What is negative temperature coefficient?

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).

Figure 1. NTCs have a steeper response compared with RTD PTCs. (Image: Dongguan Tianrui Electronics)

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.

negative temperature coefficient — Figure 2. The sharp increase in resistance of PTCs (top) can support passive inrush current limiting applications, while NTC characteristics (bottom) can be used for active inrush current limiting. Image: Amwei Thermistor Sensor)

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).

Figure 3. Examples of standard NTC materials (left and center) compared with high-density electroceramic (right). (Image: TE Connectivity)

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.