Points to Consider When Selecting an NTC Thermistor
This article explores NTC thermistor types and their critical performance criteria and provides advice on selecting the appropriate device for a given application.
Negative temperature coefficient (NTC) thermistors serve as high-accuracy temperature sensor elements for various automotive, industrial, home appliance,s and medical applications. Because a broad spectrum of NTC thermistors is available – created with different designs and made from various materials - selecting the optimal NTC thermistor for specific applications can be challenging. This article explores NTC thermistor types and their critical performance criteria and provides advice on selecting the appropriate device for a given application.
There are three main temperature sensor technologies, each having its own characteristics: resistance temperature detector (RTD) sensors and two types of thermistors, positive and negative temperature coefficient thermistors. RTD sensors are used mainly for measuring extensive temperature ranges, and because they use pure metals, they tend to be more expensive than thermistors.
Therefore, as thermistors measure temperature with the same or better accuracy, they are usually used in preference to RTDs. As the name suggests, the resistance of positive temperature coefficient (PTC) thermistors increases as the temperature rises. They are commonly used as temperature limit sensors in shut-off or safety circuits as, once the switching temperature is met, the resistance spikes. Negative temperature coefficient (NTC) thermistors, on the other hand, diminish in resistance as the temperature rises. The resistance-temperature (R-T) relationship is a flattened curve, making it highly accurate and stable for temperature measurement.
Figure 1. Resistance characteristics of PTC and NTC thermistors.
Key Selection Criteria
NTC thermistors are highly sensitive and measure temperature with high accuracy (±0.1°C), making them the ideal technology for measuring temperature in a wide range of applications. However, the choice of which type to specify depends on a few criteria – temperature range, resistance range, measuring accuracy, environment, response time, and dimensional requirements.
Epoxy resin-coated NTC elements types have a rugged construction and measure temperatures typically between -55°C and +155°C, while glass-encapsulated NTC elements can measure up to +300°C. For applications where extremely fast response time is required, glass-encapsulated elements are a more appropriate choice. They are also more compact, with diameters down to 0.8mm.
It is important to match the temperature of the NTC thermistor to that of the component causing the temperature change. They are therefore not only available in conventional leaded styles, but also incorporated in screw-type housings for attachment to heat sinks for surface mounting.
New to the market are completely lead-free (chip and element) NTC thermistors, which meet the more stringent requirements of the impending RoSH2 Directive.
Understanding Dissipation Factor
The dissipation factor is defined as the ratio of the change in power dissipation and the resultant change in the thermistor’s body temperature. It is expressed in mW/K and serves as a measure of load that causes a thermistor in steady-state to raise its body temperature by 1 K. The higher the dissipation factor, the more heat is dissipated by the thermistor to its surroundings.
As the lead length and material, encapsulating material, the mounting, and assembly each help determine the dissipation factor, it is recommended that prototypes are tested in a ‘real’ environment. These tests determine the maximum allowable input current to ensure a negligible self-heating error inside the thermistor while at maximum measuring/controlling temperature. However, there is a delicate balance between the applied current versus applied power, which needs to be as low as possible to maximize system sensitivity.
Overview of Application Examples
NTC sensor elements and systems are implemented in a broad area of fields, particularly in the automotive sector. Typical applications include heated steering wheels and seats, as well as sophisticated climate control systems. Used in exhaust gas recirculation (EGR) systems, air intake manifold (AIM) sensors, and temperature and manifold absolute pressure (TMAP) sensors, thermistors cover a wide range of operating temperatures with high shock resistance and vibrational strength, high reliability, and long-term stability. Here, the AEC-Q200 global standard for stress resistance is mandatory if the thermistor will be used in automotive applications.
In electric and hybrid vehicles, NTC sensors are used for battery safety, monitoring the electrical impulsion winding, and charging status. The refrigerant cooling system, which cools the battery, is linked to the air conditioning system.
Temperature sensing and control in home appliances cover a wide range of temperatures. For example, in clothes dryers, temperature sensors determine the temperature of hot air flowing into the drum and also the vented air as it leaves the drum. For cooling and freezing, NTC sensors measure the cooling compartment's temperature, guard against icing in the evaporator, and detect ambient temperature. In small appliances, such as irons, coffee makers, and kettles, temperature sensors are implemented for safety and to improve energy efficiency. Heating, ventilation, and air-conditioning (HVAC) appliances account for a further substantial market segment.
Growing Medical Sector
The medical electronics sector features a broad range of appliances for inpatient, outpatient, and even home care, and NTC thermistors are used as temperature sensing components within medical devices.
When small mobile medical appliances are charging up, it is essential to constantly monitor the operating temperature of rechargeable batteries. This is because the electrochemical reactions used during monitoring are mostly temperature-dependent, and fast, accurate analysis is of vital importance.
Continuous glucose monitoring (GCM) patches monitor blood sugar levels in diabetes patients. Here, NTC sensors are used to measure the temperature as this affects the results.
Continuous positive airway pressure (CPAP) therapy uses a machine to help a person who has sleep apnea breathe more easily during sleep. Similarly, for serious respiratory illnesses, such as COVID-19, mechanical ventilators take over the patient’s breathing by gently forcing air into their lungs and removing carbon dioxide. In both cases, glass encapsulated NTC sensors are integrated into the humidifier, airway tube, and air intake to measure air temperature to ensure the patient remains comfortable.
The recent pandemic is pushing requirements for higher sensitivity and accuracy of NTC sensors with long-term stability. New virus testers have stringent temperature control requirements to ensure the reaction between the sample and reagent is consistent. There are also body temperature monitoring systems being integrated into smartwatches to warn of potential illness.
Solving the Application Puzzle
Uniquely, the materials and technologies used in TDK’s NTC portfolio are developed in-house.
Highly customized thermistors are designed and developed according to the application; TDK supports the whole concept from design up to mass production. This end-to-end process includes concept reviews of new designs, construction and 3D drawings, 3D modeling, computer simulations, tooling, and prototyping including 3D printing, testing, and validation according to custom specifications.
The creation process is supported by various locations with multiple production facilities worldwide.
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