How Thermistors Work - The Learning Circuit
Summary
TLDRIn this episode of 'The Learning Circuit,' Karen introduces thermistors, temperature-sensitive resistors that vary their resistance with temperature changes. NTC and PTC types are discussed, with NTCs decreasing resistance as temperature rises and PTCs increasing resistance with temperature. The script explains how thermistors work, their non-linear resistance changes, and their applications in temperature sensing and as fuses in various appliances. The video also touches on alternative temperature sensors like RTDs and thermocouples, comparing their capabilities with thermistors.
Takeaways
- π‘οΈ Thermistors are variable resistors that change resistance with temperature changes, derived from 'thermal' and 'resistor'.
- βοΈ There are two types of thermistors: NTC (Negative Temperature Coefficient) and PTC (Positive Temperature Coefficient).
- π NTC thermistors decrease in resistance with an increase in temperature, while PTC thermistors increase in resistance with temperature.
- π¬ The resistance change in NTC thermistors is non-linear and can vary significantly across different temperature ranges.
- π PTC thermistors come in three main types: cylisters, polymer PTCs (resettable fuses), and switching PTCs made from doped polycrystalline ceramic.
- βοΈ The beta value is used to calculate the curve of NTC thermistors and is determined using readings at two different temperatures.
- π οΈ Thermistors are used in various applications such as temperature sensing, inrush current limiting, and as fuses in circuits.
- π Everyday appliances like digital thermometers, toasters, and refrigerators often use NTC thermistors for temperature sensing.
- π Encapsulation in epoxy or glass makes thermistors waterproof, rugged, and stable against environmental factors.
- π Thermistors, while not linear, can be plotted on a curve using specific equations and are chosen based on the required temperature range and resistance.
- π‘οΈ Other temperature sensors like RTDs, thermocouples, and temperature sensor chips offer different advantages and are suitable for various applications.
Q & A
What is a thermistor?
-A thermistor is a type of variable resistor whose resistance changes with temperature changes in its environment. It is derived from the combination of the words 'thermal' and 'resistor'.
What are the two main types of thermistors?
-The two main types of thermistors are NTC (Negative Temperature Coefficient) and PTC (Positive Temperature Coefficient) thermistors.
How does the resistance of an NTC thermistor change with temperature?
-In NTC thermistors, an increase in temperature causes a decrease in resistance, and a decrease in temperature causes an increase in resistance.
What is the resistive response of PTC thermistors to temperature changes?
-For PTC thermistors, as the temperature increases, the resistance also increases, and as the temperature decreases, the resistance decreases as well.
How do semiconductors in thermistors respond to temperature changes?
-When an NTC thermistor gets warmer, the electrons become more excited and move faster, increasing the current and decreasing resistance due to Ohm's law. Conversely, when it gets colder, the electrons slow down, increasing resistance.
What is the difference between a fixed resistor and a thermistor in terms of temperature response?
-Fixed resistors maintain a consistent resistance until they reach a high enough temperature where their resistance begins to degrade linearly. Thermistors, however, experience a non-linear change in resistance with temperature.
What are the three main types of PTC thermistors and how do they respond to temperature changes?
-The three main types of PTC thermistors are cylisters, polymer PTCs (resettable fuses), and switching PTCs. Cylisters have a near-linear temperature resistance curve, polymer PTCs increase resistance as they heat up due to the expansion of plastic pushing carbon grains apart, and switching PTCs show slight NTC behavior at low temperatures but dramatically increase resistance at a critical temperature.
How is the beta value of an NTC thermistor calculated and where can it be found?
-The beta value of an NTC thermistor is calculated using readings at two temperatures, with temperature measured in Kelvin. It can be found on the thermistor's data sheet within certain temperature ranges, often listed at zero to 50, 25 to 100, and 25 to 85 degrees Celsius.
What are some common applications of thermistors?
-Thermistors are used in various applications such as temperature sensing in digital thermometers, toasters, coffee makers, refrigerators, motor oil monitoring, 3D printer hot ends, and as inrush current limiters in devices like motors and transformers.
What are the advantages and disadvantages of thermistors as temperature sensors?
-Advantages of thermistors include being cheap, durable, precise, easy to waterproof, and able to work at any voltage. Disadvantages include requiring an ADC to interpret temperature values, operating within a limited temperature range, inability to withstand extreme temperatures, and potential for self-heating and damage when handling high currents.
How do thermistors compare to other temperature sensors like RTDs and thermocouples?
-Thermistors are made of semiconductor materials and change resistance with temperature. RTDs and thermocouples, on the other hand, are made of pure metals and produce a temperature-dependent voltage. While RTDs and thermocouples can measure more extreme temperatures, they are less accurate, more expensive, have slower response times, and require an amplifier.
