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| Fig. Resistive Potentiometers (POT) (source: instructables.com) |
Potentiometer
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A potentiometer consists of a resistive element having a sliding contact. This sliding contact is known as a wiper. A potentiometer is also called as POT because it is used in voltage division. It is a resistive and passive transducer.
Depending upon the motion of wiper, potentiometer are of three types
1. Translatory or linear potentiometer
2. Rotary Potentiometer
3. Helipots
In Translatory or linear potentiometer, wiper has linear motion only.
In Rotary Potentiometer, wiper has rotational motion only. The rotary potentiometer can be a single turn or multi-turn.
In Helipots, wiper use combination of the two motions, i.e. linear as well as rotational. Here, a resistive element is in the form of helix shape.
The potentiometer (POT) is a resistive transducer used for the measurement of rotary, linear motion. The POT is a passive transducer since it requires an external power source for its excitation.
A potentiometer is a resistive sensor used to measure rotary motion as well as linear displacements. In a potentiometer, an electrically conductive wiper slides across a fixed resistive element.
Potentiometer Working Principle
A voltage source is applied across the resistive element in a potentiometer. Thus a voltage divider circuit is formed. When the wiper come across any displacement, it slides across a fixed resistive element. The output voltage (Vout) is measured as shown in the figure below. The output voltage is proportional to the distance travelled by the wiper.
Important Points
The potentiometers are designed with a definite power rating which is dependent on its heat dissipating capacity. This condition limits the input excitation voltage.
The output voltage of potentiometer should be high in order to get high sensitivity. This, in turn, require to have higher input excitation voltage. Therefore, a higher potentiometer resistance requires satisfying constant power dissipation. Hence linearity will decrease because of higher potentiometer resistance.
On the other hand, a low potentiometer resistance is required in order to get high linearity. But low potentiometer resistance will require low input excitation voltage due to the limitation of power dissipation. Hence sensitivity will decrease because lower input will result in lower output.
So, Sensitivity and linearity are two conflicting requirements in potentiometers. Hence rating for potentiometer resistance should be made keeping in mind both sensitivity and linearity. There may be compromise between the two properties.
Materials used for Potentiometers
Potentiometers are of two types depending upon the type of material used i.e. Wire Wound Potentiometers and Non-Wire or Continuous Potentiometers.
1. Wire Wound Potentiometers
material used: Nickel chromium, platinum, nickel copper.
2. Non-Wire or Continuous Potentiometers
material used: Carbon film, cermet, hot moulded carbon, thin metal film.
Advantages of Potentiometer
- They are cheap.
- It is easy to use and useful in many applications where requirements are not severe.
- It gives sufficient output that does not require further amplification.
- Potentiometer efficiency is high.
- They are useful for the measurement of large displacement.
- The resolution is infinite in cermet and metal film potentiometers.
Disadvantages of Potentiometer
- The major disadvantage is that it requires a large force to move their sliding contacts i.e. wiper. There is wear and tear due to movement of the wiper. It reduces the life of this transducer.
- Also, there is limited bandwidth.
- There is inertial loading.
Applications of Potentiometer
It is used in many applications such as
- Linear displacement measurement
- Liquid level measurements using floats
- Rotary displacement measurement
- Brightness control
- Volume control
Excercise
Q. What are the factors while selecting a potentiometer?
Ans. The important parameters that are important while selecting a potentiometer are
- Humidity
- Operating temperature
- Contamination and seals
- Shock and vibration
- lifecycle
Q. Explain that sensitivity and linearity are the two conflicting requirements in a resistive potentiometer transducer.
Good explanation on the working and use of potentiometer. They are also applied in medical instrumentation as displacement transducers, and also used with resistive temperature transducers in medical applications.