A negative temperature coefficient (NTC) refers to any physical value that decreases as the temperature increases or vice versa. In electronics, NTC usually refers to a material’s decreasing resistance as the temperature rises.
This article examines how NTC device performance is quantified and compares with other technologies like resistance temperature detectors (RTDs) and positive temperature coefficient (PTC) devices. It then examines an advanced fabrication technology for NTCs and closes with a glance at a device with both NTC and PCT characteristics over different voltage regimes.
Most NTC thermistors are designed for use between about -55 and 200 °C. Specialized NTC thermistors are available that can operate to nearly absolute zero (-273 °C). An NTC sensor’s temperature sensitivity is expressed as the percentage change in resistance per degree C. Typical values of temperature sensitivity range from 3% to 6% per °C, depending on the materials used and the production process.
RTDs are a type of PTC device made of metal (e.g., platinum, Pt). NTC thermistors are usually made of ceramic or polymer. The different materials produce different temperature responses and other characteristics. NTCs have a steeper temperature response curve than RTDs (Figure 1).
Key application benefits of NTC thermistors include:
- High sensitivity to temperature changes
- Inexpensive and simple to use
- Compact size
NTC thermistors are used in a wide range of medical, automotive, home appliance, and industrial applications like:
- Medical devices, including patient monitoring systems, catheters, and body temperature probes
- Automotive engine coolant temperature sensors, air conditioning controls, and EV battery temperature monitoring
- Home appliance temperature regulation in ovens and coffee makers
- Industrial process control
PCT vs. NTC applications
PTC and NTC thermistors are typically used in different applications. PTC thermistors are primarily used for circuit protection applications like overcurrent and inrush current limiting, acting as a resettable fuse, while NTC thermistors are commonly used for temperature sensing and control.
However, both types can be used in current control applications in Inrush.
When a PTC heats to its Curie point, the resistance increases rapidly and automatically limits the current flow. When the excess current is removed, the device cools and returns to a low-resistance state. Using thermistors, NTCs can monitor inrush current in active current-limiting schemes like soft-start circuits.
Electroceramic NTCs for critical applications
Standard NTC thermistors work fine in most applications. Critical applications, however, can benefit from using advanced electroceramic NTCs made with high-stability materials.
Fabrication of high-stability materials relies on precise control of the temperature, pressure, and processing time to deliver consistent electrical and mechanical properties. The process is designed to allow voids in the ceramic to collapse, eliminating porosity and delivering near 100% theoretical material density and more consistent performance (Figure 3).
Some benefits of reduced porosity include:
- Fine-grained microstructure improves the physical properties of the ceramic
- High resistance stability for critical applications
- Improved tolerance distribution and repeatability
Zener diodes are different
Zener diodes can exhibit both NTC and PTC characteristics. Below 5 V, a Zener diode has NTC characteristics, and the Zener voltage decreases with increasing temperature. Above 5, that same diode has PTC characteristics, and the Zener voltage increases with increasing temperature (Figure 4). Specialized “temperature-compensated” Zener diodes are also available and are designed to have a very low temperature coefficient.
A low-temperature coefficient Zener can be used to design a voltage regulator with a stable output voltage over a larger temperature range. In designs where the performance of other components is temperature-related, a Zener with a known temperature coefficient can be used to provide temperature compensation and improve operational stability and performance.
Summary
NTC thermistors can be used in various applications including temperature monitoring and inrush current limiting. Specialized electroceramic NTC thermistors are available for high-performance applications. RTDs exhibit PTC characteristics, and Zener diodes can exhibit both NTC and PTC characteristics.
References
Advances in NTC Ceramic Sensor Technology, TE Connectivity
Design of a Negative Temperature Coefficient Temperature Measurement System Based on a Resistance Ratio Model, MDPI sensors
NTC Thermistor Temperature Sensors, Eurosensor
PTC NTC Thermistors Comparison, Amwei Thermistor Sensor
Thermistor basics, Wavelength Electronics
What are NTC thermistors? And its principle of operation, Murata
Which is Better for Inrush Current: PTC Thermistors or NTC Thermistors?, Sisler Companies
What is the temperature coefficient of the Zener diode (voltage regulator diode, constant voltage diode)?, Toshiba
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Filed Under: Automotive, Sensor Tips, Medical-device manufacture