What is a Thermistor?
A thermistor is defined as a type of resistor whose electrical resistance varies with temperature changes. Although the resistance of all resistors will slightly fluctuate with temperature, a thermistor is especially sensitive to changes in temperature.
Thermistors act in a circuit as a passive component. They are an accurate, low-cost, and reliable way of measuring temperature. Although they do not work well in extremely hot or cold temperatures, for many different applications they are the sensor of choice. If a precise temperature reading is required, they are ideal.
Uses of Thermistors
Thermistors have a wide range of applications. They are widely used in many different liquid and ambient air environments as a means of measuring temperature as a thermistor thermometer. Some of the thermistors ‘ most common uses include:
- Electronic thermometers (thermostats)
- Automotive applications (to monitor oil and coolant temperatures in cars and trucks)
- Household appliances (such as microwaves, refrigerators and ovens)
- Battery safety (i.e. surge protection)
- Rechargeable batteries (to ensure proper battery temperature is maintained)
- To measure the thermal conductivity of electrical materials
- Temperature control (i.e. maintenance of rechargeable batteries)
How Does a Thermistor Work
A thermistor’s working principle is that its resistance depends on its temperature. We can use an ohmmeter to measure the resistance of a thermistor. If we know the exact relationship between how temperature changes will affect the thermistor’s resistance, then we can derive its temperature by measuring the resistance of the thermistor.
It depends on the type of material used in the thermistor how much the resistance changes. The relationship between temperature and resistance of a thermistor is non-linear. The following is a typical graph of the thermistor:
If we had a thermistor with the above temperature graph, the resistance measured by the ohmmeter could simply be aligned with the temperature shown on the graph. We can therefore derive the temperature of the thermistor by drawing a horizontal line across from the resistance on the y-axis and drawing a vertical line down from where this horizontal line intersects with the graph.
There are two types of thermistors:
- Negative Temperature Coefficient (NTC) Thermistor
- Positive Temperature Coefficient (PTC) Thermistor
When the temperature rises, the resistance decreases in an NTC thermistor. And the resistance increases when the temperature decreases. The temperature and resistance in an NTC thermistor are therefore inversely proportional. These are the most frequent forms of theistor.
The resistance-temperature relationship in an NTC thermistor is determine by the following expression:
- RT is the resistance of temperature T (K)
- R0 is the resistance of temperature T0 (K)
- T0 is reference temperature (normally 25oC)
- β is a constant, its value is dependant on the character of the material. The nominal value is 4000.
If the value of β is high, then the relationship between resistance and temperature will be very good. A higher β value means a higher resistance variance for the same temperature rise–that’s why you’ve improved the thermistor’s sensitivity (and thus accuracy).
The resistance temperature coefficient can be derived from the expression (1). This is nothing other than the thermistor’s sensitivity expression.
Above we can see clearly the negative sign of the αT. This negative sign shows the NTC thermistor’s negative resistance-temperature property.
If β= 4000 K and T= 298 K, αT=-0.0045/oK. This is far greater than platinum RTD’s sensitivity. This would calculate the very minor temperature changes.
Nonetheless, alternative forms of heavily doped thermistors that have a positive co-efficientof temperature are now available (at high cost). The expression (1) is such that even over a limited temperature range, it is not possible to make a linear approximation to the curve, and therefore the thermistors are certainly a non-linear sensor.
A PTC thermistor has the reverse temperature-resistance relationship. The resistance increases when the temperature increases. And the resistance decreases when the temperature decreases. The temperature and resistance in a PTC thermistor are therefore inversely proportional.
Although PTC thermistors are not as popular as NTC thermistors, they are often used as a form of protection for circuits. PTC thermistors can serve as a current-limiting device similar to the role of fuses.
It will induce a small amount of resistive heating when the current passes through a device. If the current is sufficiently large to produce more energy than the device will lose to its environment, the system can heat up. The heating up will also increase its resistance in a PTC thermistor. It produces an effect of self-reinforcement that drives up the resistance, thereby reducing the current. It functions as a current limiting mechanism in this way–to secure the circuit.
The relationship that governs a thermistor’s characteristics is as follows:
- R1 = resistance of thermistor at absolute temperature T1[oK]
- R2 = resistance of thermistor at temperature T2 [oK]
- β = constant depending on the material of transducer
In the above equation, we can see that the temperature-resistance relationship is highly nonlinear. Usually a standard NTC thermistor has a negative temperature resistance coefficient of approximately 0.05/oC.
To order to make a thermistor, two or more semiconductor powders made of metallic oxides are combined with a binder to create a slurry. The lead wires are created by small drops of this slurry. We need to put it in a sintering furnace for drying purposes. The slurry must shrink to the lead wires in order to make an electrical connection during this process. Through adding a glass coating on it, this refined metallic oxide is sealed. This glass coating provides the thermistors with a waterproof property–helping to improve their stability.
Thermistors are available in different shapes and sizes on the market. Smaller thermistors in the form of diameter beads ranging from 0.15 mm to 1.5 mm. Thermistors can also be made in the form of disks and washers by pressing the thermistor material under high pressure into flat cylindrical forms with a diameter of 3 mm to 25 mm.
A thermistor’s standard length is 0.125 mm to 1.5 mm. Commercially available thermistors have 1 K, 2 K, 10 K, 20 K, 100 K, and so on nominal values. That value shows the value of the resistance at 25oC.
Thermistors are available in various models: rod type, bead type, disc type, etc. Thermistors ‘ major advantages are their small size and relatively low cost.
This size advantage means that the time constant of thermistors operated in sheaths is small, although the reduction in size also reduces the ability to dissipate heat and thus increases the effect of self-heating. This effect can damage the thermistor permanently.
To avoid this, thermistors must be operated at low electrical current levels compared to thermometer resistance, resulting in lower sensitivity to measurement.
Thermistor vs Thermocouple
The main differences between a thermistor – thermocouple are
- Good for sensing little temperature changes
- NTC thermistors has exponential decrease in resistance with increased temperature
- A narrower sensing range (55 to + 150oC–although this varies depending on the brand)
- Nonlinear relationship between sensing parameter (resistance) and temperature
- The sensing circuit is needs no amplification and simple and is usually difficult to get better than 1oC without calibration.
- Sensing parameter= Resistance
- Thermocouple voltage is low
- Have a wide range of sensing of temperature (Type T = -200-350oC; Type J = 95-760 ° C; Type K = 95-1260 ° C; other types are even higher)
- Sensing parameter = voltage produced at different temperatures by junctions
- Linear relationship between the temperature and sensing parameter (voltage)
- very accurate
Also Read – Resistor Colour Code