Hello friends! I am back to give your daily dose of useful knowledge. In today’s topic we will study that what is thermistor. So let us start out topic.
Definition if thermistor:
Thermister is a contraction of the term thermal resistor so we can say that it is made of two terms thermal and resistor. The word ‘thermal’ means it is a special type of resistor whose resistance changes with the change in the temperature. So the help of thermistor we can measure the variations in the temperature we will measure the change in resistance and change in resistance is proportional to the change in temperature. The word ‘resistor’ means it is the property of material oppose the flow of current through it.
History of Thermistor:
Michael Faraday in 1791 a British chemist and physicist, is known for his work in electromagnetic induction and also in electrochemistry. Because the early thermistors were difficult to produce and applications for the technology were limited, commercial manufacture and use of thermistors did not begin until 100 years later. During the early 1940s, Bell Telephone developed techniques to improve for the consistency of the manufacturing process. Some of the first commercial thermistors were the disc type, and now the thermistor their liberality was quite broad. These devices were used essentially for regulation, protection, and temperature compensation.
As the reliability of these devices enhance during the 1980s, the use of electronic thermometers in the health care industry increased. The rising rate of sterilization and concerns about cross-infection for low-cost disposable temperature probes, for which chip thermistors were suited. During the 1980s and 1990s, the use of thermistors has continued to grow in the food processing, medical, HVAC, and also in telecommunications markets.
Principle of thermistor:
In a conductor when the temperature is increased the flow of electrons in a conductor it increases and do to the increase in flow of electron the flow of current also increase so e can say that the resistance decreases. In other case when the temperature increases the resistance also increases. So the materials in which the temperature or in which the resistance increases with the increasing temperature they are supposed to have a positive temperature coefficient whereas the materials in which the resistance decreases with the increase in temperature they are supposed to have a negative temperature coefficient. Most of the metal having positive temperature coefficient resistance detectors or we can say that resistance thermometer the RTD they use the material positive coefficient temperature where as the thermistor they use the material which are having negative coefficient temperature. Mostly the semiconductors they are having negative temperature coefficient. So semiconductors are used for the construction of thermistor whereas the positive temperature coefficient materials like metals and platinum they are used for the construction of the resistance thermometer resistance temperature detectors. The change in resistance is very small that with every one degree Celsius 1C rises in temperature. So it means that the thermistors they are highly sensitive because for every 1 degree rise in temperature the resistance is decreasing for decreasing at the rate of 5 percent so these two thermistors they can be used to for precision measurements, control and compensation because they are highly sensitive elements having very high negative temperature coefficient. The range over which the thermistors can be used the temperature range from -60 Celsius to 15 degree Celsius. Also the resistance of the thermometers of these thermistors it is in the range of 0.5 ohms to 0.75 ohms so this is the value of material which the thermistor is made up of.
Drawback of thermistors:
Thermistors are highly sensitive elements but there is a drawback for the thermistors that they draw linear. In the case of the resistance thermometer or RTD and thermocouples the material it has a linear characteristic means the resistance and temperature they are having a linear relationship with each other but in the thermistors it has a nonlinear characteristic it is the drawback but it has the advantage that they are highly sensitive elements with respect to the RTD and the thermocouple.
Types of thermistors:
There are many dissilmilar types of thermistors but they all work on the same fundamental. As we know that variable resistance works on temperature. There are principally two classfications of thermistors of which all types fall into, NTC and PTC. Depending on your individual application you may know.
Negative Temperature Coefficient:
NTC thermistors are the most frequently type available for use. The determining characteristic of this thermistor is that its resistance reduces as temperature increases. These sensors are available widespread all round the HVAC industry, product manufacturing, transportation, appliances etc. By resisting current a thermistor creates the consequence of residual heat. If an NTC thermistor will be familiar to handle in temperatures that will occasion stunning heat, a rectification can be applied to sensed values to accuracy. Also, with NTC thermistors, this self-heating effect will occurs at low temperatures where it can disappear much more readily into the encircling process.
