What is a Thermistor?
Thermistor
What is a Thermistor?
A thermistor (or thermal resistance) is defined as a type of resistor whose electrical resistance varies with changes in temperature. Although the resistance of all resistors fluctuates slightly with the temperature, the thermistor is particularly sensitive to temperature changes.
Function: Thermistor functions as a passive portion of a circuit. They are an accurate, cheap and robust way to measure temperatures. Since they do not operate well in excessively hot or cold environments, they are the sensor of choice for several different applications. They are optimal if accurate temperature reading is needed.
Usage of thermistors
Working Principle of Thermistor
The basic theory of the thermistor is that its resistance depends on its temperature. We can calculate the resistance of the thermistor by using an ohmmeter. If we know the exact relationship of how temperature fluctuations influence the thermistor 's resistance, then by calculating the thermistor 's resistance, we can measure its temperature.
How much the resistance changes depends on the type of material used in the thermistor. The relationship between a thermistor’s temperature and resistance is non-linear. A typical thermistor graph is shown below:
If we had a thermistor with the temperature graph above, we could simply align the resistance measured by the ohmmeter with the temperature indicated on the graph. Thus, by drawing a horizontal line over the resistance on the y-axis and drawing a vertical line down from where this horizontal line intersects with the scale, the temperature of the thermistor can be obtained.
Thermistor Types
There are two types of thermistors:
- Negative Temperature Coefficient (NTC) Thermistor
- Positive Temperature Coefficient (PTC) Thermistor
NTC Thermistor
In an NTC thermistor, when the temperature increases, resistance decreases. And when temperature decreases, resistance increases. Hence in an NTC thermistor temperature and resistance are inversely proportional. These are the most common types of thermistor.
The relationship between resistance and temperature in an NTC thermistor is governed by the following expression:
Where:
If the value of β is high, then the resistor–temperature relationship will be very good. A higher value of β means a higher variation in resistance for the same rise in temperature – hence you have increased the sensitivity (and hence accuracy) of the thermistor.
From the expression (1), we can obtain the resistance temperature coefficient. This is nothing but the expression for the sensitivity of the thermistor.
Above we can clearly see that the αT has a negative sign. This negative sign indicates the negative resistance-temperature characteristics of the NTC thermistor.
If β = 4000 K and T = 298 K, then the αT = –0.0045/oK. This is much higher than the sensitivity of platinum RTD. This would be able to measure the very small changes in the temperature.
However, alternative forms of heavily doped thermistors are now available (at high cost) that have a positive temperature coefficient. The expression (1) is such that it is not possible to make a linear approximation to the curve over even a small temperature range, and hence the thermistors is very definitely a non-linear sensor.
PTC Thermistor
A PTC thermistor has the reverse relationship between temperature and resistance. When temperature increases, the resistance increases. And when temperature decreases, resistance decreases. Hence in a PTC thermistor temperature and resistance are inversely proportional.
Although PTC thermistors are not as common as NTC thermistors, they are frequently used as a form of circuit protection. Similar to the function of fuses, PTC thermistors can act as current-limiting devices.
When current passes through a device it will cause a small amount of resistive heating. If the current is large enough to generate more heat than the device can lose to its surroundings then the device heats up. In a PTC thermistor, this heating up will also cause its resistance to increase. This creates a self-reinforcing effect that drives the resistance upwards, therefore limiting the current. In this way, it acts as a current limiting device – protecting the circuit.
Thermistor Characteristics
The relationship governing the characteristics of a thermistor is given below as:
Where:
- R1 = resistance of the thermistor at absolute temperature T1[oK]
- R2 = resistance of the thermistor at temperature T2 [oK]
- β = constant depending upon the material of the transducer
We can see in the equation above that the relationship between temperature and resistance is highly nonlinear. A standard NTC thermistor usually exhibits a negative thermal resistance temperature coefficient of about 0.05/oC.
Thermistor Construction
To make a thermistor, two or more semiconductor powders made of metallic oxides are mixed with a binder to form a slurry. Small drops of this slurry are formed over the lead wires. For drying purposes, we have to put it into a sintering furnace. During this process, that slurry will shrink onto the lead wires to make an electrical connection. This processed metallic oxide is sealed by putting a glass coating on it. This glass coating gives a waterproof property to the thermistors – helping to improve their stability.
There are different shapes and sizes of thermistors available in the market. Smaller thermistors are in the form of beads of diameter from 0.15 millimeters to 1.5 millimeters. Thermistors may also be in the form of disks and washers made by pressing the thermistor material under high pressure into flat cylindrical shapes with diameter from 3 millimeters to 25 millimeters.
Thermistor vs Thermocouple
The main differences between a thermistor and a thermocouple are:
- A more narrow range of sensing (55 to +150oC – although this varies depending on the brand).
- Sensing parameter = Resistance
- Nonlinear relationship between the sensing parameter (resistance) and temperature.
- NTC thermistors have a roughly exponential decrease in resistance with increasing temperature.
- Good for sensing small changes in temperature (it’s hard to use a thermistor accurately and with high resolution over more than a 50oC range).
- The sensing circuit is simple and doesn’t need amplification and is very simple.
- Accuracy is usually hard to get better than 1oC without calibration.
- Have a wide range of temperature sensing (Type T = -200-350oC; Type J = 95-760°C; Type K = 95-1260°C; other types go to even higher temperatures).
- Can be very accurate.
- Sensing parameter = voltage generated by junctions at different temperatures.
- Thermocouple voltage is relatively low.
- Linear relationship between the sensing parameter (voltage) and temperature.