Thermistors.All You Need To Know.
Thermistors can be used for sensing, controlling and compensating both air, liquid and surface temperatures.
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How Do Thermistors Work?
By definition the word “thermistor” is a contraction of the words “thermal” and “resistor” (i.e. they are thermally-sensitive resistors) and they generally come in 2 types either NTC or PTC thermistors. Thermistors are manufactured from a blend of metals and metal oxide materials. After mixing, these materials are shaped and fired into the desired form.
NTC vs PTC Thermistors
As can be seen by the graph, NTC’s have a resistance that decreases as the temperature increases. PTC’s conversely have a resistance that increases as temperatures increase.
NTC thermistors are commonly used for temperature sensing and measurement because their high sensitivity to temperature change enables precise temperature readings.
NTC - Negative Temperature Coefficient
NTC thermistors are so-called because of the resistance of an NTC decreases with an increasing temperature and their R-T (Resistance – Temperature) is very accurately defined. The R-T accuracy combined with the fast rate of change (compared to RTDs and thermocouples) makes them ideal to incorporate in electronics control circuitry.
Thermistors are manufactured from a blend of metals and metal oxide materials. After mixing, these materials are shaped and fired into the desired form -they can either be used in their original disc-style form or further processed with lead wires and epoxy or glass coatings to create bead-style or diode-style thermistors
Simply put, NTC thermistors are often the ‘basic’ temperature sensing elements found in a high percentage of temperature sensors assemblies and thermal probes, extensively used across numerous industries.
PTC - Positive Temperature Coefficient
Conversely, PTC thermistors have a resistance that increases as the temperature increases. They are commonly used for overcurrent protection, solid-state temperature ‘switches’ and self-regulating heating elements.
Monitoring temperature using a Thermistor
An NTC Thermistor responds to a change in temperature of its surroundings and in doing so, its resistance either lowers or increases accordingly until it stabilises at the new temperature.
How does a change in resistance measure temperature?
The resistance change of the NTC is measured by a suitable electronics circuit that is pre-programmed with the thermistor’s resistance values which it then converts into a temperature reading.
If the circuit is part of a thermometer, the new temperature is displayed. If it is part of a control circuit, the electronics activates a heater or a cooler to bring the temperature back to the set-point on the controller.
The Semitec ET Series is commonly used within thermometers due to their small size and speed of response.
Why use NTCs for temperature measurement?
NTCs are popular choice for temperature measurement due to several key advantages:
Greater resolution due to a higher resistance change per degree of temperature.
High repeatability and stability.
Excellent interchangeability.
Fast response to temperature changes due to small size.
How is temperature calculated from the resistance of a thermistor?
The resistance of a thermistor at a given temperature is determined by its material composition and the processing techniques used to manufacture it.
This information is provided by the thermistor manufacturer, either as an RT (Resistance-Temperature) “look-up” table or as Steinhart-Hart constants which can be plugged into a 3rd order polynomial.
Please note that the Steinhart-Hart constants are only relevant to a particular temperature range. This is why RT tables are often preferred as the end user can readily look up the resistance values for whichever temperature range they are contemplating.
Contact our engineers at ATC Semitec for the RT ‘look-up’ tables relating to their range of NTC thermistors and NTC temperature sensors.
What is the Time Constant of an NTC?
The thermal time constant (Tc) is the time taken for a thermistor to reach 63% of the change in temperature experienced by the NTC.
It takes approximately 5 times the time constant (Tc) to reach the final temperature the thermistor is now exposed to.
Thus, the smaller the time constant, the faster the NTC will reach its new target temperature. The Tc of a thermistor is dictated by both the ‘format’/’housing’ of the thermistor element and it’s proximity to the heat source.
The Tc of our standard NTC thermistor is approx. 15 sec (Semitec 103AT-2), however Semitec’s range of thin-film thermistors (Semitec JT), small epoxy bead (Semitec ET) or glass bead NTC (Semitec NT-4 Series) all offer a Tc of less than 1 sec.
Fast response Tcs are important when NTC thermistors are used for temperature measurement in critical situations such as medical , fire and safety, battery, and automotive applications.
What does the B value and B value tolerance mean?
The B value between two points on the RT (Resistance-Temperature) curve is simply a rough guide to its gradient and the approximate resistance values between those two points.
It is handy as an initial consideration of whether one NTC could be replaced by another.
The B value tolerance is a measure of the material accuracy of the thermistor and the tighter the B value tolerance, the less the deviation away from the manufacturers listed RT table.
As manufacturing methods and material composition significantly improved, B value tolerances reduced accordingly so that +/-1% tolerance (or less) parts are now very standard.
Advantages of Using NTC Thermistors
NTC Thermistors are a popular and versatile component that are widely used across many applications and industries for temperature monitoring and control. The most common advantages and disadvantages of using an NTC Thermistor are listed below.
NTC Thermistors - Which is the best format for my application?
Owing to the very small size that NTC thermistor chips can be manufactured to, they can then be packaged in numerous form factors, simplifying the product design engineer's task as they have a choice of formats which can then be tailored to their design or manufacturing process.
Epoxy Coated Bead NTCs
Most widely used format – NTC element is sealed within an epoxy bead, insulated and fitted with either rigid or flexible leads.
Advantages include :
- Generally lower cost.
- The flexible leads can be TPE-encapsulated to offer either IP67 or IP68 rated protection.
- Robust construction.
Our most popular epoxy bead NTCs are the Semitec 103AT-2 10K NTC , with rigid leads and the Semitec AT-4 which is the same specification but with flexible lead wires.
