2. • A Thermistor is a type of resistor used to measure
temperature changes, relying on the change in its
resistance with changing temperature.
• Thermistors can measure temperatures across the
range of -40 150 ż0.35 C
• Typical operation resistances are in the kW range,
although the actual resistance may range from few W
to several MW.
4. • Thermistors come in two varieties; NTC,
negative thermal coefficient, and PTC, positive
thermal coefficient.
• The resistdnce of NTC therrnistors decreases
proportionally with increases in temperature.
• PTC thermistors have increasing resistance with
increasing temperature
5. • NTC Therrøistors are most commonly made tiOm the oxides OI
metals such as manganese, cobalt, nickel and copper. The
metals are oxidised thrOugh a chemical reaction, ground to a
fine ptiwder, then compressed and subject to very high heat.
Some NTC thermistors are crystallized frtirn seiuiconducting
material such as silicon and geriuanium.
• PTC Thermistors are generally made by introducing small
quantities of serniconducting material into a polycrystalline
ceramic. When temperature reaches a critical point, the
serniconductîng material forms a barrier to the flow of
electricity and resistance climbs very quickly.
6. • Electi‘ical resistance or a metal depends on the temperatui‘e.
• The basic principle of thermistors is that; change in
temperature changes its resistance, this chame can be converted
to electric› 1 signal.
7. Resistance of these types of
thermistor s increases with
the
iise in temperature.
Due to the special Resistance-
Temperature-characteristic. there
is no additional temperature
regulation or sa1'ety device
necessary while reaching high
heat-power level when using the
low resistance area
R
R
,
R
T
T
,
T
,
T
Teiiip<raøire cow cs »rutuiç tc'
Ten perøhire coitespoiuhilg to @p
=
8. • The PTC-heating element regulates the power
sensitively according to the required temperature.
The power input depends on the
requested heat output.
10. l l
T Ref
• where: T is temperature (in Kelrin),
TRef is the reference temperature, usually at room temp.
(25 C; 77 F; 298.15 K),
fi is the resistance of the thermistor (W),
RRef i8 the resistance at be/,
b is a calibration constant depending on the thermistor material,
usually between 3,1
1
1
10 and 5,0110 K.
11. • Major specifications to be considered while using a
thermistor.
Resistance tein¡ier:itiire can e: it varies from theriristoi' to thei'rni stor and
the specifications are IOVİded by the lranufactul er.
manufactui er
< Beta to e.an. e . This depends on the material bein e used for the
thermistol
12. • Thei‘rnistor is a versatile component and used in various
applications where temperature is a l'actor to be
considered.
• Depending on type of application and specific output,either
PTC or NTC therinistors are used.
• The application part is broadly divided into PTC therrnistor
applicatitin and NTC therrni stor applicatitin.
13. • They are used as resettable fit se›.
• They are used in time delay circui ts.
• PTC Ther rnistore are used in motor starting circuits.
• They are also rised in Degaussin e cii cuitl y.
• The PTC ThermiS tOr can pt Ovlde a colrbiliatioli of heater and thermostat
ili
one device
• They are used as liquid level and 14ow sensors .
15. • Merlical Applications
Fever Thermometers
Fluid temperature
è Dialysis Equipment
• Consumer/
Household
Applications
ń Burglar alarm
' Refrigeration and air
conditioning
Fire detection
Oren temperature control
16. • Thei‘rnistors have high sensitivity, better then that olÍered
by thermocouples, RTD s.
• High accuracy, -TO. 2 C ż .3 F)
• They offer a wide range oł high resistance values.
• They have a small size.
• Thcrmistors have a faster response timc then that of RTD s
17. • Limited temperature range, typically -
100
1 C -
148
02 O
F).
• Nonlinear resistance-temperature relationship, unlike RTDS
which have a very linear relationship.
• Errors can result from self excitation currents being dissipated
by the thermistors.
• They get de-calibrated on exposure to higher temperatures
18. • Based on the application, the type oł Thermistor is
decided.
• Thermistors can cost from à0.5 and above.
• Some online stores tor buying therrnistors.
19. • (Introduction to
Thermistors)
• www.thermometrics.com/assets/images/ntcnotes.pdf
• http://www.ptc-ceramics.com/principle.htm
