending on the thickness of the ink film, can allow a higher level of heat absorption and dissipation than most other composite materials used in potentiometers. Thick film carbon potentiometers are comparable to Cermet in their ability to absorb and dissipate heat when they are incorporated onto a carbon molded base material.

Conductive Plastic – Conductive plastic materials do not absorb and dissipate heat as well as Cermet or Carbon. Conductive Plastic resistive elements incorporate a conductive plastic based ink film deposited onto a plastic substrate.

In all of the above examples, the ability of the potentiometer to dissipate heat is further enhanced by other mechanical parts used in the construction of the potentiometer.
The primary dissipative path for heat generated by the resistive element is through the metal shaft and bushing. When the potentiometer is mounted in a metal panel the heat dissipation is further enhanced. The metal can that surrounds some potentiometer styles also offers additional heat dissipation. A potentiometer design that has a plastic shaft, bushing and body would therefore have a lower wattage rating

Temperature Characteristics
Temperature Coefficient (TC) is a measurement of resistance change relative to temperature change and is expressed as parts per million (PPM) per degree centigrade change, or as a percentage. The TC for a typical Cermet potentiometer is 150 ppm/°C

Cermet – Ceramic based materials exhibit a high degree of stability over a wide temperature range. Resistive elements that incorporate Cermet inks will typically have very low temperature coefficients as compared to other resistive materials. This characteristic is important for any application that has to operate in a wide temperature range. Cermet potentiometers are very stable and typically have a TC that is expressed PPM. The TC for a typical Cermet potentiometer is 150 ppm/°C (0.015%) which is significantly below Conductive Plastic or Carbon

Hot Molded Carbon – Carbon film potentiometers have a relatively high TC compared to Cermet and Conductive Plastic. One percent change is equal to 10,000 PPM. The chart below represents the typical temperature change specification for Conductive Plastic potentiometers.

Conductive Plastic – Conductive Plasitc Potentiometers are not as temperature stable as Cermet Potentiometers and as a result their TC is expressed in percentage. One percent change is equal to 10,000 PPM. The chart below represents the typical temperature change specification for Conductive Plastic potentiometers.

Mechanical Rotational “Feel”
Cermet and Carbon potentiometers tend to have a harsher feel due to the roughness of the resistive substrate and the wiper design. This “feel” is transferred to the shaft so when a user turns the shaft it may not feel smooth.

Conductive Plastic potentiometers have a very smooth feel and for that reason are widely used unless of course there are environmental or stability requirements as previously outline.

Rotational Life
Rotational life is a function of the test methods and in many cases military specs are designated as the standard. The Mil-R94 specification is the one most typically used to define rotational life and that specification is the same for Cermet, Carbon or Conductive Plastic.

The rotational life specified in Mil-94 is 25,000 cycles over which the resistance change cannot exceed a predetermined limit (percentage). However, most commercial potentiometers will far exceed 25,000 rotational cycles with some designs testing to 1- 5 million rotations. Inexpensive off-shore products typically have a rotational life <5,000 cycle.