Types of Resistors – Fixed and Variable Resistors
In the last chapter, we have seen a series and
parallel circuit resistors and Temperature effect of resistance on various
types of materials. In this content, we will discuss Types of resistors Variable
and Fixed resistors.
Types of Resistors or Classification of Resistors
1.
Linear Resistors
I.
Fixed
Resistors
A.
Carbon Composition
Resistor
B.
Wire wound
resistors
C.
Metal Film
type resistor
II.
Variable
Resistors
A.
Potentiometers
B.
Trimmers
C.
Rheostats
2.
Non-linear Resistors
I.
Thermistors
II.
LDR-Light
dependent resistors
III.
Varistors
In this content, we will see about Fixed and
Variable resistors.
1.
Linear Resistors
If the voltage and current flowing through a
resistor indicates the linear properties or relationship then those resistors
called as linear resistors. It includes variable and fixed types of resistors.
I.
Fixed
Resistors
A fixed resistor having its value constant or
value of the fixed resistor cannot change.
Following are the types of fixed resistors
A.
Carbon Composition Resistor
Carbon composition
resistors are manufactured in both insulated and un-insulated form. In which
insulated form allows better heat dissipation and whereas un-insulated form always
avoids short circuit possibility to their adjacent components and metal chassis.
Construction of carbon composition
resistor
In this process of manufacturing of pressed
carbon resistors Carbon black, resin binder and refractory fillings these are
raw materials used.
These materials will be first graded then will
be mixed in proper proportion and sited.
After that resultant black powder will be
compressed into the shape of a resistor then to solidify the unit curved in an
oven.
Finally, resistors are sorted out with respect
to the resistance values and end connections will be made
Features of Carbon composition
resistors :
1. Actual value different from printed
value that means tolerance is +/- 5% , +/-10 or +/-20% .
2. Typically -55 deg C to 150 deg C –
Wide temperature range
3. Power ratings available of 1/8,1/4
, 1/2, 1 and 2W.
4. These are capable to operate up to
800 V.
Applications
1. Potential Divider
2. Radio and TV receivers
3. High Frequency and low power
applications.
4. Biasing circuits of transistor
5. Power supplies
6. Zener voltage regulator
A.
Wire wound resistors
As compared to the carbon composition resistor
the power handling capacity of wire wound resistors is much higher.
Construction of Wire wound resistor
Carbon composition resistor normally wound with
a resistance wire on ceramic tube or rod.
The value of the resistance wire is
selected that should be capable to withstand with considerable thermal shocks.
Usually, end connections wires are welded.
And to protection, these welds will be provided
by applying vitreous enamel that to protect the electrolyte action from
corrosion.
Features of Wire wound resistor
These can handle high power but windings wire
posses a large inductance, that’s why these are not used in high-frequency applications.
Applications
1. High power applications in DC power
supplies
2. High power circuit in radio and TV
receivers
3. Low frequency and high power
applications
4. Power amplifiers
Comparison chart of Carbon
composition resistors and Wire wound resistor
Sr. no
|
Particulars
|
Carbon
Composition resistors
|
Wire Wound
resistors
|
1
|
Material used
|
Carbon black, resin binder and refractory
fillings
|
Metal wire, Ceramic rod or tube
|
2
|
Size of resistors
|
Smaller
|
Depending on the Wattage
|
3
|
Wattage ratings
|
Small wattage
|
High wattage
|
B.
Metal Film type resistor
A Metal film type resistor made by the process of
depositing the thin film metal or metal oxide or metal alloy into ceramic or
glass tube.
Features or Characteristics of Metal
Film type resistor
1. 0.01 % to 10 % of Tolerance
2. Better Stability
3. Long life and better reliability
Applications
1. Those resistors are used where the need
for better stability, reliability, and long life as per its characteristics
2. Transmitter, Modulators and
Demodulators
3. Oscillators and Feedback amplifiers
4. Also used where higher demand for
endurance
All
Applications of Fixed resistors
1. Potential Divider
2. Radio and TV receivers
3. High Frequency and low power
applications.
4. Biasing circuits of transistor
5. Power supplies
6. Zener voltage regulator
7. High power applications in DC power
supplies
8. High power circuit in radio and TV
receivers
9. Low frequency and high power
applications
10. Power amplifiers
11. Transmitter, Modulators and
Demodulators
12. Oscillators and Feedback amplifiers
13. Also used where higher demand for
endurance
I. Variable
Resistors
These are the resistances whose value can be
controlled or changed as per demand or need. The resistance value can be changed in the circuit to change or control the value of voltage and current.
Types of Variable resistors
A.
Potentiometers
It consists of three terminals in
which two are always fixed and one is movable. The two fixed terminals of the
potentiometer connected to both of the ends to the resistive element and called
as track and third terminal connected to the sliding wiper. The sliding wiper
moving along the resistive track that changes the value of the resistance of the
potentiometer. The resistance of the potentiometer will be changed whenever the
wiper will move over the resistive path.
Whenever we will increase the resistance
of the potentiometer, then the current flowing through potentiometer will decrease.
Similarly whenever we will decrease the resistance of the potentiometer, the current
flowing through the potentiometer will increase.
B.
Rheostat
The word rheostat is taken from the
Greek language word “rheos” and “statis” that means a current or stream controlling
device.
A rheostat is almost similar to the
potentiometer in construction. As like in the potentiometer, the rheostat also
consists of total of three terminals. But in
rheostat always we use only two terminals for performing the operation.
The resistance of the rheostat is
always depends on the length of that resistive element or the track through
which the current flows.
If we use the terminals A and B in the rheostat
then minimum resistance will be achieved when we moves the sliding wiper close
to the terminal of A, because the length of the resistive path will decrease. But
Only a small amount of current is blocked and a large amount of current is
allowed to pass.
Applications of Variable Resistors
1. Audio control
2. Television
3. Motion control
4. Oscillators
5. Transducers
6. Computation
7. Home Electrical Appliances
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