Designers need to rethink their ideas about positive temperature coefficient thermistors. A new generation of these devices offers all the benefits and none of the downfalls associated with them.

Positive temperature coefficient (PTC) thermistors have gotten short shrift from the design community because they have the least linear resistance vs. temperature of the major temperature-sensor alternatives. It has also traditionally required a considerable amount of effort to design networks that combine thermistors and resistors to produce linear changes in resistance or current for temperature changes over a limited range.

The resulting increase in design and manufacturing cost has ruled out PTC thermistors for many applications that require a linear temperature resistance curve, such as compensating for the positive temperature coefficient of resistance (TCR) of coil windings in relays, motors, generators and instrument movements. Another group of applications that requires linearization is designed to provide a voltage with a negative slope with respect to temperature to compensate for temperature drifts of amplifiers, oscillators and similar circuits.

The new PTC thermistors based on thin-film positive technology provide a linear thermal coefficient of reaction throughout the entire application range. They also offer a TCR of up to 5,000 parts per million, 1,000 more than previous devices. Finally, PTC thermistors offer inherent safety advantages because, by their very nature, they reduce current flow as temperature rises. Their alternatives, negative temperature coefficient (NTC) thermistors, have the opposite effect.

As operating frequencies rise, temperature control is critical in a range of applications. NTC thermistors have traditionally occupied most of the temperature control market. But highly linear PTC thermistors are beginning to change that.

The positive TCR of PTC thermistors makes them ideal as overcurrent protectors. The PTC thermistor is linear to a certain critical current, allowing a constant amount of current to flow. When a fault arises, the PTC thermistor will heat up and substantially increase its resistance, reducing current to a safe level. When the fault is fixed, the thermistor cools off and automatically resets itself. This circuit is designed so that for all currents below the desired limiting current, the power dissipated is not sufficient to self-heat the device to its transition temperature. Placing identical thermistors in two legs of a bridge circuit makes it possible to measure temperature differentials with a high level of accuracy.

In summary, the new PTC thermistors eliminate the primary disadvantage of these devices while keeping their benefits. It's time to reconsider their use.

Javed Khan is a Development Engineer at Vishay Intertechnology Inc. (Norfolk, Neb.).