Glass Coated Bead NTCs
Advantages include :
- Ideal for high temperature measurement (up to 350°C).
- Smaller bead size enable placement closer to the heat source giving faster thermal response times and higher levels of accuracy.
- Give high moisture resistance, where humidity or moisture ingress could be an issue.
- Tough & robust giving superior long-term stability and reliability.
Often used in applications such as 3D printers, e-mobility sensors, home appliances and medical devices.
Available in a SMD format (Semitec FT) with fitted leads (Semitec NT-4) or axial leads (ATC Semitec ATCD)
SMD NTCs
These surface mount temperature (SMD) NTC sensors offer reliable, very cost-effective temperature control solutions for p.c.b. and/or automated applications.
In packages from 0805 to 0201, these SMDs are ideal for use where space is limited and they are required to be positioned close to the heat source.
Available with glass passivation coating to minimise effects of humidity, and for use at higher temperatures (up to 350°C)
NTC Surface Temperature Sensors
NTCs are the most common form of temperature sensing element fitted within temperature sensors.
Depending on the final application, NTCs are found within:
Ring Terminal or Ring Lug Assemblies
Used for surface temperature sensing giving fast thermal response. Provides good electrical isolation as well as good thermal insulation, helping to minimise heat-loss away from the sensor.
Ideal for the monitoring of gas or liquid temperatures. Semitec’s STS1 delivers fast thermal response along with ease of fitting.
NTC Temperature Sensor Probes
Accurate temperature sensing of liquids or gases can be achieved by our range of TPE encapsulated probes with flexible leads or specifically designed push-pull /screw fit immersion sensors.
Over-moulded NTC thermistor probes provide ultra-reliable, waterproof temperature sensor with IP67 or IP68 integrity. Our Semitec 103AT-11 series is one of the industries best selling IP67 rated temperature sensor- widely used across many industries due to their high-quality, performance and cost-effectiveness.
Situated at the very tip of the probe, the NTC thermistor delivers fast thermal response and accuracy.
What is the difference between Thermistors and RTDs?
A Pt RTD is a temperature sensor in which the resistance changes linearly with temperature. Therefore, by simply measuring a sensor’s resistance, the RTD can be used to measure temperature. The most widely used and referenced RTD is the Pt100 temperature sensor element, which displays a resistance of 100 ohms at 0°C. Thin-film Pt100 RTD sensors are manufactured by depositing a thin layer of high purity platinum onto a ceramic substrate in clean room conditions. A pattern is then etched onto the element, thus creating the electrical circuit. They are then terminated and sealed with a glass coating, which accounts for their very long term stability and durability. Whilst the Pt100 is the most common sensor, other variations are available up to 10000ohms (Pt10000). ATC Semitec stock the world leading YAGEO Nexensos range of Pt RTDs. RTDs are a very popular choice, with engineers, across many different industries due to their characteristics of long-term stability, linearity, accuracy and repeatability.
RTD Elements vs NTC Thermistors
RTD Elements
- Linear change in resistance - giving longer-term stability
- HIgher levels of accuracy
- Available in limited range of formats - RTD element with leads, SMD, and encapsulated probes
- Wide operating temperature range from -196°C and up-to +800°C
- Higher unit cost
NTC Thermistors
- Non- linear change in resistance
- Generally lower cost
- High levels of accuracy
- Readily available in a wide variety of usable formats - NTC element, SMD, with epoxy coated leads, surface mount, probes etc
- Operates within the most common temperature ranges
NTC ThermistorsApplications in Industry
The versatility of NTC thermistors in monitoring and controlling temperatures means that they are widely used across many different types of industries, in a variety of different formats. Our technical sales team at ATC Semitec have over 30 years experience in supporting engineers in their NPD or product upgrades across a wide range of industries and applications. Examples how our range of NTC thermistors can help your application - see below
Battery / E-Vehicle
NTCs are common but important components in electric vehicles and batteries playing a critical role in monitoring and controlling systems within the vehicle
In extreme conditions – NTC thermistors will help prevent catastrophic events such as battery meltdowns, motors overheating, power component malfunction and ultimately fires
Popular products include – IP67 rated (Semitec 103AT-11), compact SMD designs (Semitec FT) , ring terminal surface temperature monitoring (ATC Semitec STS2) and flexible film Semitec JT.
Medical Devices
Miniaturisation, high accuracy and fast response times mean that NTCs are ideal for use within digital thermometers, catheters and glucose monitoring devices.
Popular products include – Semitec ET series, Semitec F-Micro, Semitec JT flexible film NTC, Semitec FT ultra-thin glass SMD
HVAC Systems
High accuracy, fast response to temperature change and robustness make NTCs a common component within heating, ventilation, air conditioning systems, and showers etc.
Not only providing control for protection and safety, but also minimise energy consumption through temperature setting and monitoring.
Epoxy bead NTCs such as the Semitec ET series are ideal for use within programmable domestic thermostats.
For fast response temperature sensors for boilers etc, ATC Semitec recommend ATC Semitec BTS5 series Immersion NTC probe or ATC Semitec BTS3 pipe-clip sensor.
IP67/IP68 NTC temperature probes such as Semitec 103AT-11 are often used within heat pumps, MVHR and underfloor heating applications.
Industrial Applications
SMDs can be directly embedded into electronic circuits for real-time temperature monitoring and control.
Ultra- thin SMDs such as Semitec FT can be placed closer to the heat source enabling TCs as little as 0.2 sec.
