Potentiometer - Construction - Working Principle - Types of Potentiometer - Symbol of potentiometer - Applications of Potentiometer - Difference Between Rheostat and Potentiometer - Resolution of Potentiometer - Conformity of Potentiometer - Advantages - Disadvantages
Potentiometer
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| Potentiometer |
Potentiometer is an electronic component. It works as a variable resistor. A Potentiometer consists of a high resistance wire of manganin
or germanium silver and a variable knob which provides variable voltage to
various electronic circuits. It is also named as voltage divider. It has three
terminals. The external two terminals are connected to the supply voltage while
the central terminal or middle terminal provide variable voltage to various
circuits. Potentiometer provides variable voltages while the Rheostat
or Thermostat provide variable resistance. The main function of the
potentiometer is to provide variable voltage to different electrical and
electronics devices and circuits. A potentiometer is also used for the
measurement of voltage across a resistive element of a circuit. It is
also used to compare the EMFs of various electrical parameters. Potentiometers
have many types according to electric current and its function like Linear
Potentiometer, Rotary potentiometer, AC Potentiometer, DC Potentiometer,
Logarithmic Potentiometer, Trimmer Potentiometer, Digital
potentiometer etc.
So, let’s discuss the types of potentiometer which are used in
many circuit boards and in electrical as well as electronics devices for
various purposes according to its various functions and operations.
Symbol
Potentiometer is a three terminal electronic component and it
consist of a resistive element and a wiper which gives variable voltage to
various electric as well as electronics circuits. The symbol of the
potentiometer is shown below:
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| Symbol of Potentiometer |
Construction of Potentiometer
Potentiometer
consists of resistive element, terminals, contact/wiper, actuator/shaft
and casing. The resistive element is the main part of any potentiometer.
The resistive elements are of two types: wire-wound and non-wire wound. The non-wire
wound element is of cermet, carbon, metal film or bulk
metal.
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| Construction of Potentiometer |
Resistive Elements
Resistive elements are of various types and these types are
explained below:
Wire-wound Elements
Wire-wound elements
are made up of nickel chromium, copper nickel, gold platinum, nickel chromium.
Nickel chromium consists of 800 ohms resistivity per circle mil foot (cmf). A
circle mil foot is equivalent to one feet of wire. It is most commonly used in
potentiometers because of its excellent efficiency and availability in various
diameters. Copper-nickel wire has a resistivity of 300 ohms/cmf. The resistive
elements depend upon the total required resistance. The smaller wire allows
higher resistance and smaller wires are difficult to wind. This wire-wound
element is cut into individual rings for single turn potentiometers and multi
turn potentiometers. Wire-wound potentiometers provide very good
stability of total resistance and these elements have low noise in static state
and have high power capabilities.
Non-wire Wound Elements
The variable resistive devices which has no resistance wire in
it are non-wire wound devices. These elements are also used in potentiometers.
These elements are divided into many sections according to the resistive
materials which are described below:
Cermet Resistive Elements
Cermet is a term used for the
materials which are made up of different elements through different sources.
Cermet materials are made up of ceramic or glass with the combination of
different precious materials to form a ceramic metal resistive material. Cermet
is also known as thick film which are the resistive and conductive films
greater than 0.0001 inch thick in the form of paste. These are deposited on a
ceramic substrate. The paste is applied to the flat ceramic substrate by silk
screening operation. Cermet elements offer very low resolution and have good
stability. These materials do not produce noise while performing the operation.
This technique is also used for the manufacturing of hybrid circuits and
fixed resistors.
Carbon Elements
Carbon elements
are made up of carbon powder and phenolic resin. It is in the form of liquid.
This liquid is sprayed on the substrate. When the resistive material is applied
on the substrate then the resistive element is transferred to an oven for
curing. This process is done by infrared curing. Carbon elements have low cost
and low noise during adjustments. Carbon elements have poor moisture resistance
and have poor load stability as compared to cermet elements.
Metal Film Elements
A very thin layer of metal alloy is applied on a substrate and
the case/casing or cover is filled with vacuum. Any metal which can be
evaporated is used as a resistive element. Normally a nickel chromium alloy is
used because the other materials cannot fulfill according to the requirements.
Metal film elements cannot produce high reactive impedance. Metal film elements
are only used for low resistance values. These elements are used from 10 ohms
to 20K ohms.
