Types of Potentiometers
There are three types of potentiometers that are used commonly: wire wound, carbon film and plastic film potentiometers. All these have been described below:
1) Wire wound potentiometers: This potentiometer comprises of several rounds of wire wound around the shaft of the non-conducting material. The turns of the coil are bonded together by an adhesive. In this case the slider, connected to the body whose displacement is to be measured, moves on the potentiometer track and it makes contacts with successive turns of the coil. In this case the wire between the two successive turns is not covered by the slider, which limits the resolution of the wire wound potentiometers. However, the larger the number of turns of the coil, more is the resolution of the coil. The resolution is measured as the reciprocal of the number of turns of the coil. This devise has low noise and is mechanically rough and tough.
2) Carbon film potentiometers: The carbon film potentiometers are formed by depositing carbon composition ink on an insulating body, which in most of the cases is phenolic resin. This is one of the most commonly used materials for the pots that is quite cheap and has resolution better than the wire wound potentiometers. They have reasonable life and tolerable noise levels.
3) Plastic film pots: These pots comprise of the specially impregnated plastic material having resistance characteristics that are controlled properly. These can be used for rotary as well as translational slider movements. They have very good life, resolution better than the wire wound pots, and low noise.
In carbon film and the plastic film pots the resolution is limited by the grain size of the particles, hence their accuracy is very high. Their resolution can be as high as 10 rest to -4 and is usually limited only by the spring connected between the slider and the body whose motion is to be measured.
Operational Problems with the Potentiometers
The pots are used for measurement of the displacement of the body, but some problems are associated with this measurement. These problems usually occur at the point of the contact between the slider and the resistance track. Some of these problems can be:
1) Sometimes the dirt gets accumulated between the slider and the resistance surface thus indicating more resistance than the actual value. This give false output of the voltage and in some cases there is total loss of the voltage.
2) If the slider is moved very fast there are chances that the contact will bounce which gives intermittent output of the voltage and not continuous.
3) Sometimes the friction between the slider and the resistance surface can be quite higher, which limits the movement of the slider against the actual movement of the body due to the frictional forces.
For most of the potentiometers the typical accuracy mentioned by the manufacturers is +-1% against the full scale reading of the devise. The instruments with the accuracy of +-2mm as well as +-1mm are available.
Figure below shows some commonly used pots or potentiometers.
Different Types of Potentiometers
1) Book: Mechanical Measurements by Thomas G. Beckwith and N. Lewis Buck
2) Book: Measurement and Instrumentation Principles by Alan S Morris.