PTC Thermistor: positive temperature coefficient

The positive temperature coefficient, PTC thermistor has two types: silistor & switching PTC have very different characteristics.

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As the name indicates a positive temperature coefficient, PTC thermistor has a response in which the resistance rises with increasing temperature.

There are two types of PTC thermistor which have very different characteristics, one showing a linear increase, whilst the other shows a sudden change in resistance.

PTC thermistor outline

PTC thermistors can be divided into two types according to their structure and the materials used. The two types of PTC thermistor have very different characteristics:

Switching PTC thermistor: This form of PTC thermistor is used in heaters, sensors and specialised versions are also used as resettable fuses. Switching type NTC thermistors have a highly non-linear curve. The resistance at first falls slightly with increasing temperature, and then at a critical temperature the resistance increases dramatically, thereby acting virtually as a switch. This makes it ideal in may protection devices.
Silistor: The silistor PTC thermistor uses a semiconductor as its base material and it is characterised by a linear characteristic and as a result, the silistor is used in temperature sensors. The silistor PTC thermistor is generally manufactured from doped silicon, the level of doping determining the exact characteristics.

Switching PTC thermistor basics

As the switching PTC thermistor is very widely used form, it warrants further explanation as it has an unusual characteristic.

Switching PTC thermistors are typically fabricated from polycrystalline materials including barium carbonate or titanium oxide with added materials including tantalum, silica, or manganese, etc.

The materials are mixed and ground to fine powders and finally compressed into the required shapes before being sintered. Contacts are then added and the thermistor is encapsulated.

Switching PTC thermistor resistance temperature graph
Notice critical temperature, Tc

The characteristic of the switching PTC thermistor shows that the device has a highly non-linear characteristic. As the temperature increases, the resistance first decreases, then rising slightly before it reaches a critical temperature, Tc. At the critical temperature the resistance increases sharply for any increase in temperature, before finally levelling of and falling slightly.

A similar type of device known as a Polymer PTC. These devices consist of a plastic element into which carbon grains are embedded. When cool, the carbon is able to conduct electricity, but as the temperature increase the carbon grains move further away as a result of expansion and conduction falls rapidly. In this way the device acts like a switch in the same way as the more traditional PTC thermistors.

PTC thermistor operational modes

There are two main ways in which switching PTC thermistors are generally used.

Self heating mode: Whe used in the self heating mode, current is passed through the thermistor, often in series with the item being controlled. As it heats up as a result of the current, it reaches a point where the critical temperature is reached and the resistance increases significantly. In this way it operates in a self-heating mode and can be used as a safety cut-out or regulator.
Sensor mode: In this mode, a minimum amount of current is passed through the device and PTC thermistor senses the surrounding temperature. Keeping the current to a minimum ensures the self heating effect is negligible and only the surrounding temperature affects the device. As the surroundings heat the device it may reach its critical temperature at which point the resistance will significantly increase.

Switching PTC thermistor applications

Switching thermistors are a very useful form of electronic component. It can perform functions with a single component that would require far more complex circuitry should any other technique be used.

In-rush protection: Some electrical items like motors and transformers have a large surge of current at switch on. This gives rise to very high peaks of current which can cause spikes on the power line, or cause damage in some instances. PTC thermistors can be used to reduce the level of the inrush current and thereby prevent the spikes or damage occurring
Overcurrent protection: In this application, the PTC thermistor is placed in series with the load and uses the self-heating effect. The current taken under normal conditions should enable the thermistor to operate in its flat resistance curve area. However if an over-current condition arises, then the thermistor will carry more current and the temperature will rise more causing it to rise beyond the critical temperature when the resistance will rise significantly causing the current to drop.

PTC thermistor circuit symbol

It is sometimes necessary to indicate the type of thermistor being used on a circuit diagram. Accordingly the IEC have a special PTC thermistor circuit symbol that can be used.

As seen the circuit symbol utilises the characters +t° to give an indication of the positive temperature coefficient.

Positive temperature coefficient, PTC thermistors are used in many electronic circuits and for a variety of functions. It is essential to check that the correct type is selected for any given circuit to ensure that its characteristic meets the requirements.