Bulk Metal Elements
These elements are made up of bulk or mass metal. This metal is
applied on the substrate. Its layer is thicker than the other elements on the
substrate. Plating technique is used to apply this resistive element on
substrate. These elements are used in trimmer applications. These are expensive
elements.
Hybrid Elements
Hybrid element
consists of wire-wound element with a conductive plastic coating. Hybrid
element exhibit the temperature coefficient and resistance stability of the
wire-wound element and the long operational life. These are costly because of
the extra processing on it.
Terminals
Terminals of the potentiometer are of different types which are
shown below:
The terminations of the potentiometers are different from each
other because of various resistive elements in it. The terminations of the
major element types are discussed below:
Terminations of wire-wound Potentiometers
Wire-wound potentiometers have four different methods for the
terminations of resistive elements. These methods are:
- Single wire type
- Silver braze type
- Pressure clips
- Solder
Single Wire Type
The element is of single wire is unwounded on left side. This
wire is attached to the external terminal with soldering. The length of the
wire is small and provide low resistance.
Silver Braze
It is a preferable method. In this method, the element is brazed
with a small metal tab. This method increases reliability of the element and
also bears severe shock and vibrations.
Pressure Clips
In this method, pressure clips rely on mechanical connection
between the clip and the element wire. This clip produces problems in potential
source its position changing and cause noise and sudden output variations. So,
this method is not used in industries.
Solder
A single wire is soldered directly to the external element in
this method. This is very effective method for wire-wound potentiometers.
This method is used commonly by various manufacturers.
Termination for Cermet Potentiometers
In this termination, thick film conductive pads are used as cermet
elements. This termination has two methods.
The first method uses conductive precious metal paste like
paladiul-silver. The substrate is heated at 950oC and then the
resistive element is screened on it. The solid electrical bond made between
them.
The second method is that the resistance and the termination
materials are fired at a same time. In this process, the termination ink is
applied first and when it is dried then the other material is screened on the
substrate.
Contact / Wiper
Contact is a part of shaft of
potentiometer and this part is named as contact or wiper. This wiper is used to
make a connection between the potentiometer’s terminals and resistive element.
It plays important role in every potentiometer. This contact must be connected
with resistive element and external terminals in ceramic potentiometers.
These contacts are made up of metal alloy which have its own spring force.
Actuator / Shaft
A component or a part of a potentiometer which moves the
wiper/contact across the resistive element is known as actuator / shaft of a
potentiometer. These shafts come in many types according to its shape and
sizes. So let’s discuss some types of actuators / shaft.
Rotary Shaft Type
These actuators are cylindrical shaped with a knob on its
head. These actuators rotate in circular position that is why these actuators
are named as rotary shaft type. The wiper is connected to its end which is used
to make contact with the external terminals and a resistive element. This wiper
is insulated with the shaft. The wiper is connected to the additional sliding
contact. This sliding contact is connected to the resistive element and the
wiper easily make contact with the resistive element and the external terminals.
Lead Screw Actuators
Lead screw actuators
are used to fix the adjustments of resistive element according to its
requirements. These are cylindrical shaped. These actuators are made up of
steel and the wiper is attached on its end. This wiper make contact with the
external terminals and the resistive element directly. These actuators have
knob on its head for the adjustments of the resistive element. These actuators
were used on old age potentiometers.
Worm Gear Actuators
The Potentiometer which consists of Greater length of resistive
element consists of worm gear Actuator. If the element is formed in a circular
manner, then the adjustment screw worm engages the teeth of a small plastic
gear. The Wiper is placed between the plastic gear. When the gate is rotated by
the worm gear actuator adjustment screw, the viper moves long to the
element. If the direction of the adjustment
screw rotation is reversed, the Viper correspondingly begin to move
immediately.
The Single Turn Direct Drive Actuators
It consists of a simple router with the slot and mechanical
stops which are used in rotary adjustments of the wiper position in a single
turn unit. In this actuator, ring seal is used to prevent from moisture and it
also provides friction which serves as a mechanical restraint. this actuator is
a lower cost unit and it cannot provide sealing feature.
Linear Actuator
It is used for servo applications. The viper is directly
connected to it and it make contacts between the external terminals so that a
linear motion causes a direct linear travel of the wiper these actuators are
used as Precision linear position feedback transducer. These are several
feet long. These types of potentiometers are frequently used on audio level
control panel which is used in audio recording studios. It has the capacity
to make Rapid changes and visually compare relative settings in an
instant.
