Topic: A review of variable resistors for different application in
measurement system
1. Introduction to Measurement System(PHASE I)
1.1 Definition and Importance
1.2 Basic Functional Elements
1.3 Types of Measurement Systems
2. Resistors(PHASE I)
2.1 Introduction
2.2 Ohm’s Law and Joule’s Law
2.3 Importance of Resistors in Measurement Systems
3. Variable Resistors(PHASE II)
3.1 Introduction
3.2 Types of Variable Resistors
3.3 Specification of Variable Resistor
3.4 Applications of Variable Resistor
4. Variable Resistors in Measurement Systems(PHASE II)
4.1 Objectives of using Variable resistors
4.2 Devices using Variable Resistors
4.3 Application of Variable resistors in Everyday life
5. Evolving Use of Variable Resistors(PHASE III)
5.1. Recent research trends regarding variable resistors
5.2. New domains of application for variable resistors in
measurement systems
Conclusion (PHASE III)
Bibliography and References (PHASE III)
Introduction to Measurement Systems
❖ Definition and Importance
Measurement system refers to an assembly of component(s) meant for
the purpose of gathering data in a standardized manner for various
purposes such as data analysis, input to machines, experimentation, etc.
Some of the various applications signifying importance of measurement
system are as follows:-
1. For Process or Operation Control- Data obtained through
measurement is critical in deciding the state and magnitude of a
process or operation
2. Simulation of System- The measurements of features of a system
aid –in conducting virtual trials or experiments as cost-effective
measures.
3. Experimental Studies- The experimental studies involve
establishing parameter dependence of a subject under observation
and thus, measurement of parameters is indispensable part of
experimentation
4. Quality Control- Product quality is ensured by defining standard
parameters which are measured for all products to keep them all
nearly identical in performance.
5. Product Specification, Testing and Maintenance- Most features
of product are measurement quantities and hence measurement
systems are of utmost importance for its testing and maintenance
and description(specifications)
6. Verification Studies- In order to verify theoretical results,
parameter measurement is required to identify validity constraints
of theoretical sciences.
❖ Basic Functional Elements of a Measurements
1. Transduction Element
2. Signal Processing Element
3. Data Presentation Element
Fig 1: A Schematic for basic functional elements of measurement
system
Resistors
Resistors are electrical circuit elements which have the ability to hinder
the flow of electric current. This property is called resistance.
● A resistor is a two-terminal device that is used to resist the flow of
current. It is one of the most commonly used components in
electrical circuits.
● Resistance of any resistor is described in ohms. Ohm is denoted by
the Greek letter omega (Ω). More the value of resistance more is
the capability of resisting the current.
● Resistance will be considered as 1 ohm if the potential difference
between the two ends of the conductor is 1 V
and a current flowing through it is 1 Ampere.
● Resistance can be derived from Ohm's law
which states voltage is directly proportional to the current flowing
through the conductor.
V= I * R
A resistor dissipates power when a current passes through it. The energy
is released in the form of heat. The power is a function of the current I
and the applied voltage V:
where P is the power in watts. Combined with Ohm’s law, the
power law can be rewritten into:
● The process by which the passage of an electric current through a
conductor produces heat is Joule heating, also known as resistive,
resistance, or Ohmic heating. Joule's first law, also known as the
Joule–Lenz law states that heat energy generated by an electrical
conductor is proportional to the product of its resistance and the
square of the current.
● Each resistor comes with two wires, also called as leads. Between
these two leads there lies a ceramic part which actually resists the
flow of current. Resistor consists of three colored strips that
indicate the value of resistance.
● Some resistors come with four colored strips. In such cases, the
fourth strip indicates the value of tolerance. Tolerance is the value
of the deviation of resistance from its given value on the resistor.
Gold color of the fourth strip indicates tolerance is 5% and silver
color indicates tolerance is 10%. Where there is no forth strip,
tolerance is considered as 20%. Suppose, if resistance has 50-ohm
resistance with no forth strip. Then tolerance of such a resistor can
be 50 ±20%.
● Resistance of any resistor also depends on its resistivity, its length
and cross-sectional area.
