What is a Positive Temperature Coefficient (PTC) | Positive Temperature Coefficient Thermistor

PTC is an initial for Positive Temperature Coefficient PTC thermistors are resistors with a positive temperature coefficient, meaning their resistance rises as the temperature rises.

PTC thermistors are classified into two types depending on the materials used, the structure of the device, and the production method. Solicitors are the earliest type of PTC thermistors, and they employ silicon as the semiconducting material. Because of their linear feature, they are employed as PTC temperature sensors. 

The switching type PTC thermistor is the second category. The resistance-temperature curve of a switching type PTC thermistor is very nonlinear. When a switching type PTC thermistor is heated, the resistance initially decreases until a critical temperature is achieved. As the temperature rises above the critical point, the resistance rises substantially. This kind of PTC thermistor is commonly utilized in PTC heaters, sensors, and other applications. This second group includes polymer PTC thermistors, which are composed of a specific material and are frequently employed as resettable fuses.

A polymeric positive temperature coefficient device (PPTC, commonly known as a resettable fuse, poly fuse, or poly switch) is a passive electronic component used to protect against over-current faults in electronic circuits. They are similar in function to PTC thermostats in certain situations but operate on mechanical changes instead of charge carrier effects in semiconductors. These devices were first discovered and described by Gerald Pearson at Bell Labs in 1939, and later patented as US patents.

Operation

A polymeric PTC device is made up of a non-conductive crystalline organic polymer matrix that is loaded with carbon black particles to make it conductive. While cool, the polymer is in a crystalline state, with the carbon forced into the regions between crystals, forming many conductive chains. Since it is conductive (the "initial resistance"), it will pass a current. If too much current is passed through the device the device will begin to heat. As the device heats, the polymer will expand, changing from a crystalline into an amorphous state.

PTC And NTC Working

The expansion separates the carbon particles and breaks the conductive pathways, causing the device to heat faster and expand more, further raising the resistance. This increase in resistance substantially reduces the current in the circuit. A small current still flows through the device and is sufficient to maintain the temperature at a level that will keep it in a high resistance state. The device can be said to have latching functionality. The hold current is the maximum current at which the device is guaranteed not to trip. The trip current is the current at which the device is guaranteed to trip.

When power is removed, the heating due to the holding current will stop and the PPTC device will cool. As the device cools, it regains its original crystalline structure and returns to a low resistance state where it can hold the current as specified for the device. This cooling usually takes a few seconds, though a tripped device will retain a slightly higher resistance for hours, slowly approaching the initial resistance value. The resetting will often not take place even if the fault alone has been removed with the power still flowing as the operating current may be above the holding current of the PPTC. 

The device may not return to its original resistance value; it will most likely stabilize at a significantly higher resistance (up to 4 times the initial value). It could take hours, days, weeks, or even years for the device to return to a resistance value similar to its original value, if at all. A PPTC device has a current rating and a voltage rating.

Applications

PTC Working

These devices are often used in computer power supplies, largely due to the PC 97 standard (which recommends a sealed PC that the user never has to open), and in aerospace/nuclear applications where replacement is difficult. Another application for such devices is protecting audio loudspeakers, particularly tweeters, from damage, when overdriven: by putting a resistor or light bulb in series with the PPTC device it is possible to design a circuit that limits the total current through the tweeter to a safe value instead of cutting it off, allowing the speaker to continue operating without damage when the amplifier is delivering more power than the tweeter could tolerate.

While a fuse could also offer similar protection, if the fuse is blown, the tweeter cannot operate until the fuse is replaced.

PTC NTC Thermistors Comparison

PTC NTC Thermistor Comparison

• PTC, abbreviated for Positive Temperature Coefficient.
• NTC, abbreviated for Negative Temperature Coefficient.

• PTC thermistor resistance increases with a temperature rising. 
• NTC thermistor resistance decreases with a temperature rising.

• PTC Thermistor major material BaTio3, 
• NTC thermistor major material is Mn, Ni, Cu.

• PTC thermistor is mainly applied in over-current overload and short circuit protection, telecom protection, lighting soft-switching time delay, motor starting, temperature sensing & protection, self-regulation heating, etc.
• NTC thermistor is mainly applied in inrush current suppressing limiting, temperature sensing measurement, temperature compensation, temperature control, etc.