Housing / Casing / Case
Housing of a Potentiometer is very important in potentiometers
because it holds various components of Potentiometer in it. The casing of Potentiometer has a stability
and quiet performance by shielding the resistive element and contact/ wiper
surfaces from dust and dirt. It is a
mechanical structure which holds the external terminals and it provides
protection to external terminals from mechanical destructions. The casing
prevents mechanical and electrical installation from various affects.
Types of Potentiometer
The major types of potentiometers are Linear Potentiometers and
Rotary Potentiometers. All the types of potentiometers according to their
function and operation are mentioned below:
Linear Potentiometer
Linear potentiometers
use wire-wound resistive element in it. These are not rotated in a
circular position. These potentiometers consist of a slider which moves
from top to bottom or from bottom to top in a linear position. The wire-wound
resistive element is connected to the wiper and this wiper is connected to the
slider and this slider gives variable voltage on its central terminal when it
moves from one end to other end in a straight line.
The battery supplies the
current through the rheostat and a slide wire. The current of slide wire
changes when the rheostat is adjusted.
Types of Linear Potentiometer
Membrane Potentiometer
Membrane potentiometers
are the type of Linear potentiometers and these potentiometers move in a
linear direction with the help of a slider. Membrane potentiometers uses
membrane material in it. Three types of membrane materials are used in these
potentiometers which are as follows:
- Membrane as a wiper
- Membrane as a resistive element.
- Adhesive film
All these membranes are separated with spacers. These membrane
layers are connected to each other with the mechanical wiper. The contacts of
membrane potentiometers are achieved by the mechanical wiper / contact.
Multi Turn Potentiometer
Multi turn potentiometers
have multiple turns of resistive element and that is why these are named as
multi turn potentiometers. These potentiometers are a type linear
potentiometer. In multi turn resistive element, the wire is wrapped with each
other and the element consists of multiple turns of wire of resistive element.
These multi turn potentiometers are used for large electric supplies. The
potentiometers consist of a slider which moves in a straight line and provide
variable voltage.
Single Turn Potentiometer
These potentiometers consist of a slider which moves on a
resistive element. The resistive element consists of some turn of nichrome wire
and these turns are separated to each other. These potentiometers are used on
low power circuits and provides minimum voltage.
Rotary Potentiometer
Rotary potentiometers have rotatable shaft and this shaft is
connected to the cermet resistive element with a wiper/contact.
When the shaft/actuator rotates, it provides resistance on its
circuit on which it is connected. It also gives the divided voltage. Rotary
potentiometers usually have carbon material resistive element because it
provides stability and efficient in dividing voltages.
Types of Rotary Potentiometer
Logarithmic potentiometer
Logarithmic potentiometer
is a type of rotary potentiometers and these potentiometers work according to
the logarithmic scale. These potentiometers consist of cermet
resistive element and a wiper which makes contact between the resistive
element and rotatory knob of potentiometer. The whole construction of
logarithmic potentiometer is same as rotary potentiometer. Logarithmic
potentiometers provide logarithmic functions on its output voltages. These give
logarithmic values / fixed values on different position of resistive element. Some
potentiometers cannot provide accurate logarithmic voltages.
Logarithmic
potentiometers are used in volume control panels, audio voice control circuits
and tone controls etc.
Arduino Potentiometer
In Arduino Circuit board, rotary potentiometer is used in
it. As the Rotary potentiometer is used in Arduino circuit board so
these are named as Arduino potentiometers. Rotary potentiometers are
used to control the speed of DC motor. The circuit diagram of DC motor
speed control is shown below:
Digital Potentiometer
It is a mixed signal device which enable digital control of a
variable voltage. Digital potentiometers are also known as digitally
adjustable potentiometer, digi-pot or digital programmed
potentiometer. This is an integrated circuit chip which works by means of
programming. These potentiometers are used for the adjustment of the pulse width
of an oscillator. Digital potentiometer consists of a ladder resistances
and it allows access to each end of the ladder through two pins as a high and
low. The high pin is numbered 1 and the low pin is numbered 1. These
potentiometers are designed for 5v supply or less.
Most microcontrollers contain one or more analog
digital converters that convert analog input into numeric value. Microcontrollers
cannot create analog output but the problem is that these converters are operated
on low currents.
