Thermistors

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Thermistors (construction of thermal resistor) are semiconductors which behave as resistors with a high negative temperature coefficient of resistance.

Thermistor word derived from Thermal resistor. It became available in early 1960’s. They are based on oxides of semiconductors which has high temperature coefficients (mostly NTC) and have high resistance.

Construction of Thermistor

Thermistors are composed of sintered mixture of metallic oxides such as manganese, nickel, cobalt, copper, iron and uranium to form them in the form of beads, rods or discs.

Different forms of construction of thermistors
Figure 1: Different forms of construction of thermistors

Sometimes, a glass envelope is provided to protect a thermistor from contaminations.

Epoxy encapsulated bead thermistors, Thermistor temperature sensor,
Figure 2: Epoxy encapsulated bead thermistors

Some parameters

  1. Temperature range: -250 °C to 700 °
  2. Resistances: typically, 100Ω (higher available).
  3. Sizes: from a few mm to a few cm.
  4. Compatibility: Glass or ceramic encapsulation.
  5. Available in ready-made probes.
  6. Accuracy: ±0.01 °C to ±0.05 °
  7. Calibration: usually not necessary beyond manufacturing.

The resistance RT of a thermistor at temperature T (Kelvin) can be written as

{R_T} = {R_0}\exp \left[ {\beta \left( {\frac{1}{T} - \frac{1}{{{T_0}}}} \right)} \right] …(1)

Where,

RT and R0 are the resistances in ohms of the thermistor at absolute temperatures Tand T0.

β is a thermistor constant ranging from 3500 K to 5000 K.

The reference temperature T0 is usually taken as 298 K or 25°C.

Now the temperature coefficient of the resistance

\alpha = \frac{1}{{{R_T}}}\frac{{d{R_T}}}{{dT}} = - \frac{\beta }{{{T^2}}}

At T = 298 K, the value of α is

\alpha = - \frac{{4000}}{{{{298}^2}}} = - 0.045\Omega /{}^oC assume β =4000 K …(2)

This is evidently a rather high temperature coefficient because for a platinum resistance thermometer the corresponding figure is 0.0035/°C.

The plot of resistivity (ρ) versus temperature (Figure 3) also demonstrates this comparison.

temperature vs resistivity of thermistors,
Figure 3: Comparison of resistivity of platinum with that of thermistors

Equation (1) can be rearranged to the form where A and B are constants.

\frac{1}{T} = \left( {\frac{1}{{{T_0}}} - \frac{1}{\beta }\ln {R_0}} \right) + \frac{1}{\beta }\ln {R_T} …(3)

\frac{1}{T} = A + B\ln {R_T} …(4)

Equation (4) may alternatively be used to find temperatures by evaluating A and B from two pairs of known values of RT and T.

Thermistor Properties

  • Most are NTC devices
  • Some are PTC devices
  • PTC are made from special materials
    • Not as common
    • Advantageous when runaway temperatures are possible
  • Self heating errors as in RTDs but:
    • Usually lower because resistance is higher
    • Current very low (R high)
    • Typical values: 0.01°C/mW in water to 1°C/mW in air
  • Wide range of resistances up to a few MΩ
  • Can be used in self heating mode
    • To raise its temperature to a fixed value
    • As a reference temperature in measuring flow
  • Repeatability and accuracy:
    • 1% or 0.25°C for good thermistors
  • Temperature range:
    • – 50 °C to about 600 °C (Depending on type)
    • Ratings and properties vary along the range
  • Linearity
    • Very linear for narrow range applications
    • Slightly non-linear for wide temperature ranges
  • Available in a wide range of sizes, shapes and also as probes of various dimensions and shapes.
  • Some inexpensive thermistors have poor repeatability – these must be calibrated before use.

Advantages of Thermistor

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 Thermistors

  1. They are non-linear over large temperature range.
  2. The thermistor 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 Thermistor

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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