Types of Resistors – Non-linear
Resistor
In the last chapter, we
have seen a type of linear resistor including fixed and Variable resistor. In
this content, we will discuss Types of resistors Non-linear Resistors.
Types of Resistors or
Classification of Resistors
A.
1.Linear Resistors
I. Fixed Resistors
B.
A. Carbon Composition
Resistor
C.
B. Wire wound resistors
D.
C. Metal Film type resistor
E.
F. II. Variable Resistors
G.
H.
A. Potentiometers
I.
B. Trimmers
J.
C. Rheostats
K.
L.
2.Non-linear Resistors
M.
N.
I. Thermistors
O.
P.
II. LDR-Light dependent resistors
Q.
R.
III. Varistors
In this content, we will
learn about Non-linear Resistors.
2.Non-linear Resistors
A nonlinear resistor is one whose resistance
depends on the voltage or the current for that particular matter. Non-linear
resistor indicates the properties of non-linearity. If you want to look at the
current-voltage relationship of a transistor, you can see that generally, a
transistor can act as a non-linear resistor.
1. Thermistors
Concept:
Thermistors are generally called as ‘Thermal
Resistor’ which are temperature sensitive semiconductors. These are variable
resistor resistance varies with respect to temperature.
Thermistors
are available in various shapes and size.
These
are available in a range of 0.5 Ω to 0.75 MΩ.
Following are the types of Thermistors
Types based
on the operating principle
1.
NTC
2.
PTC
1.
Negative Temperature
Coefficient ( NTC )
Resistance
of NTC type thermisters decrease with increase in Temperature.
Resistance
is inversely proportional to temperature.
R α 1/T
1.
Positive Temperature
Coefficient ( PTC )
Resistance
of PTC type thermistors increase with the increase in Temperature.
Resistance
is directly proportional to temperature.
R α T
Distinguish between NTC and PTC
NTC
|
PTC
|
Negative Temperature Coefficient
|
Positive Temperature
Coefficient
|
The resistance of NTC type
thermisters decrease with increase in Temperature.
R α 1/T
|
The resistance of PTC type
thermistors increase with the increase in Temperature.
R α T
|
NTC Thermisters are
composed with a sintered mixture of a semiconductor material such as metal oxides
as manganese, nickel, cobalt, iron, and uranium.
|
PTC thermistors are
manufactured from barium titanate, titanium oxide, and powdered barium
carbonate
|
Types Based
on the Size and Shape of Thermistors
1.
Disc Type
2.
Bead Type
3.
Rod Type
4.
Probe Type
5.
Washer Type
Operating
Principle of Thermistors :
Some
of the semiconductors exhibits the properties that, there is a change in
resistance as there is a change in temperature.
Construction
and Working :
Thermisters
are composed with a sintered mixture of a semiconductor material such as metal
oxides as manganese, nickel, cobalt, iron, and uranium.
Thermistors
have leads to form the connectivity with the circuits.
Bead
Type thermisters are smallest in size having the diameter of 0.015 mm to 1.25
mm.
Generally
Bead type thermisters are sealed in the tips of solid glass rods that to form
probe type thermisters.
Glass
the probe has a diameter about 2.5 mm and having the length that may form 6 mm to
50 mm.
Disc
type thermisters are made by using pressing material under the high pressure in
to cylindrical disc shape with the diameters that range from 2.5 mm to 25 mm.
As
per requirement washer types thermisters also available.
Relation
between Resistance and Temperature for the Thermisters
Where
,
R = Resistance of the thermister at T deg.K
R0 = Resistance
of the thermistor at T0 deg.K
β =
Constant that depends on the material of thermister typical value of β is between 3500 to 4500 deg.K
Temperature
operating range : -150 deg.C to + 300 deg.C
Advantages
of Thermisters :
1.
Small Size
2.
High Sensitivity for Change in Temperature
3.
These are available in various shape.
4.
There is no necessity of Cold junction compensation
5.
Fast response
6.
Low cost
Disadvantages
of Thermisters :
1.
There is a non-linear relationship between resistance and temperature.
2.
These are not suitable for wide temperature range.
3.
At higher temperature approximately 316 deg.C working is unstable.
4.
Limited process applications
5.
To avoid self-heating low excitation current.
Applications
of Thermisters:
1.
These are widely used in the application range of -60 deg.C to +15 deg.C
2.
Can be used as sensing device (Sensor) in liquid level, Pressure and in
Flow measurement systems.
3.
Measurement of Power in high frequency
4.
Used for temperature control , Temperature compensation and in Time
delay Circuits .
2. LDR - Light Dependent Resistor
What is
Light Dependent Resistor or a Photo Resistor?
A Light Dependent Resistor or a photoresistor
is a device that their resistivity is a function of the incident electromagnetic
radiation. Hence, We can be called they are light sensitive devices. These are
also called as photoconductors, photoconductive cells and simply photocells.
Generally, these are made up of semiconductor
materials having high resistance. There are many different symbols are used to
indicate an LDR, one of the most commonly used symbol is shown below in which
The arrow indicates light falling on it.
Types of Light Dependent Resistors
Intrinsic photo
resistors ( Undoped semiconductor): Generally
these are made from pure semiconductor materials such as silicon or germanium. In
these Electrons get excited from valance band to conduction band whenever photons
of enough energy falls on it and the number of charge carriers is increased.
Extrinsic photoresistors: Generally these are semiconductor materials doped
with impurities which are also called as dopants. Theses dopants create new
energy bands on above the valence band which filled with electrons. Hence that
these reduce the band gap and less energy is required in exciting them.
Applications of LDR
1. These are used as light sensors.
2. These are used whenever there is a need to
detect absences or presences of light like in a camera light meter.
3. Also can be used in street lamps, burglar alarm
circuits, alarm clock, light intensity meters, for counting the packages moving
on a conveyor belt, etc.
3. Varistor
What is a varistor?
VDR - A varistor is a voltage-dependent resistor
. The resistance of a varistor is always variable and that depends on the
voltage applied. The word ‘Varistor’ is
composed of parts of the words “variable resistor”. Resistance decreases when
the voltage increases. In case of excessive voltage increases, then their
resistance drops. This behavior makes them suitable to protect circuits during
the voltage surges.
Applications Of Varistors
Radio communication equipment transient
suppression
Surge protector power strips
Cable TV system surge protectors
Telephone and another communication line
protection
Power supply protection
Microprocessor protection
Electronics equipment protection
Low voltage board level protection
Transient voltage surge suppressor (TVSS)
Car electronics protection
Industrial high energy AC protection
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