An Up/Down digital potentiometer is controlled by the
pair of push buttons. One push button increases its resistance value and the
other push button decreases its resistance.
The resistance of microcontroller /
digital potentiometer increases and decreases step by step. Once pressing the
push button, the resistance increases slightly and same as decreasing. These
push buttons are not so successful and a rotational encoder is used
which emits stream of pulses when its shaft is turned. In this case, another
component is used to interpret the pulse stream and change it to a format that
the digital potentiometer can understand.
Advantages of Digital Potentiometer
These potentiometers are reliable because these are used as many
times and these are capable of adjustments of voltage.
These potentiometers have digital interface and works properly
and so fast.
Digital potentiometers reduce capacitive affects from signals.
These potentiometers are weightless in size.
Disadvantages
of Digital Potentiometer
Its internal resistance is affected by the temperature.
It cannot pass significant current from it.
Users prefer a rotational knob rather than the push buttons or
rotational encoder.
Trimmer Potentiometer
Trimmer potentiometers
are one of the applications of potentiometer. This potentiometer is used in
electronic circuit boards and these are also used in electronic chips of
various electronic devices. These trimmer potentiometers are used to
provide variable voltages to various electronic components and according to
their requirements. These are very useful in electronic circuits. Trimmer
potentiometers consist of a knob which is adjusted to the requirement of
various input values. These also uses cermet material resistive element.
Types of Potentiometers with respect to
Measurement
DC Potentiometer
A DC Potentiometer is used for the measurement of EMF’s
of different cells and for calibrating the voltmeters, ammeters, wattmeters
etc. These potentiometers consist of a germen silver or manganin wire. This
wire is one meter long and it is stretched between two terminals. This wire is
connected with Rheostat in series. As shown in figure below:
Measurement of EMF by DC Potentiometer
The switch and the slide wire are set to the standard cell
voltage. The standard voltage are 1.01 volt. The switch S is in calibrate
position and the galvanometer switch K is pressed when the rheostat is adjusted
to zero. A 10Kw resistance is included in a circuit to protect the Galvanometer
from overloading. When the null deflection on the galvanometer is come near,
then the protective resistance is shorted to increase the sensitivity of the
galvanometer. The rheostat is adjusted for the deflection of galvanometer. Now,
the switch is closed to connect the unknown emf with the protective resistance
in the circuit. The potentiometer is adjusted by means of a main dial and a
slide wire. The balance is obtained and the value of unknown EMF is measured.
Applications of DC Potentiometer
DC Potentiometer
is used for the measurement of various electric parameters and for the
calibration of different instruments. The uses of DC potentiometer are
described below:
- Measurement of Electric Current
- Measurement of Voltage
- Measurement of Resistance
- Measurement of Electric Power
- Calibration of Ammeter
- Calibration of Voltmeter
- Calibration of Wattmeter
So, let’s discuss all these measurements and calibrations of
instruments with the help of DC Potentiometer one by one:
Measurement of Electric Current
In this method, the current which is to be measured by the DC
potentiometer is passed through the standard resistance / resistor R as shown
in figure. The standard resistor should be of a value by which voltage drop are
caused with the flow of electric current which is to be measured and its value
may not be exceeded to the value of the potentiometer. The following formula is
used to measure the unknown electric current in amperes from this
circuit which is shown in figure is given below:
Measurement of Voltage
In this method of measurement, DC potentiometer measures the
high voltage. These high voltage are measured by the volt-ratio box with the
potentiometer. This box consists of simple resistance with the various tapping
on its input side as shown in the figure. Each input terminal has its own
maximum voltage and these are corresponding to the multiplying factor for the
voltage scale.
High voltage are applied to the input terminal of the volt-ratio
box/voltage divider and these high voltage leads to the potentiometer through
the voltage divider and the two points are taken by the potentiometer from the
volt-ratio box. The high voltage are measured by the potentiometer. The formula
to measure the high voltage is given below:
Where, v are the voltage measured by the potentiometer and k is
the multiplying factor of the volt-ratio box.
