Temperature Dependent Resistor (TDR)

The Temperature Dependent Resistor (TDR) is nothing but a thermistor. Thermistor word derived from Thermal resistor. It became available in early 1960’s. This describes the action of the thermistor particularly well.

Today, thermistors are used in a wide variety of devices from temperature sensors through to providing temperature compensation in electronic circuits. As such thermistors are widely used in electronic, although they are obviously not as commonly used as ordinary resistors, capacitors and transistors.

Types of TDR

There are a number of ways in which thermistors can be categorized. The first is dependent upon the way they react to heat. Some increase their resistance with increasing temperature, while others exhibit a fall in resistance.

Accordingly, it is possible categorize them accordingly:

  • Positive temperature coefficient (PTC): Where the resistance increases with increasing temperature
  • Negative temperature coefficient (NTC): Where the resistance decreases with increasing temperature.

Depending on shape they are classified as follows.

Temperature Dependent Resistor , thermistor types, tdr
Figure: Types of TDR/ thermistors

Comparison between PTC thermistors and NTC thermistors is as follows.


NTC Thermistors

PTC Thermistors

Resistance with temperature

Decreases as temperature increases

Increases as temperature increases

Temperature coefficient




Temperature sensing, current limiting, temperature compensation in circuits

Overcurrent protection, inrush current suppression in circuits

Response time

Generally faster


Operating range

Wider range of operating temperatures

Limited operating temperature range

Voltage dependency

Relatively low

Higher dependency on voltage changes

Sensitivity to temperature

Highly sensitive

Highly sensitive

Overload protection

Less effective as overload protection

Effective for overcurrent protection


Relatively less stable at extreme temperatures

More stable across a range of temperatures

In addition to the nature of the resistance change, thermistors can also be categorized according to the type of material used. Typically, they use one of two materials:

Semiconductor Type Thermistors:

    • Metal Oxide (NTC) Thermistors: These are composed of ceramic materials like oxides of manganese, nickel, cobalt, iron, or various other metal oxides. They exhibit a negative temperature coefficient (NTC), meaning their resistance decreases as temperature increases. Commonly used materials include mixtures of manganese, nickel, and cobalt oxides.
    • Polycrystalline Ceramic (PTC) Thermistors: These are typically made from barium titanate and other materials. PTC thermistors have a positive temperature coefficient, meaning their resistance increases as the temperature rises.

Non-Semiconductor Type Thermistors

    • Bead Thermistors: These are typically made of metals or metal alloys like nickel, copper, or platinum. They are usually small, bead-shaped components.
    • Glass Thermistors: These thermistors are made of a mix of metal oxides and glass and are encapsulated in glass.
    • Silicon Based Thermistors: Silicon thermistors are made of silicon material, which can exhibit either NTC or PTC characteristics based on the doping materials and production process.

Advantages of TDR

Thermistors are very popular as temperature transducers because

  1. They are compact, rugged, inexpensive.
  2. Their calibration is stable.
  3. They have a small response time.
  4. They are friendly to remote measurements.
  5. Their accuracy is high.

Disadvantages of TDR

  1. TDR is non-linear over large temperature range.
  2. It is not suitable for a large temperature range.
  3. Due to noise problem, they need shielding of power lines.
  4. It is a passive device.
  5. More fragile as they are semiconductor devices.
  6. They are prone to self-heating.

Applications of TDR

  • Thermistors are used as temperature sensors. They can be found in home appliances such as boilers, fire alarms, microwaves ovens and refrigerators. They are also used in digital thermometers and in many automotive applications to measure temperature.
  • Some more commercial uses for thermistors include applications in manufacturing facilities as circuit breaker, medical applications, Food and beverage industries, Aerospace, 3D printer etc.

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