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What is a Thermistor?
Thermistors are thermally sensitive resistors, which show a precise and predictable response to resistance change in direct proportion to temperature change.
There are 2 main types of Thermistor:
NTC Thermistors (Negative Temperature Coefficient) whose resistance falls when the temperature increases.
PTC Thermistors (Positive Temperature Coefficient) whose resistance increases when the temperature increases.
The differences are easily explained in the diagram below.
When would you use a Thermistor?
Typical applications for thermistors include:
Temperature Measurement
Temperature Control
Temperature Compensation
Inrush Current Limiting
Overtemperature Switches
What is an NTC Thermistor?
NTC Thermistors (Negative Temperature Coefficient) whose resistance drops when the temperature increases. When the term thermistor is used, engineers generally refer to NTC thermistors.
These are generally used for temperature measurement as their RT (Resistance – Temperature) characteristic is very well defined and can be readily programmed.
They are used across a wide range of industries and products from the miniature Semitec F Micro within catheters, to the Semitec 103AT-11.They are manufactured from metal oxide semiconductor materials encapsulated in an impermeable material such as epoxy or glass. These coatings are used to protect both the thermistor bead and wire connections mechanically whilst providing protection against humidity.
An NTC thermistor has a high resistance at low temperature, and as the temperature increases the resistance decreases rapidly. These therefore these offer the benefit of highly accurate and quick response times to very small changes in temperature.
What are the key features of an NTC Thermistor?
Highly stable and highly reliable, excellent repeatability
High accuracy (Tc ~1s)
Fast response (± 1% tolerance or better)
Compact designs – allowing them to be fitted into very tight spaces.
Low-cost – by purchasing a thermistor with an accurate RT curve no further calibration is required over the life of the product.
Interchangeability – no recalibration required when replacing thermistors.
Point matching – Thermistors can be supplied calibration at exact working temperature.
Wide temperature range (-60°C/+300°C).
Highly customisable to best fit your application.
Which applications would you typically use an NTC Thermistor?
NTC thermistors are used in a large number of temperature sensors in everyday life and include :-
Home appliances
Medical device
Smart devices /IOT products
Battery monitoring
Automotive and E-Vehicle applications
Temperature control within HVAC appliances
Fire & Security market
Office automation products
Some larger NTC disc thermistors are used for current inrush limiting as they offer a neat and cost- effective solution to surge currents arising from switching on certain electronic or electrical products, e.g. S.M.P.S.s (Switch Mode Power Supplies), LED driver circuits and halogen lamps/ heaters.
Do you offer a choice of housings/configurations?
NTC thermistors can be supplied in different configurations/housings depending on your applications:
Epoxy coated generally for lower temperature use (-60°C to +150°C)
Glass encapsulated ideal for use in high temperature applications ( -50°C to +300°C)
In temperature probe assemblies – with IP67 or IP68 ratings if required.
SMD (surface mount) packages down to 0201 size
Threaded or boss mounted, metal encapsulated temperature sensors.
What is a PTC thermistor?
PTC stands for “Positive Temperature Coefficient“. PTC thermistors are thermally sensitive resistors with a positive temperature coefficient, which means that the resistance increases with increasing temperature.
This is in contrast to an NTC thermistor whose resistance decreases with the increase in temperature.
How do you choose the correct NTC Thermistor for your application?
Ring ATC Semitec on 01606 871680.
There is no easy formula in deciding which is the best NTC thermistor for your application as there are many different factors to take into consideration. Factors to take into consideration include levels of accuracy, resistance values, shapes and size.
At ATC Semitec we have more than 25 years of experience in providing thermal solutions for industry. Give us a call today for expert technical advice.
Which Thermistors are suitable for Medical Applications?
We have an array of thermistors suitable for medical temperature sensing, with our F-Micro Miniature Thermistor having a multitude of applications due to its small size and precise design. These allow it to be easily supplied in tubes as small as 0.28mm, and combined with a high thermal accuracy of 0.14K at 37°C make it our most advanced thermistor available.
For non-contact applications, the NC temperature sensor utilises the latest FT thermistor technology to sense infrared energy with a high signal strength.
For a full range of our products suitable for medical applicaitons, click here.
What is a Thermal Fuse?
A thermal fuse (or thermal cut-off or “tco”) is a one-shot/one-time, non-resetable, temperature-sensitive device that provides ultimate overheat temperature protection in small appliances, electric heaters and electronic circuits.
