What are PTC Thermistors and How Do They Work?

A PTC thermistor is like an NTC thermistor but works oppositely.

Whereas an NTC thermistor reduces in resistance as it heats up, a PTC thermistor increases in resistance as it heats up. This difference is seen in their names:

• NTC = negative temperature coefficient
• PTC = positive temperature coefficient

What are PTC thermistors used for?

PTC thermistors have several uses:

• Like NTC thermistors, PTC thermistors can be used for inrush current limiting. Inrush current is the abnormally high flow of electricity that flows through an electrical circuit in the immediate moments after it is switched on.
• They can be used for overcurrent protection. Overcurrent is when the electricity flow causes the PTC thermistor to exceed a specific temperature, thus becoming more resistant.
• They are commonly used in the telecoms and security industries, specifically to protect systems against damage from electrical surges if they’re hit by lightning during storms.

How do PTC thermistors work?

Like NTC thermistors, PTC thermistors are a type of temperature-dependent resistor; they use semiconductor ceramics with a positive temperature coefficient (PTC), hence their name.

PTC thermistors have low resistance at room temperature. When an electrical current flows, the heat energy this generates raises the temperature of the PTC, which subsequently increases its resistance.

This feature is why PTC thermistors are most commonly used for overcurrent protection.

Advantages of using PTC thermistors

The most significant benefit of using PTC thermistors is they effectively work in the same way a fuse does but won't need replacing each time it does its job. A PTC thermistor limits the electrical current when it gets too hot. As this will lead to a noticeable drop in performance or total shutdown, you’ll be alerted to a problem with whatever it is you're using. You can then investigate and fix the problem, after which the operating temperature should drop, and the PTC thermistor will lower its resistance accordingly.

At the same time, a PTC thermistor also protects electrical circuits and components from blow out or gradual failure in the same way NTC thermistors do.

Where are PTC thermistors used?

For inrush current limiting

PTC thermistors are sometimes used as an alternative to NTC thermistors in:

• Switch Mode Power Supplies (SMPS), which are commonly used as power supplies for a variety of electronic devices.
• Induction motors in industrial inverter systems often found within air conditioning systems in office blocks or retail units.

For overcurrent protection

• In motor vehicles, to prevent onboard systems becoming overheated and overstressed.
• In solenoids, which are present in things like office printers and electric locks, where they help to prevent driver circuit damage.

In telecoms

PTC thermistors are also commonly used in telecoms and security systems to ensure they can maintain their operation as a result of electrical surges, which in most cases will be caused by lightning strikes during storms. PTC thermistors can be found in surge protective devices deployed within crucial systems, limiting the current flow while maintaining basic operations of essential systems.