Measurement of Resistance
The resistance which is to be measured is connected in series
with the standard resistor. The rheostat is connected in this circuit to
control the electric current and an Ammeter is also connected in series with
the rheostat and it is used to indicate the value of current is within the
limit of the potentiometer or not. The two pole double throw switch is
connected between the unknown resistance which is to be measured and the
standard resistance. When this switch is put into 1 position, the unknown
resistance is connected to the potentiometer. The reading of the potentiometer
is in VR. then,
And when the switch is thrown to the 2 position, the standard
resistance S is connected to the potentiometer. This reading of the
potentiometer is in VS then the value of unknown resistance is
measured correctly. The formula is given below for the measurement of
resistance:
Measurement of Electric Power
In this method, the standard resistance is connected across the
load. The voltage drop are divided across the standard resistor then the load
current will be,
Where VS are voltage drop across standard resistor as
measured by the potentiometer. The total power consumed in a circuit is given
below in the form of formula:
Calibration of Voltmeter
In this calibration, a stable DC voltage supply is used and a
potential divider network is used which consists of two rheostats. One rheostat
is used for coarse and the other is used to control the calibrating voltage. These
controls are used to adjust the supply voltage. The volt-ratio box steps down
the voltage across the voltmeter. These voltage are suitable for the voltmeter
reading. The potentiometer measures the accurate value of voltage and if the
reading of the voltmeter and potentiometer does not match with each other then
there will be opposite connections of the voltage supply in the circuit.
Calibration of Ammeter
In this method, standard resistor is connected in series
with the ammeter. This resistor
carries high value of current. The voltage
drop and the current is measured across the standard resistor with the help of potentiometer
by dividing the voltage across the standard resistor with the standard resistor
value. The following formula is used to measure the current from potentiometer:
Where VS is the voltage across the standard
resistance and S is the value of standard resistance.
Calibration of Wattmeter
In this method of calibration, the current coil of the wattmeter
is connected to the low voltage supply and the voltage coil is connected
to the normal supply through the voltage divider. The voltage across the
voltage coil is measured by the potentiometer directly. The current of
the current coil of wattmeter is measured by measuring the voltage drop across
the standard resistance.
The power will be VI where V is the voltage across the voltage
coil and I is the electric current through the current coil of the wattmeter.
AC Potentiometer
The working principle of AC Potentiometer is same as the
working principle of DC Potentiometer. The main difference in AC and DC
Potentiometer is that the DC Potentiometer measures the magnitude of EMF by
standard cell but the AC Potentiometer measures the magnitude and the phase
angle of the unknown voltage to compare the achieved balance.
Classification of AC Potentiometer
AC Potentiometers are classified into its two types which are as
follows:
Polar Potentiometer
In Polar Potentiometer, the EMF is measured in polar
form. In other words, the EMF is measured in terms of its magnitude and its
phase angle Θ. The magnitude and the phase angle of the EMF is indicated by separated
scales. Polar potentiometers are classified into its one type:
Drysdale Potentiometer
In this potentiometer, a slide wire S is used and the terminals
of slide wire S1 and S2 is supplied from a phase shifting circuit for AC
measurement. The voltage supplied by the phase shifting circuit remains
constant and the slide wire current is constant but it varied in phase. The
phase shifting circuit consists of two stator coils which are connected in
parallel to the same supply. A mechanism is used to differ these currents by 90o in phase shifting circuit.
The two windings produce rotating flux and induces a secondary emf in the rotor
winding of a constant magnitude. The phase of the rotor EMF is observed from
circular dial which is attached in the potentiometer. The potentiometer is
calibrated by the DC Supply for slide wire and standard cell for test terminals
T1 and T2. The unknown AC Voltage are measured across the
test terminals. The ammeter is connected to the slide wire circuit and it gives
the magnitude of the unknown emf and the circular dial in the rotor circuit
gives the phase angle of it.
