PTC Thermistor

- the PTC thermistor is the less widely used of the two forms of thermistor - the abbreviation PTC standing for positive temperature coefficient.

The PTC thermistor is widely used in a number of applications for a variety of purposes.

The PTC thermistor, as the name indicates has a positive temperature coefficient, having an increasing resistance for an increasing temperature.

PTC thermistor basics

PTC thermistors cane be divided into two types according to their structure and the materials used:

  • Silistor:   This type of PTC thermistor uses a semiconductor as its base material and it is characterised by a linear characteristic. As a result this type of device is used in temperature sensors. The silistor PTC thermistor is generally manufactured from doped silicon. It iexhibits a resistance / temperature characteristic that is virtually linear, the exact characteristic depending upon the semiconductor used and the level of doping.
  • 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 with increasing temperature, and then at a critical temperature the resistance increases dramatically, thereby acting virtually as a switch.

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.

The circuit schematic symbol for a PTC thermistor
PTC thermistor circuit symbol

Switching PTC thermistor basics

As the switching PTC thermistor is the most widely used form, it warrants further explanation.

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.

The typical characteristic of a PTC thermistor showing the almost flat section of the characteristic below the switching temperature
PTC thermistor characteristic

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 it then reaches a critical temperature, Tc, is reached. At this point the resistance increases sharply for any increase in temperature.

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:   In this 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, virtually no current is passed through the device and PTC thermistor senses the surrounding temperature. The self-heating effect is negligible and it is only the surrounding temperature that 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.

  • 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.
  • 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

These form some of the more widely used applications for PTC thermistors. Naturally they can be used in a variety of ways and circuits where a sudden increase in resistance is required above a given temperature.

By Ian Poole

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