Thermal fuses are fail-safe devices which activate when other safety measures fail to operate. They help prevent temperatures rising to dangerous levels, and thus stop product fires from occurring. The original fault in the appliance could have been caused by either control malfunction or customer misuse.
Thermal fuses therefore offer the ultimate thermal protection. They are single use protection devices that cannot be reset or repaired. They have to be replaced once the original fault which caused the product failure has been identified and rectified. In the case of many consumer products, this would mean replacing the complete appliance.
When would you use a Thermal Fuse?
Thermal fuses are generally used to protect electric heaters, household electrical appliances and electrical products used in hazardous areas.
Specific examples include washing machines, tumble dryers, , convector heaters, coffee makers,hair dryers laser printers and rechargeable battery packs..
When designing with a thermal fuse, you need to consider the operating current, applied voltage and the duty hours of the appliance that it will protect. The normal running temperature needs to be a minimum of 25°C below the fuse opening temperature not including any self-heating due to I²R/ current self-heating. The Th (holding temperature) and Tm (maximum temperature) values stated for the fuse should also be taken into account. This is to ensure reliable operation over the required product life, and that any IEC/EN standards they are approved to, are complied with.
What are the different types of Thermal Fuse available?
ATC Semitec stock a number of different types of thermal fuse:
Organic pellet – see SEFUSE SF/R types for details
Eutectic solder alloy – see SEFUSE SM/A types for details
Bimetal element – see PEPI H types for details
How do Organic Pellet Thermal Fuses work?
SEFUSE SF series pellet thermal fuses are housed in a metal casing. The organic thermal pellet inside responds to external high temperatures and close to its set-point, will start to melt. As the pellet melts, it triggers a spring-loaded contact mechanism which breaks the electrical circuit as shown in the diagram below;
The SF-R thermal fuse contains a sliding contact, springs, and a thermal pellet inside a metal case. When spring B is compressed, contact between lead A and the sliding contact is maintained. At normal temperatures, current flows from lead A to the sliding contact and then through the metal case to lead B.
When the ambient temperature rises to the SEFUSE operating temperature, the heat transferred through the metal case melts the thermal pellet. When the thermal pellet melts, spring A and B expand, moving the sliding contact away from lead A. The electrical circuit is opened by breaking contact between the sliding contact and lead A.
How do Eutectic Solder Alloy Thermal Fuses work?
SEFUSE SM series solder-based thermal fuses are housed in a ceramic case. The solder alloy inside responds to external high temperatures and at its set-point, will start to melt. As the solder melts, it separates to either end of the case (helped by the surrounding flux) and thus breaks the electrical circuit. See diagram below;
In the SM type, leads are connected by a fusible alloy. The current floes directly from one lead to the other. The fusible alloy is coated with a special flux.
When ambient temperature rises to the SEFUSE operating temperature, the fusible allow melts and condenses into a drop around the end of each lead because of the surface tension and the coating of special flux. The electrical circuit then opens.
How do Bimetal Element Thermal Fuses work?
PEPI H series bi-metal fuses use a snap-action bi-metal temperature-sensing element instead of a melting solder or organic pellet.
At its set-point, the specially designed bi-metal element inverts (i.e. “snaps” from concave to convex) and opens a set of contacts, thus breaking the electric circuit.
Bi-metal fuses do not suffer from ageing (e.g. shrinking of pellet fuses or sagging in solder fuses) and so are more reliable, especially at temperature settings above 200°C. These bi-metal fuses will not reset even at ambient temperatures as low as -35°C.
Also available in ½”disc (button) thermostat housings suc
h as our Asahi US-622 series.To view our full range of thermal fuses
What are Circuit Protectors?
Circuit protection devices protect the circuit from excessive temperatures, currents or short circuit in an electrical conductor. They are essentially ‘weak links’ within the circuit that are designed to fail when a fault in the system is encountered. By breaking at low currents, they limit the amount of energy that is released during electrical failure, leading to safer devices.
From PTC resettable fuses, to CRDs, to MOV’s, we have an array of different devices to that can be implemented.
To see our full range of Circuit Protection Devices, click here.
What are the different types of Circuit Protection?
There are many different types of circuit protection devices due to the multiple different ways that current can be limited in a circuit. At ATC Semitec, we have a range of different circuit protects to suit your appliance.
From our repeat use Inrush Current Limiters from Thinking TKS,
designed to protect front-end mains voltage supplies against current spikes at switch-on, to our SEFUSE Thermal Fuses from Schott which offer one-shot 
ultimate thermal protection from 70°C to 240°C, our range can tailor to your requirements.For more information, and to see our full range of devices, click here.