Coordinate Potentiometer
This potentiometer measures the unknown EMF in certasian form. The
two components along and perpendicular to some standard axis which are measured
by two different scales which are known as in phase V1 and quadrature V2
scales. The formula to find the unknown voltage by this potentiometer is given
below:
These potentiometers are classified into its one type:
Gall Coordinate Potentiometer
Gall coordinate potentiometer
consists of two separate potentiometer circuit. One is the In Phase
Potentiometer and the other is the Quadrature Potentiometer. The two
currents are supplied to these two potentiometers. The slide wires of these two
potentiometers are balanced to obtain the value of unknown voltage. The
magnitude of the unknown voltage is measured by the following formula:
And the phase angle of the unknown EMF is:
R and R’ are the two Rheostats used to control the two
slide wire currents. The slide wire of In phase potentiometer is
supplied from a single phase supply and the quadrature potentiometer is
supplied from a phase splitting device. These supplies create the phase
difference of 90o between the two slide wire currents. The two Transformers
T1 and T2 are used in this circuit. These transformers are used to isolate the
potentiometer from high voltage. R and C are the variable resistance and
capacitance for phase splitting purpose. VG is the Vibration Galvanometer
and K is the galvanometer key. SW1 and SW2 are the two sign changing switches
which are necessary to reverse the direction of the unknown EMF applied
to the slide wires. SW3 is the selector switch and it is used to apply the
unknown voltage to the potentiometer. Before using the potentiometer for the
measurements of AC, the current in the In Phase Potentiometer slide wire is
standardized using the a DC Cell. The vibration galvanometer is replaced by the
D’Arsonval Galvanometer. For AC calibrations, the DC supply is replaced
by the AC Supply and the D’Arsonval Galvanometer is changed by Vibration
Galvanometer.
The magnitude of the quadrature potentiometer slide wire is
equal to the In Phase potentiometer slide wire current and the two currents are
in quadrature. The switch SW3 is placed by the test position and the EMF induced
in the secondary winding of mutual inductance is overwhelmed across the In
Phase Potentiometer wire through the vibration galvanometer. The induced EMF in
the secondary of mutual inductance M is equal to the 2pf Mi volt in magnitude.
Where f is the supply frequency, I is the current in the
quadrature slide wire, then the value of EMF is calculated as e’ = 2pf Mi. the
polarity difference between the two circuits is corrected by changing switches
Sw1 and SW2.
Advantages of AC Potentiometer
By using the volt-ratio box, these potentiometers measure wide
range of voltage, current and resistance.
It is able to measure the phase as well as magnitude of two
signals and it is used to measure the Inductance, Power and phase
angle of a coil.
These AC Potentiometers are used to measure errors in Current
Transformer (CT).
Disadvantages of AC Potentiometer
The mutual inductance is affected by the magnitude of the
current of quadrature wire.
Inaccuracy is occurred in the measurement of frequency value.
Harmonics are produced in the input signal which occurs
operating problem and the vibration galvanometer turned to the fundamental
frequency which will not show full null position at all.
Applications of AC Potentiometer
The following are the major applications of AC Potentiometer:
Measurement of Self - Inductance
In this method of measurement, a non-inductive resistor
is connected in series with the coil and two potential differences V1 and V2
are measured in magnitude and phase by the potentiometer. The voltage drop
across the standard resistor RS, V2 = IRS
Where,
I = current flowing through the circuit,
RS = resistance of the standard non-inductive
resistor
Voltage drop across inductive coil = V1
Phase angle between voltage and current through the coil = Θ
Voltage drop due to the resistance of coil IR = V1
cos Θ
Voltage drop due to inductance of coil, IwL = V1 Sin
Θ
Calibration of Ammeter
In the method of calibration of AC Potentiometer, the AC
Ammeter is connected in series with a non-inductive variable resistance.
The non-inductive standard resistor and a voltage drop across standard resistor
are measured by AC potentiometer.
Calibration of Voltmeter
This method of calibration of AC voltmeter is similar to the
calibration of DC voltmeter by using AC potentiometer.
Calibration of Wattmeter
This procedure is same as the calibration of wattmeter by DC
Potentiometer. The current coil of the wattmeter is supplied through the stepdown
transformer and the voltage coil is supplied from the secondary winding of
a variable transformer whose primary winding is supplied by the rotor of
a phase shifting transformer.
Difference Between Rheostat and Potentiometer
Rheostat
are the variable resistors while the potentiometers act like voltage
divider. The potentiometer provides variable voltage on its output while
the rheostat is different from potentiometer and provides variable resistance
on its output. A rheostat consists of 2 terminals while the potentiometers
consist of three terminals and a sliding wiper. In rheostat, one terminal is
connected to supply while the other contact is open and is used for output
variable resistance with rotation but in potentiometer, two terminals of
resistive element are connected to the supply and one terminal provides output
variable voltages by rotating it.
Resolution of Wire-wound Potentiometers
Potentiometers are of two types according
to its resistive element. These are wire-wound type and non-wire wound type. The
resolution effects occur in wire wound potentiometers only and non-wire wound
potentiometers are free from these effects. The resolution is used to measure
the changings in output value of the potentiometer. So, lets discuss the
resolution effects of wire wound potentiometers.