● Resistors also indicate temperature coefficient. Temperature
coefficient is known as a resistance due to the change in
temperature. There are two types of temperature coefficients.
Positive temperature coefficient and negative temperature
coefficient. If resistance increases with the increase in temperature
then it is called positive temperature coefficient and if resistance
decreases with the decrease in temperature then it is called
negative temperature coefficient.
❖ Importance of Resistor in a Measurement System
Resistors are the basic components in almost all electrical or electronic
circuits. Resistors control the amount of current that flows through them.
They control the voltage in individual components connected to them.
Without resistors, the individual components can’t handle the voltage
and might result in overloading. Hence, a measurement system without
resistors would result in the weak and unreliable instruments with
frequent fault occurring in them. To overcome all these problems,
resistors are obligatory for the measuring instruments.
Input Transduced Conditioned Data Outpu
Transducer Signal Intermediat signal
presentatio t
Element e element n element
Resistors are used in circuits for various purpose,
usually in most of the elements to reduce current
flow, adjust signal levels, use resistance as a electrical
output for the next element , among other uses.
Fig 3: A Schematic of importance of resistors in measurement
system
❖ Resistors used in Transducer elements
Transducer elements convert the input physical variable to usable form .
In most cases, it is in the form of an electrical signal. This is the reason
why we use resistors in most of the places in transducers. They are
connected in electrical circuits as per their requirement. So, here we will
be discussing the purpose of resistors used in some of the transducing
elements.
In Potentiometric Resistance type transducers, we use
resistors to measure effective resistance due to the motion of the object
and hence the voltage output is directly proportional to dynamic
displacement of the moving object.
In Resistance strain gauges, the principle of working is that
if a conductor is stretched or compressed, its resistance will change,
because of change in its length, area and resistivity and hence resistance
varies with applied force; It converts force, pressure, tension, weight,
etc., into a change in electrical resistance which can then be measured.
Even in Opto-Electrical Transducers, which convert a light
beam into an electrical signal , we use resistors in parallel to cathode and
anode to measure the output and hence among other transducers also we
use resistors as per their requirement.
❖ Resistors used in Intermediate Elements
The output signal of any transducer usually needs to be modified by
elements known as intermediate elements, such as amplifiers,
compensating devices, differentiating elements or integrating elements,
filters etc. These elements usually require electrical setup and hence
resistors play a key role in each of them.Some of the intermediate
elements in which resistors are used are discussed here.
In Electrical Amplifying Elements, the magnitude of the
signal from a transducer is increased so that it can be displayed or
recorded and hence for the gain of the signal, for example voltage gain
we adjust the resistors in accordance to the desired output.
Filters are meant to remove unwanted signals from the
desired transducer signal so as not to obscure the effect of the latter.
Resistors are used in filters for different purposes such as in electrical
circuits for controlling current flow or with resistance strain gauge for
filtering the unwanted noise signals.
In A-D and D-A Converters, the potentiometer resistances are
used to mark reference potentials according to binary values assigned
and that's how we convert an analog signal to digital form or vice-versa
especially where a digital computer has to be used. Similarly
differentiating and integrating elements, and many other intermediate
elements require resistors.
❖ Resistors used in Data Presentation Elements
The final stage in the measurement system comprises an indicating
and/or a recording element , which gives an indication of the input being
measured. These elements also require resistors for circuit purposes.
Some of these elements are discussed here such as potentiometric type
DVM , Cathode ray oscilloscopes (CROs) and other elements.
Potentiometric Type DVM convert analog signals into digital
presentations which may be an indicator or may give an electrical digital
output signal. DVMs measure input signals with the voltage obtained
from an internal reference, which is applied to a potentiometer. The
position of the slider on the potentiometer indicates the voltage and
hence the signals are displayed.
Cathode Ray Oscilloscopes (CROs) are essentially a high input
impedance voltage measuring device, which indicates voltage signal
from the intermediate elements as a function of time. Here clearly
resistors are used as high input impedance.
Even in display devices such as LCDs, LEDs etc., resistors are
used for current flow control. For example, the larger the resistor the
dimmer the backlight on LED and hence resistors are the key electrical
components used in various electrical elements used in measuring
systems.