What is a Metal Oxide Varistor (MOV)?
Metal oxide varistors are used to control high transient voltage surges in electronics and electrical circuits to safe levels. When a voltage surge exceeds the specified voltage, the varistor supresses the voltage to protect the circuit.
In general, a varistor (variable resistor) is an electronic component with an electrical resistance that varies with the applied voltage. Also known as a voltage-dependent resistor, it has a nonlinear, non-ohmic current–voltage characteristic that is similar to that of a diode. But, unlike a diode, it has the same characteristic for both directions of traversing current.
At low voltage it has a high electrical resistance which decreases as the voltage is raised.
The advantages if using a varistor over other transient suppressors include;
Improved peak handling capability
High speed of response
High level of reliability – operating even at high temperatures
Offers cost- effective and compact solution
What are PTC Resettable Fuses?
PTC or Polymer PTC (PPTC) fuses carry current up to a threshold temperature. At this point, the crystalline polymer structure will change to an amorphous state, leading to a massive increase in resistance, preventing charge from being carried across the device.
The device will then remain in this high resistance state until the applied voltage is removed and they have cooled down, allowing the polymer structure inside to return to its original crystalline form. Hence, they are considered resettable.
At ATC Semitec, we have a range of different PTC resettable fuses for use over many currents and operating temperatures.
To have a look at our current range of devices, click here.
Where are Circuit Protectors used?
Circuit protectors offer circuit protection in devices in a manner that is far superior to using fuses. This makes them ideal for applications containing relay circuits, motors, heaters, transformers, solenoids and semiconductors.
Our CRD‘s from Semitec have a wide range of operating voltages that they can steady current over, and their small size makes them a neat solution in tight spaces.
What is a Thermostat?
Thermostats are used to control temperature and are present in many household appliances.
They are thermally sensitive switches which sense the temperature of either a heat source or a cooling device. The thermostat then controls this heat/cool source at a pre-set temperature within a predetermined differential, i.e. the difference in temperature between the thermostat switching on and off, on and off, etc.The primary function of a thermostat is to control temperature. Thermostats can either be pre-set, adjustable and are either electro-mechanical or electronic in form.
When would you use a Thermostat?
Thermostats can be used anywhere that requires heating or cooling controlled to a set temperature.This could include;
Kitchen appliances – e.g. refrigerators, ovens, kettles, etc.
Water heaters, electric showers
Air conditioning and heat pumps
Catering equipment
Home automation
What is the difference between Electro-Mechanical and Electronic Thermostat?
The most common form of thermostat is the electro-mechanical thermostat, which is often made up of a bimetal sensing element and contact switch mechanism. The bimetal element expands differentially with the change of temperature. When the thermostat reaches its set-point, the bimetal element activates the contact mechanism to stop the heating process.
Mechanical thermostats are therefore unable to control to an exact temperature but only within a temperature range defined by the thermostat. They are often used as simple On/Off switches.
Electronic thermostats use electronic sensors such as thermistors, platinum sensors or thermocouples to sense the change in temperature. Any temperature change is then transmitted back to the electronic control circuit, where a relay is activated to help control to the pre-set temperature.
Electronic thermostats are more complex and therefore more expensive but can often accurately control a set temperature to within 1°C or less. Whilst this could readily translate into money saved through energy savings, their use is often limited to ‘premium’ products and critical applications
What are Adjustable Thermostats?
Adjustable Thermostats work in a similar way to regular bimetal thermostats, but have an adjustable spindle, which can be used to accurately set the temperature that the thermostat will activate.
ATC Semitec sells an array of Adjustable Thermostats from Campini, that can be used for both air and surface sensing. With a variety of temperature ranges these thermostats can be adjusted to, they provide ideal solutions for whatever your application is.
What are the different types of Thermostat available?
ATC Semitec offers various types of thermostat depending on your application, accuracy and expected life-time of your product (duty hours).
High accuracy Matsuo MQT series precision thermostats offer unrivalled accuracy and switching differential compared to other electro-mechanical controls. They use a unique twin bimetal system to maximise accuracy, contact life and performance
Disc-Type Thermostats
Button or disc type bimetal thermostats are ubiquitous and are more widely used than any other type of bimetal thermostat. Capable of switching high loads (up to 16A/230VAC) for over 10,000 cycles and also control within a 10K temperature differential, they offer low-cost control solutions for millions of home appliances and automotive uses.
