17 Mar 2016

Engineers guide to PTC thermistor technology

Yuki Fujii, product engineer, Murata Japan and Yumin Saigo, product engineer, Murata Japan look at PTC thermistor technology for use in industrial automation equipment.

Smart factory initiatives have been growing in popularity across most manufacturing-based economies. Many other European countries have adopted the German government’s Industry 4.0 approach as a way to speed up the automation processes and improve operating efficiency of factories.

The demand for industrial, or factory automation equipment has increased significantly as manufacturers can see the financial benefits a far more agile manufacturing approach will bring. However, as the number of wired connections increases between controller and sensors so does the possibility of mis-wiring. This may cause abnormally high inrush currents with the result that machinery being damaged.

Protecting machinery such as a motor drives, PLC controllers, sensors and actuators from over-load conditions is usually the job of the humble thermistor or PTC component. Modern thermistor PTC devices offer a resettable function to restore to normal operation once the malfunction has passed and offer a high reliability of operation. For the future, the reliability of a thermistor PTC will become more important, since it is anticipated that such abnormal current malfunctions will increase in frequency. In this article we introduce the use of a ceramic PTC for industrial automation due to its higher reliability.

Comparison of thermistor over-current protection solutions

Figure 1 Comparison of over-current protection solutions

Figure 1, above, illustrates some of the protection devices available, each highlighting pros and cons of each type. Thermistor PTCs operate by increasing their resistance exponentially as it starts to warm up due to the current passing through it. Operating in a similar manner to a fuse, during normal operation the devices resistance value is steady. As soon as an inrush current occurs the effect of the current flow increases the devices temperature that in turn causes the resistance to increase exponentially. As this happens the current falls significantly and protects the supplied circuit. See Figure 2.

Operation of a thermistor PTC for over-current protection

Figure 2 Operation of a thermistor PTC for over-current protection

Thermistor PTCs fabricated out of either polymer and ceramic can be commonly found on the market. Which type should an engineer choose? There are a number of considerations that should be taken into account when selecting a PTC. These include the change of the PTC’s characteristics due to reflow soldering, reliability, and the mechanical construction method used.

Of the two materials, polymer is the most susceptible to changing the PTC’s resistance due to applied temperature. A change of resistance between 100% to a maximum of 190% is possible based on two reflow soldering operations. The resistance tolerance of polymer is also very wide. By contrast ceramic is much more stable. With a typical resistance change in the order of -1 to a maximum of 0.5% with two reflow soldering operations, the characteristics for volume production of a thermistor PTC device show that a ceramic device is much more suitable.

In terms of reliability a ceramic PTC is also more reliable that those constructed from polymer. Figure 3 illustrates this with intermittent load test conducted at room temperature

Thermistor PTC resistance change

Figure 3 Resistance change rate of intermittent load test

Another significant difference between the two types of PTC is that of the change in resistance characteristic due to a prolonged abnormal load. The resistance of a polymer-based PTC can increase up to more than 100% should a overload condition be in place for over 100 hours. After such a malfunction situation it is also possible that a polymer PTC device may not function properly due to an increased ambient resistance value. Again, ceramic is a far more stable material and in similar conditions the resistance hardly changes at all.

Construction of polymer and ceramic thermistor PTC

Figure 4 Construction of polymer and ceramic PTC

Figure 4 above illustrates the different material behavior of a ceramic and a polymer PTC. While they have similar characteristics the way in which the resistance change occurs is different. A ceramic PTC changes due to a chemical affect resulting from the resistivity changes of the ceramic boundaries.

At the trip condition, where the resistance increases exponentially, the resistance of the ceramic boundary increases significantly with an increase in temperature. As the current flow reduces and the temperature reduces the resistance of the ceramic element returns to normal without any affects of hysteresis. When we examine the behavior of a polymer PTC device we find that the change is resistivity is down to mechanical properties.

Constructed of a polymer resin and carbon chain, as the heat resulting from the overload current increases so does the thermistor PTC device expand slightly, causing the carbon chains to disconnect and increase the overall resistance.

As the current reduces and the temperature falls so does the resin shrink back and the carbon chain reconnects. However, in the same situation not all the carbon chain reconnects. So the nature of this movement causes a polymer PTC to exhibit some degree of hysteresis, such that over a long-term life the characteristics are rather unstable.

Use case examples of ceramic thermistor PTC

Figure 5 Use case examples of ceramic thermistor PTC

Figure 5 highlights a number of use cases for a ceramic PTCs typically found in industrial and factory automation systems such as PLC controller, servo drivers and sensors. As highlighted within this article, a ceramic PTC is far more reliable and offers much better performance over the whole design, manufacture and deployment cycle for industrial and factory automation systems.

An example of a ceramic PTC series is that of Murata’s PRG series, with a typical nominal resistance value of 3.3 ohms, the PRG21BC3R3MM1RK device has a maximum voltage of 30V, hold-current of 180mA at 25°C, and an operating temperature range from -40 to +85°C.

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About the author

Yumin Saigo (pictured), is Manager of the Sensor division, Murata Japan. He has a Master of Engineering and has ten years' experience in the development and design of Thermistor products and five years experience in ceramic material R&D.

Yuki Fujii, Product Engineer, Sensor division, Murata Japan, has a Master of Engineering degree with 6 year’s experience in the development and design of thermistor products.

Murata is a leading manufacturer of electronic components, modules, and devices. The complete range of this Technology house includes ceramic capacitors, resistors / thermistors, inductors / chokes, timing devices, buzzers, sensors and EMI suppression filters. Whilst the company is known as a global ceramic capacitor manufacturer, it is also the world leader in Bluetooth® & WiFi™ Modules, the world's no.1 manufacturer of board-mount DC-DC converters and is a key manufacturer of standard and custom AC-DC power supplies. Established in 1944, Murata is headquartered in Japan and has European offices in Finland, France, Germany, Hungary, Italy, the Netherlands, Spain, Switzerland and the UK.

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