The applications of resistors in measurement system include the
following:
● High-Frequency Instruments
● Oscillators
● Voltage Regulators
● Medical Instruments
● Digital Multimeter
● Amplifiers
● Telecommunication
● Modulators and Demodulators
● Feedback Amplifiers
All these applications indicate how resistors play an important role in
each element of the measurement system .
All electrically conductive materials are somewhat resistive, too.
Because of this, even a good electrical conductor, such as metal wire,
can be used as a resistor.
Economic Advantage: Some metals, such as iron, tin, or platinum, do
not conduct electrical current very well because of their higher
resistance values and other metals, such as gold, silver, and copper, are
excellent electrical conductors and have a lower resistance value . Most
common, modern resistors are made out of either a carbon, metal, or
metal-oxide film and hence out of all other alternatives, resistors are
more economically reliable to use .
Availability: The components such as nichrome, ceramic, plastic, or
fiberglass core are commonly used to make resistors which makes them
more available to the market at cheaper cost.
Ease of Application: Resistors are usually added to circuits where they
complement active components like op-amps, microcontrollers, and
other integrated circuits Resistors are used in series or parallel
combination and any value of resistance can be created by using series
or parallel combinations which makes the application of resistors more
easy due to developed scientific studies of its value and combinations
and laws related to it.
Variable Resistors
Variable resistors are integral to most electronic circuits and are
used extensively as two or three terminal devices for calibration
purposes. They have the ability to change their resistance as per
requirement in circuits.
Variable Resistors are also called linear variable resistors due to
their linear voltage-current characteristic. And also to differentiate
Fig 4counterpart
: Classification Of Resistors
them from their similar varistors which behave non-
linearly.
❖ Variable Resistors and Varistors
Variable Resistors which can automatically vary their resistance as
a function of voltage are called “varistors”. Variable Resistors
require manual action to vary resistance unlike varistors. Varistors
are quite useful for device protection by suppressing high voltage
in transient systems.
❖ Types of Variable Resistors
1. Variable Wire-wound Resistors
� A resistance wire (nichrome usually)
is uniformly wound on a circular ring
made up of ceramic.
It has two fixed terminals and one
variable arm. The variable arm
makes contact with the wound wire
Fig 5: A variable resistor
and is moved to vary resistance as
per requirement.
� Range: 1 Ω to 150 kΩ.
� Power Ratings: 3 to 200 W
2. Carbon Composition Variable Resistor
Fig 6: Carbon Composition Variable
Resistor
� The resistance element consists of a thin carbon coating with
filler and binder on a circular ring of pressed paper or moulded
circular discs. The variable arm (slider arm) is connected to two
fixed terminals and the resistance element as shown in the
adjacent figure where,
A & B represent fixed arm
C is connected to variable arm
S is the slider (variable) arm
� Range: 100 Ω to 10 MΩ
� Power Ratings: 0.5 to 2.5 W
3. Potentiometers or POTs
� A three-terminal variable resistor in which two terminals are
fixed and the third is variable (wiper/slider) terminal.
The relative motion between shaft and wiper along the shaft
axis leads to continuous variation in resistance. They are of two
types: Linear and Angular, as shown in the above figure.
� Potentiometers are used as trimmers in electronic circuits to
calibrate equipment after manufacturing, by adjusting
(trimming) the resistance of circuits.
❖ Specifications of Variable Resistor
� The specifications are defined in the following syntax:-
(Resistance value in numeral) Ω (Type of Scale)
For linear scale as found in straight track resistors, LIN symbol
is used, e.g. 10KΩ LIN => A 10 kΩ(kiloohms) variable resistor
with linear scale.
For logarithmic scale as found in circular track resistors, LOG
symbol is used, e.g. 5M Ω LOG=> A 5 MΩ(megaohms) resistor
with logarithmic scale.
❖ Applications of Variable Resistors
1. Potentiometers : The potentiometer, commonly referred to as
a “pot”, is a three-terminal mechanically operated rotary
analogue device which can be found and used in a large variety
of electrical and electronic circuits. The term potentiometer and
variable resistor are often used together to describe the same
component, but it is important to understand that the
connections and operation of the two are different. However,
both share the same physical properties in that the two ends of
an internal resistive track are brought out to contacts, in addition
to a third contact connected to a moveable contact called the
“slider” or “wiper”.
