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Muhimmiyar Thermistor?

Takardar Muhimmiyar Thermistor

Thermistor (ko kuma thermal resistor) ya ake takarda a matsayin resistor da yake canza ciki da karfi ga tsari a lokacin da hawa ta gina.

Thermistors suna aiki a cikin circuit da suka yi aiki a kan kompoenent passive. Suna zama tare da shirya, kadan, da kuma inganci wajen bincike hawa.

Babu da cewa thermistors ba su da muhimmiya a hawa masu karfi, amma ana son sa a fili a duk fannin aiki.

Thermistors suna fi yawa idan an bukata bincike hawa mai kyau. Alamar circuit na thermistor ya a bayyana da zuwa:

Fannin Aiki na Thermistors

Thermistors suna da fannin aiki daban-daban. Suna da amfani a matsayin binciken hawa a cikin manyan abubuwa da kuma alamun jikin. Duk fannin aiki mafi yawan amfani a kan thermistors sun haɗa da:

• Digital thermometers (thermostats)

• Fannin aiki na motoci (don bincike hawa na oil da coolant a cikin motoci da kamyon)

• Abubuwan gwamnati (kamar microwave, fridges, da ovens)

• Circuit protection (i.e. surge protection)

• Battery rechargeable (don hakuri hawa na battery ya bambanta)

• Don bincike thermal conductivity na electrical materials

• Yana da amfani a manyan basic electronic circuits (e.g. as part of a beginner Arduino starter kit)

• Temperature compensation (i.e. maintain resistance to compensate for effects caused by changes in temperature in another part of the circuit)

• Used in wheatstone bridge circuits

Principle of Working

Principle of working na thermistor shine cewa resistance ita yana iya sama da hawa. Zan iya bincike resistance na thermistor da amfani a ohmmeter.

Ta haka da magana game da yadda hawa yake canza resistance na thermistor, zan iya bincike resistance ita don tabbatar da hawa.

Yadda resistance yake canza yana iya sama da abin da ake amfani a cikin thermistor. Relationship bayan hawa da resistance na thermistor ba ce linear. Graph na thermistor na biyu ya a bayyana da zuwa:

Idan ban samu thermistor da graph na hawa na biyu, zan iya line up resistance da aka bincika a cikin ohmmeter da hawa da aka nuna a cikin graph.

Da drawing 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, we can hence derive the temperature of the thermistor.

Types of Thermistors

Akwai abubuwa biyu na thermistors:

• Negative Temperature Coefficient (NTC) Thermistor

• Positive Temperature Coefficient (PTC) Thermistor

NTC Thermistor

A cikin NTC thermistor, resistance yake rage da hawa yake rage, da kuma vice versa. This inverse relationship makes NTC thermistors the most common type.

The relationship between resistance and temperature in an NTC thermistor is governed by the following expression:

• RT is the resistance at temperature T (K)

• R0 is the resistance at temperature T0 (K)

• T0 is the reference temperature (normally 25oC)

• β is a constant, its value is dependant on the characteristics of the material. The nominal value is taken as 4000.

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 equation, we can determine the resistance temperature coefficient, which indicates the thermistor’s sensitivity.

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 co-efficient.

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 device.

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 will 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.

Characteristics of Thermistors

The relationship governing the characteristics of a thermistor is given below as:

• 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 (e.g. an oscillator 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.

Construction of Thermistors

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, the 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 a diameter from 3 millimeters to 25 millimeters.

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