Low-Profile Thermostats
PEPI bimetal thermostats are some of the smallest available worldwide and can offer accurate control from 5°C to 200°C. Both creep-action and snap-action contact options help to make these one of the widest thermostat ranges, with control options for both low voltage and mains voltage applications.
These types of thermostat use a relatively large bimetal blade which is flexed by means of an adjusting spindle. Turning the spindle moves the bimetal blade either closer to or further away from the switching mechanism, thus creating a temperature “range”. By varying the bimetal materials and thermostat design, control from 5°C to 300°C can be readily achieved whilst switching loads of up to 16A/230VAC
What is a Thermal Cut-Out?
Thermal cut-outs (or thermal switches) are a resettable, temperature-sensitive device, which open at high temperatures, stopping current flow, and the re-close as the temperature decreases.
The opening and closing of the device is due to a bi-metallic element, which changes shape instantaneously with a change in temperature. They can either reset automatically, or manual reset options are available when an automatic and unattended restart could create a hazardous condition.
Thermal cut-outs are therefore used as fail-safe devices, preventing temperature from rising to dangerouse levels, whilst their ability to reset makes them ideal for applications where high temperatures are reached frequently, without the need to replace the appliance.
How do Bimetallic Elements work?
Bimetallic elements (or Bimetals) are comprised of 2 metals with differing thermal expansion coefficients, that are fixed together. As the element is heated, one side will expand at a faster rate than the other, resulting in unidirectional bending.
By altering the shape of the element at ambient temperature, the shape change can be made instantaneous, hence bimetals are often found as either curved disks or strips, which will ‘snap’ into shape.
What types of Thermal Cut-Out are available?
There are 2 types of thermal cut-outs that we sell here at ATC Semitec:
Snap Action Disc Thermal Cut-Outs, supplied by either ASAHI or EAW, which use a bimetallic disc.
Probe Thermal Cut-Outs, supplied by PEPI, which contain a bimetallic strip element.
PEPI’s JS Self-Hold Thermal Cutout includes an integral “self-hold” thermal and current sensitive cut-ouallowing the protector latches to open after tripping and then reset remotely. This offers the same benefit as fitting a manual-reset cut-out but without the disadvantage of having to access it.
When would you use a Thermal Cut-Out?
Thermal cut-outs are generally used in appliances that expected to frequently reach high temperatures, where the temperature needs to be carefully controlled up to a specific limit. The re-settable nature of thermal cut-outs make them ideal for repeated use in devices with cyclic thermal loading.
A prime example of this is kettles, but they are also frequently used air conditioners, motors, and an array of other kitchen appliances.
As they rely on bimetal elements, their thermal cut out temperature can vary significantly depending on the metals used and shape of the element. This results in thermal protection of devices over significant temperature ranges. The ASAHI US-622, for example, has an impressive operational temperature range of 0°C to +185°C.
What is a Temperature Sensor?
A temperature sensor is a device that collects information about the temperature of the surrounding environment by relating the changes in temperature to the change of a physical characteristic. This information can then be used to interpret the temperature of the device in an understandable form that can be ‘read’ by the device or observer.
Different types of temperature sensors are as follows:
Thermocouples
Resistor temperature detectors
Thermistors
Infrared sensors
Which Temperature Sensors are suitable for use in E-Vehicles?
With the continual growth of the electric automotive industry, at ATC we ensure that we stock a range of temperature sensors suitable for the market. E-vehicle battery sensors need to be small, accurate, robust and fast responding, and our surface temperature sensors cover all these key parameters and more.
Our range includes :
Ring Terminal Sensors, such as our top seller the STS2 Series Ring Terminal.
Flexible Film Sensors, like Semitec’s ultra thin JT NTC Thermister.
SMD NTC’s, encompassed in our Miniature SMD NTC Thermister range.
To see our full range of products suitable for e-vehicles, click here.
What different types of Temperature Sensors are there?
Temperature sensors can be split into either:
Contact Temperature Sensors – which are required to be in physical contact with the object being sensed, using conduction to monitor changes in temperature.
Non-Contact Temperature Sensors – which use convection and radiation to monitor changes in temperature of an object without the need to be in contact with it.
What are typical Temperature Sensor applications?
Temperature sensors are required in any application where temperatures need to be carefully monitored. At ATC Semitec, we have a whole range of sensors suitable for an array of applications, such as:
Miniature Medical Temperature Sensors
And many more. To see our full range of temperature sensors, click here.