When used as a potentiometer, connections are made to both ends as
well as the wiper, as shown. The position of the wiper then provides an
appropriate output signal (pin 2) which will vary between the voltage
level applied to one end of the resistive track (pin 1) and that at the other
(pin 3).
The potentiometer is a three-wire resistive device that acts as a voltage
divider producing a continuously variable voltage output signal which is
proportional to the physical position of the wiper along the track.
2. Rheostat: Thus far we have seen that a variable resistor can be
configured to operate as a voltage divider circuit which is given
the name of potentiometer. But we can also configure a variable
resistor to regulate a current, and this type of configuration is
commonly known as a Rheostat.
Rheostats are two-terminal variable
resistors which are configured to use one
end terminal and the wiper terminal only.
The unused end terminal can be either left
unconnected or connected directly to the
wiper. They are wirewound devices which
contain tight coils of heavy duty enamelled
wire that changes resistance in step-like increments. By
changing the position of the wiper on the resistive element, the
amount of resistance can be increased or decreased thereby
controlling the amount of current.
Then the rheostat is used to control a current by changing the
value of its resistance making it a true variable resistor. The
classic example of the use of a rheostat is in the speed control of
a model train set or Scalextric where the amount of current that
passes through the rheostat is governed by Ohm’s Law. Then
rheostats are defined not only by their resistive values but also
by their power handling capabilities as P = I2*R.
3. Presets : Presets are just smaller versions of a variable resistor.
They can be easily placed on a PCB and can be adjustable when
needed. The value of resistance is commonly adjusted with the
help of a screw-driver. They are mostly used in applications like
adjusting the frequency of an alarm tone or to adjust the
sensitivity of circuits. Since this device is the cheapest among
all the three they are more commonly used amongst all of them.
There are also highly precise presets which have multi turn
options. In this type, the resistance will increase/decrease only
slowly and hence the screw has to be rotated many times. Here
also the basic slider and track mechanism is used. The track
mechanism is always linear. Take a look at the preset symbol.
Most of the variable resistors are placed directly on the PCB. Some are
mounted by drilling a hole in the case containing the circuit and is
connected to the terminals with the help of a wire.
In recent years, the size as well as the weight of such devices have been
greatly reduced which make them more suitable for any field of
electronic applications.
Variable Resistors in Measurement Systems
Variable resistors find their application in electrical and electronic
measurement systems.
Objectives of Using Variable Resistors
1. In electronic circuits
For trimming (reducing a parameter to calibrate a new instrument)
In operational amplifiers (extensively used as signal processor)
For setting trip points (to prevent thermal overloading)
Adjusting voltage, current and frequency, etc.
2. In electrical circuits
Limiting current flow
Adjusting power input and output
Variable resistors used as Transducers
Variable resistors are used as secondary transducer in electro-
mechanical devices. That is they transform mechanical input signal
into electrical quantity.
1. Sliding Contact Devices
The resistance of a conductor is directly proportional to its length
as suggested by the below given relationship.
L
R=ρ
A
where, L is the length of the conductor
and A is the area of cross-section of the conductor
Using this principle, the sliding contact devices used for
displacement measurement (linear as well as angular) possess a long
conductor with one fixed arm and other variable arm (wiper) which
is moved along the length of conductor to determine corresponding
resistance value in the circuit or divide the resistance as shown in
figure,
2. Wire Resistance Strain Gauge
-> The elongation of wire on application of load results to change in
resistance of wire. The
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REFER ABOVE LINK AND COMPLETE 2 3 4 POINT IN THIS
SECTION SIMILAR TO POINT 1 WRITTEN ABOVE. ALSO
INCLUDE PICTURES WHEREVER REQUIRED WITH
NUMBERING.
Variable Resistors in Signal Conditioning Devices
Variable Resistors in Data Presentation Devices.
ABOVE TWO LINKS ARE TO BE COMPLETED IN THE
SIMILAR MANNER.