Resolutions are of three types and these all three types are
discussed below:
Theoretical Resolution
This resolution is for the linear wire-wound potentiometers and
it is also named as nominal resolution. The linear wire-wound potentiometers
consist of a movable contact which is used to set for any turn of the resistance
wire. Assume that N is the number of active turns in the resistive element, then
the theoretical resolution in percent is given by:
The active turns of the resistive element represent the total
resistance. Higher active turns of resistance in potentiometers have better
theoretical resolution. The better theoretical resolution is the low
theoretical resolution. Higher resistance values potentiometers have lower
theoretical resolution.
Travel Resolution
Travel resolution is the maximum movement of the mechanical
input in one direction. It is required to add incremental step in the output
voltage. It is specified in degrees in case of rotating input and it is
specified in inches in case of linear actuating shaft. The output of these wire-wound
potentiometers are in a staircase pattern as shown in figure. Theoretical
resolution and travel resolution are the output response of the wire-wound
potentiometers.
Voltage Resolution
It is defined as the huge incremental change in output voltage
in any portion of resistive element with the movement of actuator/ shaft of the
potentiometer. This resolution is applied to wire-wound potentiometers only. A
circuit is shown in figure. As shown in figure the voltage source of 10v is
applied to the potentiometer as input
voltage.
Conformity of Potentiometer
Conformity is defined as the application of potentiometer that
requires output voltage of a nonlinear potentiometer with respect to input
voltage. Its formula is
Eo is the output voltage, E1 is the input voltage and
f (Θ) is the theoretical output function of the potentiometer. This function is
specified with deviation from the theoretical function. This deviation of the
output curve is conformity.
The figure below demonstrates the factors which affect the
conformity of potentiometer.
Mechanical Travel
Mechanical travel is the amount of angular input rotation Θm
which is necessary to move the wiper from one end to the other end. The output
ratio of various wiper positions along mechanical travel is measured as shown
in figure (B) above.
Electrical Travel
This is the amount of angular input rotation ΘA on
which the output ratio actually varies. This range is observed by measuring the
high and low end of points on the curve as shown in the above figure (C).
Theoretical Electrical Travel
It is the amount of angular input rotation ΘT. It is
defined as the operational range of the potentiometer. This travel range is
between the high and low theoretical end points as shown in figure (D). Mathematically
its formula is given below:
Where K is the conformity, Θw is the wiper position
and ΘT is the theoretical electrical travel.
Absolute Conformity
Absolute conformity
is defined as the total applied input voltage and voltage measured over theoretical
electrical travel. It is maximum vertical deviation of the actual response
from the theoretical curve. The following figure shows the absolute conformity.
Applications of Potentiometers
Potentiometers
are used in various field and for various purposes. Potentioimeters have
different function which are used in a vast range of electric circuits for the
purpose of controlling, operating, measuring any value etc. Some of the
applications of the potentiometers are given below:
Power Supplies
Potentiometers are used in power supplies for the adjustment of
output voltages for various devices or circuits connected to the power
supplies. Potentiometers are mostly used in variable Supplies for the adjustment
of output voltage of supplies according to the load requirements. Potentiometers
are also used for limiting the current in power supplies.
Operational Amplifiers
Potentiometers are used for the adjustments of voltage to set
the gain of the operational amplifiers. These amplifiers are used for
the adjustment of frequency of various channels, radios and for the
audio control systems. These amplifiers are also used to increase the
capacitance of various circuits with the help of potentiometers.
Digital Circuits
Trimmer potentiometers
are used in all the digital circuits to adjust the time delay, clock
frequency, thresh hold levels etc. These potentiometers are also used in integrated
circuits for the use in digital circuits. 555 IC timer is the
example of the time delay due to trimmer potentiometers.
Electronic Instruments
Trimmer potentiometers play a vital role in the electronic
circuits for their use. Trimmer potentiometers are used in communication,
computer, medical, manufacturing and in automotive instruments. Trimmer
potentiometers are used in digital voltmeters to control the power supply, zero
adjustment, amplifier gain control. Signal generators require oscillators, timing
circuits, triggering circuits and trimmer potentiometers provide all these characteristics
in signal generators. Trimmer potentiometers are also used in oscilloscope to
control the power supplies, amplifiers, timing circuits etc.