Shree Swami Atmanand Saraswati Institute
Of Technology
DEPT. OF ELECTRICAL
ENGINEERING SSASIT, SURAT
• 130760109011
• 130760109012
Kathiriya Krishna
Kheni Vaidehi
Submitted to
Prof.Pratik Desai
Prof.Hetal mem
7/31/2017
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Thermistor
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Basic characteristics of thermistors
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Thermistor are also temperature dependent resistors.They are
made of semiconductor materials which have a negative
temperature coefficient of resistivity.
The variation of resistance with changes in temperature is
nonlinear. Thermistors can be used to measure temperatures in the
range of -100c to 300 c.
The resistance decreases with increase in temperature, hence this is
called as the negative temperature coefficient thermistor(NTC)
Zero power resistance of thermistor
R=R0exp[ β (1/T-1/T0)]
T, T0 are ambient temperatures,
R, R0 are corresponding resistances
b is Characteristics temperature
Since thermistor is a resistor, power dissipation
P=C(T2-T1), where C is the thermal dissipation constant (mW/ºC).
This causes self-heating.
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R-T characteristics of thermistor
A common 10kOhm NTC thermistor
• It is nonlinear!!
• Temperature goes up
more charges in
semiconductor resistance
goes down! (NTC)
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Construction
• Thermistor are constructed using the material such as sintered
mixtures of metalic oxides of Mn,Ni,Co,Fe,Cu etc.
• Thermistor are available in various configuration such as small
beads,disks,rods,etc.
• The bead type thermistors have a diameter of 0.015 mm to 1.25mm
and are smallest in size. The beads may be sealed in the tips of solid
glass glass roads to form probes that are easier to mount.
• Glass probes have diameter of approximately 2.5mm and their
length varies from 2.5mm to25mm.
• The discs are made by pressing thermistor material under high
pressure in to cylindrical flat shapes with diameter varing from
2.5mm to 25mm.
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bead thermistors
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Advantages
• They are suitable for
temperature measurement,
temperature control and
temperature compensation
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• High Resolution
• Small
size,compact,rugged and
inexensive.
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Disvantanges
• Temperature versus
resistance characteristic is
non linear.
• Not suitable for wide
range operation
• Low excitation current to
avoid self heating.
• Needs external dc power
supply for its operation.
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Potentiometer
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Construction
• The resolution of the potentiometer influences the construction of
their resistor elements .
• Normally the resistive element is a single wire of conducting
material which gives a continuous step less variation of resistance as
the wiper travels over it .
• Such potentiometer are available but their length restrict their use on
account of space considerations.
• The resolution of potentiometer is dependent upon the construction
of the resistive element and in order to get high value of resistance
in a small wire wound potentiometer are used extensively.
• The resistance wire is wound on a mandrel or a cart for translational
displacement.
• For the measurement of rotational motion these mandrels or cards
are formed in to a circle or a helix.
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Principle of Working of Potentiometers
• The potentiometer is also called as pots and it one of the most
commonly used devise for measuring the displacement of the body.
The potentiometer is the electrical type of transducer or sensor and it
is of resistive type because it works on the principle of change of
resistance of the wire with its length. The resistance of the wire is
directly proportional to the length of the wire, thus as the length of
the wire changes the resistance of the wire also changes.
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• The potentiometer are used for voltage division. They
consist of a resistive element provided with a sliding
contact. The slide contact is called as wiper.
The contact motion may be linier (traditional)or rotational
or combination of the two.
The combinational potentiometer have their resistive
element in helices form and are called helipots.
The resistive body of the potentiometer may be wire
wound.On the insulated former a thin film of 0.01 mm
diameter of platinum or nickel is wound.
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A potentiometer with a resistive load, showing equivalent fixed
resistors for clarity.
The potentiometer can be used as a voltage divider to obtain a
manually adjustable output voltage at the slider (wiper) from a fixed
input voltage applied across the two ends of the potentiometer. This is
their most common use.
The voltage across RL can be calculated by
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• If RL is large compared to the other resistances (like the
input to an operational amplifier), the output voltage can be
approximated by the simpler equation:
• (dividing throughout by RL and cancelling terms with RL as
denominator)
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Advantage
• They are easy to operate.
• They are cheap.
• Their electrical efficiency is
high.
• They are useful for
measurement of large
amplitude of displacement.
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Disadvantages
• They require large force to
move their sliding contacts.
• Environmental factors
greatly affect the
measurement.
• Use in laboratories but
seldom used in industries.
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References
• Electrical and Electronic measurements and
instrument by R.K.Rajput.
• www.eme.ubc.ca
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Thank You
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