Outlines
π Understanding Thermistors
This segment introduces thermistors as variable resistors whose resistance changes with temperature. It explains the difference between NTC (Negative Temperature Coefficient) and PTC (Positive Temperature Coefficient) thermistors. NTC thermistors decrease in resistance with an increase in temperature, while PTC thermistors increase. The segment also discusses how semiconductors in thermistors respond to temperature changes, affecting the flow of electrons and thus resistance. It contrasts thermistors with fixed resistors, which maintain consistent resistance until a critical temperature is reached. The non-linear resistance change in thermistors is highlighted, with NTC thermistors showing varying resistance changes over different temperature ranges. The segment also covers the three main types of PTC thermistors: cylisters, polymer PTCs (resettable fuses), and switching PTCs, each with a unique temperature resistance curve. The use of the beta value equation to calculate the curve of NTC thermistors is mentioned, along with the importance of considering the Celsius to Kelvin conversion. The segment concludes with a brief on the physical protection of thermistors through encapsulation and their applications in various devices for temperature sensing and as fuses.
π Applications and Limitations of Thermistors
This part of the script delves into the practical applications of NTC thermistors as inrush current limiters in circuits with inductive devices like motors and transformers. It explains how the resistance of NTC thermistors can restrict high inrush current and how resistive heating works. The segment also touches on the use of PTC thermistors as current limiting devices or fuses due to their increasing resistance with temperature. The script compares thermistors with other temperature sensors like Resistance Temperature Detectors (RTDs) and thermocouples, noting their differences in material composition and temperature measurement capabilities. It points out the advantages of thermistors, such as affordability, durability, precision, and ease of waterproofing, but also their limitations, including the need for an ADC to interpret temperature values, limited temperature range, and susceptibility to self-heating errors under high currents. The segment advises on the selection of thermistors based on temperature application needs and the temperature resistance of different coatings. It ends with an invitation to the Element14 community for further discussion and a teaser for the next video on a fun thermistor project.
Mindmap
Keywords
π‘Thermistor
π‘NTC (Negative Temperature Coefficient)
π‘PTC (Positive Temperature Coefficient)
π‘Ohm's Law
π‘Semiconductor
π‘Resistance Temperature Detector (RTD)
π‘Thermocouple
π‘Inrush Current
π‘Resistive Heating
π‘Voltage Drop
π‘Beta Value
Highlights
Thermistors are variable resistors whose resistance changes with temperature.
NTC thermistors decrease in resistance with an increase in temperature, while PTC thermistors increase.
NTC stands for Negative Temperature Coefficient, and PTC stands for Positive Temperature Coefficient.
Semiconductors in thermistors have free electrons or holes that respond to temperature changes.
NTC thermistors exhibit a non-linear change in resistance with temperature.
Fixed resistors maintain consistent resistance until a high temperature is reached, unlike thermistors.
Cylisters, a type of PTC thermistor, have a near-linear temperature resistance curve.
Polymer PTCs, also known as resettable fuses, change resistance as the plastic expands with heat.
Switching PTC thermistors show slight NTC behavior before a critical temperature is reached, after which resistance increases dramatically.
The beta value of an NTC thermistor is used to calculate its resistance curve and can be found on data sheets.
Thermistors come in various shapes and are typically encapsulated with epoxy or glass for protection.
NTC thermistors are used in common appliances for temperature sensing, such as digital thermometers and toasters.
PTC thermistors are used as fuses and current limiting devices in circuits.
Thermistors are compared to other temperature sensors like RTDs and thermocouples, which use different principles.
Thermostats and RTDs can measure extreme temperatures but are less accurate and more expensive than thermistors.
Temperature sensor chips offer analog or digital output options and are similar to thermistors.
Thermistors have limitations such as requiring an ADC for temperature interpretation and operating within a limited temperature range.
Epoxy-coated thermistors can withstand temperatures between -50Β°C and 150Β°C, while glass-coated ones can go up to 300Β°C.
Thermistors are a cheap, reliable, and durable solution for sensing temperature in circuits.
