Displacement Measurement

# What is a potentiometer?

The potentiometer can be described as a device to measure the potential difference, this device is capable to measure the unknown voltage by comparing it with a known voltage. The known voltage could be supplied by a standard cell or it could also be supplied from another voltage source. Basically, a potentiometer is a resistor that has three terminals that have a sliding contact at the center creating a voltage divider that could be adjusted. This device produces a voltage output that is less than its input. A potentiometer can give smooth voltage level transition and it can be either in linear or rotary form.

A potentiometer is a three-terminal device and it would measure the voltage by varying the resistance. The potentiometer has a manually adjustable, variable electrical resistor. It has a resistive element that is attached to the circuit by three contacts or the terminal.

### Where do we use potentiometers?

Potentiometers are used where the precision required is very high, the potentiometers won’t draw any current from the source under measurements. The potentiometers can be used to adjust the level of analog signals and also they are used as control inputs for electronic circuits. The potentiometer is used in a loudspeaker to adjust the volume, it is also used in television sets, computer monitors, to control the screen brightness. It is also used in light dimmers to control brightness. So we use a potentiometer in our day-to-day life.

### How is a potentiometer constructed?

The potentiometer has a sliding and a non-sliding part, the sliding part is the wiper and their motion could be translational or rotational. The potentiometer has three terminals in that two terminals are connected to the resistor and the third one is connected to the wiper and it is movable with the wire. Due to this moving wire the variable potential is tapped off. The variable resistor is controlled by the third terminal. The potentiometer body is made with a resistive material and the wire is wound on it.

### How does a potentiometer work?

The working of a potentiometer is based on the principle that if a constant current flows through a wire of uniform thickness if it is so then the voltage difference between the two points will be proportional to the length of the wire between the two points. So a uniform high resistance wire will be connected to the insulating support and this would be marked with a linear measuring scale. A regulated voltage source is used and this would have a greater magnitude than the potential to be measured. This voltage source can be regulated and it will be connected across the wire so a steady current can be passed through it. Between the end of the wire and any point along it will be a potential proportional to the length of the wire to that point. So by comparing the potentials at the points along the wire with unknown potential, we can determine the magnitude of the unknown potential.

### What are the types of potentiometer?

Wire wound type potentiometer

In this type of potentiometer, many rounds of wire will be wounded around an insulator. So if the number of turns of the coil is high then more will be its resolution. This type of potentiometer has good tolerance of linearity and the stability of the resistance wire is high. The disadvantage of this type of potentiometer is that it doesn’t have good resolution due to the gap between the windings

Plastic film type potentiometer

In this type of potentiometer, the plastic materials that have resistance properties that are controlled properly are used. This can be used for translational or rotary slider movements. This type of potentiometers has a good life and better resolution than the wire wound type.

Linear potentiometer

This type of potentiometer has a linear taper, which means that the resistance measured from the wiper to one of the ends of the terminal will vary when the shaft is moved.

Logarithmic potentiometer

This type of potentiometer has a logarithmic taper or a reverse logarithmic taper. This means that there is a highly non-linear change of resistance when the shaft is moved.

### How potentiometers are used as transducers?

The potentiometer can be used as a displacement measurement device, so in order to measure the displacement of an object, the object must be connected to the sliding part of the potentiometer. So when the body moves the slider position will change too and because of this the resistance between the fixed point and the slider will change, this would cause a voltage variation across these points. This change in voltage or resistance will be proportional to the change in the displacement of the body, so the change in voltage is the displacement of the object. We can use a potentiometer for the measurement of translational and rotational displacements.

### What are the advantages of a potentiometer over a voltmeter and how are they different from each other?

By using a potentiometer we can measure the internal resistance of a cell but it is not possible by a voltmeter. By using a long wire and varying the potential gradient, the potentiometer can be used to measure very low potential difference accurately and it is not possible by the voltmeter. A potentiometer is more sensitive than a voltmeter, we can increase the accuracy of the potentiometer but it is not possible for a voltmeter. We can use a potentiometer to measure the e.m.f of a cell and it is not possible by a voltmeter. If a voltmeter is connected to a circuit then it will absorb current through the circuit and thus it measures the potential difference across a cell. But when a potentiometer is connected in a circuit it won’t draw any current, when the null point is obtained and thus it measures the e.m.f of the cell. A voltmeter is a direct reading instrument but in the case of a potentiometer, it is not possible we have to do some calculations for this.

### What are the advantages of potentiometer?

• The potentiometer can be used for the measurement of voltage and current
• This device can be used for the measurement of power and resistance
• The detection of the voltage is done independently of the source resistance
• It won’t use any power from a circuit that has unknown e.m.f when it is balanced
• It has high accuracy
• High power
• Heat tolerance
• It has a long life
• This device can also be used to set controls

### What are the disadvantages of potentiometers?

• It could be susceptible to wear
• Dirt could get accumulated between the slider and the resistant surface
• The movement of the slider is very fast and the friction between the slider and resistance surface is high

### What are the applications of potentiometer?

• This device is used in many electronics like fan regulators and volume knob in music systems
• The potentiometers can be used for the calibration of the voltmeter
• This device can be used for the calibration of the ammeter
• By using a potentiometer we can do the testing of a wattmeter and energy meter
• It is used for the measurement of the self-reactant of the coil
• This device is used for magnetic testing and it can also do the testing of instrument transformers
• This device is used for the volume controls and it is also used for position sensors