What is PTC Thermistor : Working & Its Applications

The term “thermistor” is a combination of two terms like “thermal” & “resistor”, which is one kind of resistor or a resistance thermometer where the resistance of this mainly depends on temperature. Thermistors are available in two types; NTC (negative temperature coefficient) and PTC (positive temperature coefficient). The working of these two resistors mainly depends on temperature. Once the temperature increases in NTC, the resistance will be decreased and temperature decreases then resistance will be increased. Similarly, PTC is quite opposite of NTC. So this article gives brief information on one of the types of thermistor namely PTC thermistor or positive temperature coefficient thermistor.


What is PTC Thermistor?

PTC Thermistor Definition: The PTC thermistor is one type of resistor including a positive temperature coefficient, which means the temperature will be increased once the resistance increases. So the main relationship between resistance (R) & temperature (T) is linear, as expressed in the below equation.

PTC Thermistor
PTC Thermistor

PTR thermistor formula is ΔR = k(ΔT)

Where ‘ΔR’ is the change within the resistance,

ΔT is the change within the temperature.

‘K’ is the temperature coefficient.

When ‘k’ is positive, then it increases resistance when the temperature increases.

PTC Thermistor Symbol

Sometimes, it is very important to specify the type of thermistor being used on any circuit diagram. So PTC thermistor circuit symbol is shown below which uses the ‘+t°’ character on the symbol to represent the PTC or positive temperature coefficient.

PTC Thermistor Symbol
PTC Thermistor Symbol

Working Principle of PTC Thermistor

Most PTC thermistors are designed with doped polycrystalline ceramic that includes BaTiO3 (barium titanate) & other compounds. These materials have the property that is, at a particular critical temperature, the resistance will be increased suddenly. Barium titanate (BTO) material is ferroelectric where its dielectric constant changes with temperature.

The working principle of the PTC thermistor mainly depends on the change within resistance because of a change within the temperature. Once the temperature increases, then resistance will be increased.& temperature reduces then resistance will be decreased. So in this type of thermistor, both the resistance & temperature are directly proportional.

Types of PTC Thermistor

These thermistors are available in three types based on the structure, materials, and processor of manufacturing like ceramic switching, silistor silicon, and polymer PPTC.

Silistor Silicon PTC Thermistor

This kind of thermistor mainly includes silistors, which use silicon semiconducting material. These are linear devices that show significant PTC (positive temperature coefficient) resistance. Once the temperature exceeds 150 °C, then they will show NTC (negative temperature coefficient). These thermistors are used as PTC temperature sensors due to their linear characteristic and also for temperature compensation.

Silistor Silicon PTC Thermistor
Silistor Silicon PTC Thermistor

Ceramic Switching PTC Thermistor

The second type is the ceramic switching PTC thermistor which has an extremely nonlinear resistance-temperature curve. Once this thermistor is excited then resistance will decrease initially, until a fixed temperature level is reached. Once the temperature is increased above the fixed level, then resistance will be increased radically. This kind of thermistor is extensively used in sensors, PTC heaters, and also for protecting over temperature, current, time delay, and temperature compensation.

Ceramic Switching
Ceramic Switching

Polymeric PPTC Thermistor

The Polymeric positive temperature coefficient thermistor or PPTC thermistor is also called a Resettable Fuse because they exhibit a nonlinear PTC effect. This type of thermistor is a thermally activated device, so any fluctuation within ambient temperature will have an effect on the thermistor performance. This thermistor displays minimal resistance in normal operating conditions as compared to the rest of the circuit & it has less control over the performance of the circuit.

Polymeric PPTC
Polymeric PPTC

But, if the circuit system moves into a fault condition, then PPTC immediately responds by going into a tripping condition. Once the fault conditions are eliminated, then the thermistor will reset itself & the circuit system will return to its regular operating conditions.

These thermistors are used in automotive, telecom, consumer electronics, process control & protection of medical devices.

Steinhart–Hart Equation

The formula ΔR = k(ΔT)  holds good only for a certain temperature beyond which it is difficult to correct temperature. So we go for a the Steinhart-Hart equation . It helps in designing temperatures of thermistor precisely & effortlessly.  The Steinhart–Hart Equation is shown below.

B(T1/T2) =  (T2xT1/T2-T1) x In(R1/R2)

Where,

‘T1’ is the primary temperature point within Kelvin.

‘T2’ is the second temperature point within Kelvin.

‘R1’ is the thermistors Resistance at ‘T1’ within Ohms.

‘R2’ is the thermistors Resistance at ‘T2’ within Ohms.