Transcripts
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hello and welcome back to the learning
circuit i'm karen and it's time to learn
about another sensor
thermistors
[Music]
thermistors are variable resistors whose
resistance changes
based on temperature changes in its
environment the word comes from a
combination of thermal
and resistor there are two types of
thermistor
ntcs and ptcs ntc
stands for negative temperature
coefficient while ptc
stands for positive temperature
coefficient with ntc
thermistors a rise in temperature causes
a decrease in resistance
therefore a decrease in temperature
causes a rise in resistance
so warmer equals less resistance colder
equals more resistance
positive temperature coefficient or ptc
thermistors
have a resistive response that aligns
with the temperature change
as the temperature increases the
resistance also increases
as the temperature decreases so does the
resistance
in previous lessons we've learned that
semiconductors may have a free
floating electron in negative n-type
regions or holes for those electrons to
fill
in positive p-type regions when
semiconductor components are connected
to an electric current
the particles are drawn to the opposing
charges with the free electrons moving
into the holes
and continuing to be drawn along towards
the positive power terminal
when an ntc thermistor gets warmer the
electrons become
more excited moving faster the current
increases
therefore by ohm's law the resistance
decreases
when the thermostat gets colder the
electrons slow down
making it harder for current to flow
with an increased resistance
fixed resistors may also be affected by
temperature changes
but as you can see here they maintain a
consistent resistance
until they reach a high enough
temperature here about 70 degrees
celsius
where their resistance begins to degrade
in a linear fashion
the resistance changes thermistors
experience tends to be more
non-linear with ntc thermistors the
nonlinear change in resistance is not
consistent with the change in
temperature
in certain temperature ranges the
resistance may change a large amount
from one degree to the next
while in a different temperature range
the change may be less significant there
are three main types of ptc thermistors
each with a different makeup and each
experiencing a different non-linear
response to temperature changes
the first but less common type are
cylisters
due to their doped silicon makeup
cylisters have a near-linear temperature
resistance curve
which is determined by the amount of
doping used the second type
polymer ptcs are called resettable fuses
they're made of a slice of plastic
embedded with carbon grains
at room temperature the carbon greens
are in close contact with each other
forming a conductive path through the
device
as the thermistor heats up the plastic
expands
pushing the grains farther apart
increasing its resistance
pptcs have near-linear temperature
resistance curves
the majority of ptc thermistors are the
third type
which are made from doped
polycrystalline ceramic
they're called switching ptcs because at
low starting temperatures they observe
slight
ntc behaviors but once they reach a
certain critical temperature
their resistance increases dramatically
while thermistors don't have a linear
change
this equation can be used to calculate
the curve of ntc thermistors
the beta value is calculated using the
readings at two temperatures
with temperature measured in kelvin
on an ntc thermistor data sheet you can
find the beta value within a certain
temperature range
here are examples on data sheets that
list beta values at temperature ranges
of zero to 50
25 to 100 and 25 to 85.
these numbers can be put into the
equation factoring the conversion from
celsius to kelvin
to plot the points of the curve as
you've seen so far
thermistors come in a variety of shapes
their semiconductor material is
typically encapsulated with either epoxy
or glass protecting them from humidity
corrosion and mechanical stress
making them waterproof rugged and quite
stable
some thermistors are color band coated
with their resistance value
similar to fixed resistors thermistors
have a variety of uses
most often ntc thermistors being used
for temperature sensing
and ptc thermistors being used as fuses
ntc thermistors are used in everyday
common appliances that require
temperature sensing
like in digital thermometers toasters
coffee makers
refrigerators motor oil monitoring
3d printer hot ends and more
ntc thermistors can also be used in
series with a circuit
as inrush current limiters inductive
devices such as motors and transformers
experience a high inrush current when
they first turn on
when placed in series the resistance of
the ntc thermistor
can restrict that in rush current as the
device runs
the current causes the ntc to heat up
lowering its resistance
electrical current passing through a
device often causes resistive heating
when we talk about a voltage drop it is
frequently energy lost to heat
devices are limited at how quickly they
can dissipate heat
so too large of a current can cause the
device to heat up
as we've learned the resistance of a ptc
increases with its temperature
making them useful for protecting
circuits as current limiting devices or
even fuses
thermistors are only one type of
temperature sensor there are also
resistance temperature detectors or rtds
thermocouples and other temperature
sensing chips
thermocouples and rtds are very similar
to thermistors
except they are made of pure metals
rather than changing resistance
their two dissimilar metals produce a
temperature dependent voltage
which can be used to determine
temperature compared to thermistors
they can measure more extreme
temperatures measuring a temperature
range from negative 270
up to 3000 degrees celsius but they are
less
accurate more expensive have slower
response times
and require an amplifier to properly
interpret readings
there are also temperature sensor chips
of varying types with either analog or
digital output options
thermostars are cheap durable precise
easy to waterproof can work at any
voltage and basically have the same
advantages as fixed resistors
the downside of thermistors is that they
require an adc to interpret temperature
value
they operate within a limited
temperature range they cannot withstand
extreme temperatures
and when handling high currents they can
self heat and potentially become damaged
or show errors
so they are often used with low level
currents
when choosing a thermistor low
temperature applications generally use
lower resistance thermistors while
higher temperature applications
generally use
higher resistance thermistors epoxy
coated thermistors can withstand
temperatures between negative 50 and 150
degrees celsius
while glass-coated thermistors can
withstand temperatures up to 300 degrees
celsius
when it comes to needing to sense
temperature in a circuit thermistors are
a cheap
reliable and durable solution be sure to
check out my next video
where i show a fun way to use a
thermistor in a project
put a lot of heart into that one in the
meantime
if you have any questions or comments
about thermistors you can post those on
the element14 community
where you can find me as maker karen on
element14.com forward slash the learning
circuit
happy learning
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