Thermistor Working Principle

Thermistors

• Thermistors are another type of temperature-measuring device used to measure temperature.
• Thermistors are abbreviated as THERMally sensitive resiSTOR.
• These manifest large resistance variations for small temperature changes.
• A thermistor is a type of resistance thermometer that reveals similar function like an RTD to measure process temperature.
• In Short, This thermistor is known as a ‘Thermal Resistor’.
• A thermistor consists of a semiconductor instead of a metal.
• A thermistor is a solid-state device with large sensitivity compared to RTD.
• Thermistor exhibits non-linear temperature-resistance characteristics.
• These cannot be specified by a single coefficient.
• Thermistors are not used for high-temperature measurements.
• The maximum operating temperature of the thermistor is limited to the 100 or 200°C range.
• It includes a non-metallic resistor as a sensing element.
• In the case of thermistors, both resistance and temperature are inversely proportional to each other.
• When temperature increases, the resistance of a thermistor decreases, and vice versa

Construction of Thermistor

• Thermistors are made up of sintered mixtures of metal oxides like manganese, nickel, cobalt, and iron.
• The resistance of thermistors varies from 0.4 ohms to 75 mega-ohms.
• Thermistors are fabricated in different shapes and sizes.
• Small-sized thermistors resemble the shape of beads with a diameter of 0.15 mm to 1.5 mm
• These beads are sealed at a solid glass rod tip to form a probe for easier mounting.
• These thermistors may also be available in the form of disks and washers.
• These disks and washers type thermistors are made into flat cylindrical shapes with diameters of 3mm to 25mm by pressing thermistor material at high pressure.
• These washers can stack in series or parallel to increase power.

Resistance vs. Temperature Curve of Thermistor

• Thermistors do not possess standard resistance vs. temperature characteristics or curves like RTDs and thermocouples.
• Every thermistor has dissimilar resistance vs. temperature curves.
• Some thermistors furnish good stability, but some bear high resistance.
• Due to this stability or resistance, these thermistors can be fabricated into a larger or smaller size.
• The large resistance of the thermistor varies for a small change in temperature.
• This thermistor includes a bulk semiconductor device that bears a resistor with a high & negative temperature coefficient of resistance.
• The temperature coefficient of resistance exceeds as high as -6% per degree Celsius for the rise in temperature.
• The thermistor is mainly applicable in precision temperature measurement, control, and compensation from -100 to +300 degrees Celsius due to its highly sensitive properties.

Difference between Thermistors and Resistance Temperature Detectors (RTD)

• The working principle for both thermistors & RTD is the same the flow of current through the resistor is constant.
• In thermistors, the electrical resistance of resistance changes with respect to temperature in a non-linear manner.
• The operating temperature range of both devices is not identical.

Resistance V/s Temperature Characteristic Curve

• In temperature measurement, control, and compensation, using a thermistor this resistance versus temperature curve is the main characteristics curve.
• This characteristics curve of resistance versus temperature is shown below.
• The characteristics curve of a typical thermistor, the resistivity varies from 107 ohm-cm to 1 ohm-cm for a change in temperature from -100 degrees Celsius to +400 degree Celsius.
• This thermistor is considered an ideal temperature transducer, due to this high negative temperature coefficient.

Thermistor as Temperature Sensor

• The thermistor is linked to a battery along with a micro-meter in series.
• The thermistor’s sensitivity can be increased by using a bridge circuit.
• The resistance of the thermistor at 25 degree Celsius or at ambient is 2 kilo-ohms.
• The temperature difference of the thermistor causes a change in the resistance value.
• For a change in temperature of the thermistors temperature coefficient of -4% per degree Celsius reduces about 80 ohms per degree Celsius.
• A change in a change if thermistor and the corresponding      micro-ammeter current reading is noted.
• Generally, a       micro-ammeter is calibrated with a resolution of 0.1 degree Celsius.

Thermistor Types

To know about various types of thermistors, it is essential to know the linear relationship between temperature, & resistance.This linear relationship is shown as

dR = K.dT

where,         

• dR –Resistance Change
• K –Temperature Coefficient of Resistance (First Order))
• dT –Temperature Change

• A change in resistance equals first order temperature coefficient      of resistance times temperature change.
• If the temperature coefficient of resistance (K) is Positive, such devices are known as Positive Temperature Coefficient thermistors (PTC).
• Here, both temperature & resistance are in direct proportion to each other.
• If the temperature increases then resistance also increases and vice versa.
• If the temperature coefficient of resistance (K) is Negative, such devices are known as Negative Temperature Coefficient thermistors (NTC).

Here both temperature & resistance are inversely proportional to each other. If the temperature increases then resistance decreases and vice versa.

Classification of Thermistors

Thermistors are classified into two categories

1. Positive Temperature Coefficient (PTC)
2. Negative Temperature Coefficient (NTC).

Positive Temperature Coefficient (PTC) Thermistors

Industrial Positive Temperature Coefficient Thermistors are classified as Silistors, & Switching Type PTC Thermistors.

Silistors

• These are known as Sensitive Silicon Resistors.
• These have a positive temperature coefficient of about 8% per degree Celsius.
• If the temperature exceeds more than 175 degree Celsius, the device shifts to a negative temperature coefficient regionSwitching Type PTC Thermistors:
• These are made from ceramic materials.
• These have very high resistance.
• These materials represent semi-conductive behavior due to the addition of Dopants.

Switching Type PTC Thermistors

• It represent negative temperature co-efficient patterns in their resistance-temperature characteristics.
• These devices have a transition or “Curie” temperature until it reaches some value.
• When this device reaches some value, these PTC Thermistors indicate an increasing positive temperature coefficient of resistance to develop resistance.

The figure shown below represents the difference in the temperature-resistance curve between a Silistors and a switching PTC Thermistor    

Negative Temperature Coefficient (NTC) Thermistors

• Industrial Negative Temperature Coefficient Thermistors are classified as Bead type thermistors, & metalized surface contacts.
• The classification of Industrial Negative Temperature Coefficient Thermistors is based on

1. Electrodes were placed on the ceramic body.
2. Type of geometry, shapes, and processing methods.

Bead Type Thermistors

• Bead-type thermistors are common types of Negative Temperature Coefficient (NTC) Thermistors.
• Bead-type thermistors are again classified based on shape and manufacturing methods such as
  • Bare Beads
  • Glass Coated Beads,
  • Ruggedized Beads, and
  • Bead in glass Enclosures.

Metalized surface contacts

• These thermistors are fixed by using
• Spring contacts & Surface mounting.

Comparison of PTC & NTC Thermistors

Difference between Thermistors and Resistance Temperature Detectors (RTD)

Applications of Thermistors:

• The temperature is at a narrow span and low-temperature range.
• Incubator Temperature.
• NTC thermistors measure and monitor batteries during charging hours.
• NTC determines the temperature of oil and coolant inside automobiles.
• It is used to limit the sudden over-current flowing in supply circuits.

Advantages of Thermistors:

• Large resistance change with temperature,
• Rapid response time,
• Good stability,
• High resistance vanishes problems occurred by lead resistance.
• Low cost and interchangeable
• They are compact in size, allowing them to fit into extremely tight spaces.
• Choices that allow for personalization and adjustment
• They are compatible with a wide variety of devices due to their standard connection, which consists of two wires.
• Easily compatible with electronic apparatuses

Disadvantages of Thermistors:

• Non-linear,
• Limited operating temperature range,
• Inaccuracy due to overheating,
• Current source required.