Operational Modes

These are normally used in two modes of operation based on the application like the self-heating and sensor mode.

Self Heating Mode:

When PTC thermistor is used in self-heating mode the current starts flowing throughout the thermistor. Once it gets heated up, it reaches a critical temperature level and the resistance will be increased drastically. In this way, it operates in this mode, so it can be used as a regulator or safety cut-out.

Sensor Mode:

In sensor mode, a small amount of current supply is passed throughout the thermistor & device. The thermistor detects the temperature in its surrounding area. By maintaining the current to the smallest amount, the self-heating effect will be negligible & the surrounding temperature will affect only the device. When the surroundings heat up the device, then it reaches to its critical temperature point then the resistance will increase significantly.

PTC Thermistor Vs PTC Fuse

The difference between PTC Thermistor & PTC Fuse includes the following.

PTC Thermistor

PTC Fuse

A PTC thermistor is a thermally sensitive resistor used in a circuit for its protection. A PTC fuse is used to protect electronic devices when heat or excessive current is produced.
These are not rated above 60 volts. These are rated up to 600Volts.
The resistance of these resistors mainly depends on the temperature.

 

These have less resistance so it breaks the circuit once excess current supplies throughout it.
For a PTC thermistor, the leakage current can range from 100 mA at rated voltage to several 100 mA at lower voltages.

 

The leakage current for the fuse disrupts the flow of current.
The highest operating temperature for this thermistor is 85 degrees Celsius. The highest operating temperature for this fuse is 125 degrees Celsius.
The resistance of this thermistor is more or doubles as compared to a fuse. The resistance is low as compared to the thermistor.

PTC Thermistor Characteristics

This is a positive temperature coefficient resistor and the materials used to make this resistor is polycrystalline ceramic. When the temperature is increased then the resistance of a  thermistor can also increase in a non-linear manner. So this thermistor will show simply a small change in resistance through temperature until the TR point (switching point) is reached.

PTC Thermistor Characteristics
PTC Thermistor Characteristics

The characteristics of this thermistor in between temperature and resistance diagram are shown above. Generally, thermistors have rated resistance (TR). So PTC thermistors have rated resistance at 25°C.

How to use PTC Thermistor/PTC Thermistor Circuit Diagram

The wiring diagram of the PTC thermistor is shown below. This application circuit is used for protecting transistor circuits from overvoltage. In this transistor amplifier circuit, a PTC thermistor is used in an easy & convenient way to defend transistor circuits from an increase in an ambient temperature otherwise from heat generated through the transistor.

Over Voltage Protection Circuit
Over Voltage Protection Circuit

In the above circuit diagram, if the temperature is higher than the rated temperature then it may degrade the transistor characteristics. So, this thermistor is used in this circuit to detect an increase within the ambient temperature & goes into a high ohmic condition. Consequently, the voltage at the base-emitter (BE) of the transistor in the circuit falls & the transistor will cut off the load current. Once the ambient temperature comes back to the normal level, then the thermistor comes to its original low-resistance condition.

Advantages

The advantages of PTC thermistor include the following.

  • These thermistors are stable and powerful.
  • They have a compact size, so it allows them to work in less space.
  • They give a quick response.
  • These are not expensive as compared to other types of temperature sensors.
  • If this Thermistor has the correct resistance-temperature curve, then no other calibration is necessary throughout its fixing.

Disadvantages

The disadvantages of PTC thermistors include the following.

  • The temperature range is limited, so not applicable for a large range of temperatures.
  • The output is curved.
  • It requires shielding within power lines.
  • It is non-linear at maximum temperatures, so better to use them below 100°C.
  • It is delicate.
  • Its nonlinear characteristics will create a problem frequently for the measurement of temperature.

PTC Thermistor Applications

The applications of PTC thermistors include the following.

  • These thermistors are used in self-regulating heaters.
  • These are used to protect electrical circuits from overcurrents.
  • These Thermistors are used for motor winding protection.
  • These resistors play a key role in sensing liquid levels.
  • Switched PTC-type thermistors are mainly used as resettable fuses & over-current limiters in different circuits.
  • These resistors are used as current limiting devices for protecting a motor.
  • These are used for measurement & control of temperature, detection & regulation of temperature.
  • They can also be used as a thermal switch.
  • This thermistor limits the Inrush current for onboard chargers and industrial inverters.
  • It is used to protect DC motors & solenoids from over current.

Thus, this is all about an overview of the PTC thermistor & its working with applications. Here is a question for you, what is an NTC thermistor?

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