A variable resistor is a type of resistor whose electrical resistance value can be adjusted on demand. Variable resistors are used in electronic circuits to adjust circuit resistance and thereby control voltage or current. There are different types of variable resistors that work on the same principle, including potentiometers, rheostats, photoresistors, thermistors, and humistors. Potentiometers are position sensors that can linearly or rotationally measure displacement using a wiper moving along a resistive track between two fixed terminals. Rheostats are similar to potentiometers but only use two terminals, not three. Photoresistors and thermistors have a resistance that changes based on light intensity or temperature, respectively.

1. Variable
Resistor

2. What is Variable
Resistor?

3. A Variable Resistor- is a type of resistor
whose electrical resistance valur can be
adjusted of demand. Variable resistor are
used in electronic circuit to adjust circuit
resistance as a means to control the voltage
or current within the circuit(as per ohms law).

4. Construction of a Variable
Resistor

5. When the inside of a variable resistor is
examined such as Figure 7, there is a
fixed resistance called the resistive track
which is between terminals 1 and 3.
Terminal 2 is connected to the knob and
the slider (wiper) has a direct contact
with the knob.

7. Types of Variable Resistors
There are different types of variables
resistors which all have almost the
same working principle that was
illustrated in the previous sections.

8. 1. Potentiometer
2. Rheostat
3. Photoresistors
4.Thermistor
5. Humistors

9. 1. Potentiometer
The potentiometer consists of three terminals among
which two are fixed and one is changeable. The two
fixed terminals of the potentiometer are connected to
both ends of the resistive element called track and third
terminal is connected to the slider or sliding wiper. The
slider or wiper moving along the resistive track changes
the resistance of the potentiometer. The resistance of
the potentiometer is changed when the wiper is moved
over the resistive path.

10. What POTENTIOMETER used for?
A Potentiometer is a type of position
sensor. They are used to measure
Displacement in any direction. Linear
potentiometers linearly measure
displacement and Rotary potentiometers
measure displacement and rotary
potentiometers measure rotational
displacement.

14. 2. Rheostat
The word rheostat is derived from the Greek word
“rheos” and “-statis” which means a current
controlling device or a stream controlling device.
The construction of rheostat is almost similar to the
potentiometer. Like the potentiometer, the rheostat
also consists of three terminals. However, in
rheostat we use only two terminals for performing
the operation.

15. A rheostat, you can see the wiper which moves over a
wirewound resistor. Rheostats are very similar in construction to
potentiometers, but are not used as a potential divider, but as a
variable resistance. They use only 2 terminals instead of the 3
terminals that potentiometers use. One connection is made at
one end of the resistive element, the other at the wiper of the
variable resistor. In the past, rheostats were used as power
control devices in series with the load, such as a light bulb. Today,
rheostats are not often used for power control because this is an
inefficient method. For power control, rheostats have been
replaced by more efficient switching electronics. Preset variable
resistors, wired as rheostats, are used in circuits to perform
tuning or calibration.

18. 3.
Photoresistors
The word photoresistor is derived from the combination of words:
photon and resistor. When light energy is applied to the
photoresistor, its resistance changes. The resistance of the
photoresistor decreases when the intensity of applied light increases.
Photoresistors are of two types based on the material used to
construct them: intrinsic photoresistors and extrinsic photoresistors.
The resistance of the photoresistor decreases when the intensity of
applied light increases. Photoresistors are also known as light
dependent resistors, semiconductor
photoresistors, or photoconductors.

21. 5. Thermistor
The word thermistor is derived from
the combination of words:thermal and
resistors.It is a type of resistor whose
resistance changes rapidly with a
small temperature change.the
international standard symbol of the
thermistor is shown below.

22. 5. Thermistor
Thermal resistor also is a type of
resistor whose depends on its
surrounding temperature.it either
decreases or increase with the
temperature depending on the
type of thermistor.

23. They can be found in everyday
appliances such as fire alarms,ovens
and refrigerators.they also used in
digital thermometer and in many
automotive applications to measure
temperature.

25. 5. Humistors
The name humistor is derived from the
combination of words: humidity and resistor.
Humistors are very sensitive to the humidity. The
resistance of the humistor changes with the
slight change in the humidity of the surrounding
air. Humistors are also known as resistive
humidity sensors or humidity sensitive resistors.

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