Resistors are components that are very commonly found in many electrical circuits, and they act as an element that resists the flow of current with their resistance values and other specifications varying based on their design. PTC thermistors in particular are a form of positive temperature coefficient resistor, meaning that the rise of its resistance is directly tied to a rise in temperature. PTC thermistors may either come in the form of silistors or switching type PTC thermistors, each category differing in their material composition, structure, and manufacturing process. In this blog, we will provide a brief overview of PTC thermistors, allowing you to have a basic grasp on their characteristics, modes of operation, applications, and more.
With a silistor, one will find that it has a very linear resistance-temperature characteristic, its slope mostly consistently remaining minor across the entire operational range of the device. Nevertheless, they have the chance of exhibiting a negative temperature coefficient when temperatures exceed a value of 150 degrees Celsius. On the other hand, switching PTC thermistors are made from polycrystalline ceramic materials, and while these substances are highly resistive, doping allows for a semiconductive state to be attained. Switching PTC thermistors are commonly relied on as self regulated heaters as their general transition temperature lies between 60 and 120 degrees Celsius. If a much higher or lower transition temperature is desired, specialized versions may be procured.
When discussing the transition temperature, one is referring to the temperature where the resistance of a switching type PTC thermistor begins to spike. In some instances, individuals may also use the term “Curie” temperature, which is the temperature where the resistance of the device is twice the value of its minimum resistance. When shopping around for various options, it can be very useful to garner information on a particular device’s minimum resistance, rated resistance, dissipation constant, maximum rated current, and maximum rated voltage as each of these specifications will determine how good of a fit the device is for your particular project.
To achieve the different properties of silistors and switching type PTC thermistors, varying materials are required for construction. For example, silistors are made from silicon wafers that are of high purity, and they may be doped to varying degrees to achieve a particular temperature-resistance curve. Meanwhile, switching type PTC thermistors are made from polycrystalline materials that consist of barium carbonate, titanium oxide, and additives. Once the mixture is made, contacts are established on the component before coating or encasing is carried out.
While presenting a fairly simple method of operation, PTC thermistors find use in a number of applications and settings. For example, they can act as self-regulating heaters with their ability to auto-stabilize at a specific temperature when a current is running through them. Additionally, PTC thermistors regularly act as time delay devices as their switching is tied to temperature fluctuations, allowing them to conduct delayed reactions. Beyond these examples, PTC thermistors also serve for over-current protection, motor starting, liquid level sensing, and more.
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