Positive Temperature Coefficient
PTC thermistors behave in the opposing way of an NTC thermistor. Positive Temperature Coefficient method that as temperature increases the resistance of the thermistor also increases. This classification of thermistors is not customary but they do execute a specific niche function that of a well being fuses. In some procedure the presence of intemperate heat means an unacceptable situation is come about. If a PTC thermistor is attendence within a circuit it can behave like a category of throttle. The increase of resistance that comes with an increase in heat is like a usual safety valve and the circuit that is intense will reach a classification of upper limit. The graph below high point the opposite curves of PTC and NTC thermistors.
Thermistor Curves and Ranges:
To one side from the two distinct classifications of NTC and PTC, thermistor types diverge by curve and range. Generally, they are typically recognized by their resistive capacity at 25℃. We have already briefly introduced the common 10K thermistor, for example. It withstands 10,000 ohms of current when the ambient temperature is 25℃. The 10K thermistor might be a familiar standard but there innumerable other thermistors out there that are more accurate to use for other diversified tasks. Let’s look at the graph of different NTC-type thermistors and also study a couple of important points.
From these conceive curves, you can tell the indefectible range of a thermistor. Here you can see a large different in resistance but small change in temperature. This means that each small increase in temperature can be definite measured because the resistance change is huge and easily sustained. Thermistors don’t automatically manage better the colder it in. Below temperatures of -50℃, the resistive amplitude of most thermistors is too intemperate without diversified monitoring and circuitry.
On the other hand of the chart, the curves over and above 50℃. There is little change in resistance but large changes in temperature in this segment. The curve is apprrozimately flat. That measure it is easy to get defective temperature readings because the resulting resistance changes are so tiny. You’ll need a very accurate instrument to estimate the minute changes in resistance or else it will appear like your temperature is swinging wildly everywhere. Only idiomatic thermistors can operate precisely above 100℃.
Constructions and Types:
Thermistors are made up of materials which having the negative temperature coefficient generally the semi conductor like manganese, nickel, cobalt, copper, iron and uranium. So they are made up of materials and also the metallic oxides of the metals up. The thermistor they are available in various sizes and shapes and according to their shapes there are various types of thermistorsthey may be in the forms of beads in the form of rods and in the form of disc in the form of probe. So in these shapes thermistors are available.
There are four types of thermistors. Te thermistors which are in the shapes of bead they are the smallest in size. It has two terminals. The beads having the diameter of 0.015mm to 1.25mm. The sides of bead they may be sealed in a glass envelope so you have seen in the diagram that over the bead they are having the glass coating as to protect the thermistor material.
The glass probe type of thermistors their diameter is 2.5mm and the length varies from 6mm to 50mm. An oxide-binder is used in making probe thermistors. The cast material is permit to dry into a flexible tape, which is turn into piece of smaller sections and kept at high temperatures. After a thick film electrode material is applied, the wafers are dip into chip. The chips can be used as surface amount devices. Typical chip sizes range from 0.04 in square and rectangular shapes. Coated chip thermistors commonly measure from 2.5mm in diameter. Very small coated chip thermistors 0.02 in. to 0.06 in dies are available for applications requiring small size, fast response and interchangeable.
Disc are made up pressing material under high pressure. So they are shaped into a cylinderical flat shape with the diameter ranging from 2.5mm to 25mm. Disc thermistors are made with metal oxide powders, blending them with in a suitable binder, and then compressing all the small amounts of the mixture under tons of pressure. The discs are cept at high temperatures to form solid bodies. A layer of epoxy, phenolic, or glass is applied to each device to comeup with protection from mechanical environmental.
This type of shaped thermistors are primarily a variation of the disc type excluding for having a hole in the middle, and are normally use as surface mount devices. Rod-shaped thermistors are made by release a adherent oxide-binder mixture through a die,then through a heat-treating machine to form a material applying electrodes on it and also attaching leads. Rod thermistors are used primarily for applications essentially very high resistance and also high power dissipation.
