Epcos, a world leader in passive components, is one company that has years of experience, competence and innovation in developing and designing protective components such as gas-filled surge arresters, thermistors and varistors, for electronics.
Following our brief look at NTC thermistors in Part II, this article continues now with a look at PTC thermistors.
PTC thermistors
PTC thermistors protect equipment against overcurrent and excessive temperatures. PTC, for positive temperature coefficient, are thermistors that have the unique property in that their resistance is low when they are cold and rises abruptly when they are heated. So in the cold state, they do not affect the functions of the device protected. The PTC thermistor is connected in series with the load to be protected against overcurrent. When an overcurrent flows through the PTC thermistor, its resistance rises steeply, limiting critical currents to safe levels.
Annette Justl, product marketing manager for ceramic semiconductors at Epcos, sums up their benefits: "PTC thermistors are superior to conventional fuses because they are self-resetting. As soon as the fault has been eliminated, they cool down, their resistance drops, and they resume their full protective function without having to be replaced. Smaller electric motors, line cards, output stages of small amplifiers and many other loads can thus be economically protected against overcurrent without time-consuming replacement of fuses or resetting of circuit breakers."
The 'telecom pair protector' is an innovative module from Epcos for subscriber line protection. It comprises two PTC telecom thermistors in a common four-pin package. Capable of surface mounting, it saves up to 40% of the space required by conventional solutions. The configuration with two PTC thermistors in a single package version permits cost-efficient design and placement. The insulation voltage between the two disks is more than 3000 V. And the two PTC thermistor disks are resistance-matched, thus ensuring balance between the lines. The telecom pair protector can be equipped with various PTC thermistor disks.
PTC thermistors for thermal protection are operated with only a low evaluation voltage. They are used in electric motors, transformers and many other applications in which high temperatures represent a hazard to components or users. Progressive miniaturisation of power supplies is taking its toll. Their designs become more compact, but their outputs remain constant or even rise. This results in problems of temperature management. Epcos has developed a solution - PTC thermistors as SMDs. If a certain limit temperature is exceeded, the resistance of the PTC thermistor rises sharply. The voltage drop across the thermistor then reduces the output, effectively protecting the power supply from overheating.
PTC thermistors from Epcos for thermal protection are, of course, also available in leaded versions or with stranded-wire and screw terminals. In Part IV of this series to follow, we will look at a very versatile component called a varistor.
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