A temperature sensor is a device that evaluate the temperature of its surroundings and transform the input data into electronic data to record, the signal temperature changes. There are many hybird of temperature sensors. Some temperature sensors require direct associate with the physical object that is being observe while others incidentally measure the temperature of an object. Non-contact temperature sensors are normally infrared (IR) sensors. They remotely discern the IR energy emitted by an object and send a signal to a measured electronic circuit that decover the object temperature. In between the proximity temperature sensors are thermocouples and thermistor. A thermocouple is encompass of two conductors, each made of a dissimilar type of metal, that are connected at the end to form a junction. When the junction is reveal to heat, a voltage is give rise that straight corresponds to the temperature input. This transpire on account of the occurrence called the thermoelectric effects. Thermocouples are normally low price as their design and substances are simple. Where as the contact temperature sensor is called a thermistor. In thermistors, resistance decine as temperature increases. There are two main types of thermistors: Negative temperature coefficient and Positive temperature coefficient. Thermistors are more accurate than thermocouples (range for measuring with 0.05-15 celisus) and they are made of ceramics. Resistance Temperature Detectors are fundamentally the metalopposite number of thermistors, and they are the most accuraate and high cost type of temperature sensors.
Temperature sensors are used in automobiles, medical devices, computers and many other types of machinery items.
Thermistor VS Thermocoupler
|A narrow range of sensing||A wide range of temperature sensing|
|Non linear relationship between sensing parameter and temperture||Linear relationship between sensing parameter and temperature|
|Thermistor voltage is relatively high||Thermocoupler voltage id relatively low|
|Thermistor has poor sensitivity||Thermocoupler has good sensitivty|
|DC supply power is not required||DC power supply is required|
Characteristic of thermistors:
There are three type of ccharacteristic of the thermistor.
As we already know that thermistors are having the negative temperature coefficient so as their temperature increases their resistance decreases also the relationship between the resistance and temperature of the thermistors there is a non linear relationship between them. So as you that temperature increases the resistance decreases but thermistor it is showing a very good amount of change in the resistance for this temperature changes so we can see that the thermistors they are highly sensitive as compared to the platinum which is used for the construction of RTS and thermocouples. The mathematical expression for the relationship between the resistance of thermistor and temperature is,
RT1= RT2 exp [ß]
RT1= resistance of thermistor at temperature T1
RT2=resistance of thermistor at temperature T2
ß= constant depending upon the material of thermistor.
T1= initial temperature
T2= final temperature
Thermistor is the measurement of resistance at one temperature as compared to different temperature. Its value may be calculated by the formula shown degree in Kelvin.
The characteristic between the voltage and current of thermistor. Voltage os going to drop across the thermistor with the increasein the current. The current increase the voltage first increase after that it will starts decreasing. So the reason is when the temperature increased to a level this temperature is not enough that it is going to produce the heat in the thermistor. It will also produce the change in the resistance in the thermistor. So in this characteristic the voltage drop a thermistor increases with increasing current until it reaches a peak value. According to ohm’s law
The voltage and current showing the linear relationship in this range. After ppeak value, the voltage drop decreases with increase in current.
So if we see the graph the time delay to reach the maximum value of the currrent it is a function of the applied voltage when we decreasing the voltage the time delayis also decreasing. When the heating occurs in a thermistor, a definite time is necessary for the thermistor to heat and the current to increase to a maximum dependable state value. So this is called the time delay.
Ardino Temperaature Sensor:
Thermistors are effortless, low price, and precise components that make it uncomplicated to get temperature data for your projects. Remote weather stations, home automation systems, and equipment control and protection circuits are some applications where thermistors would be consummate. They are analog sensors, so the code isapproximately simple contrast to digital temperature sensors that essential special study. Thermistors are changeable resistors that reorder their resistance with temperature. They are assorted by the way their resistance counter to temperature changes. In NTC thermistors, resistance reduce with an increase in temperature. In (PTC) thermistors, resistance rise with an increase in temperature.
NTC thermistors are the most ordinary. NTC thermistors are made from a semiconducting substance such as a metal oxide that has been heated and squeeze to form a temperature sensitive conducting substance.
The conducting substance contains charge carriers that permit current to pass through it. High temperatures source the semiconducting substance to emancipate more charge carriers. In NTC thermistors construct from ferric oxide, and electrons are the charge bearer. In nickel oxide NTC thermistors, the charge bearer are electron holes.
The thermistor is a variable resistor, so we have to calculate the resistance before we can calculate the temperature. However, the Arduino can not calcalate resistance directly, it can only estimate voltage.
The Arduino will calculate the voltage at a point between the thermistor and a well known resistor. This is called a voltage divider. The equation for a voltage divider is
Vout= Voltage between thermistor and resistor
R1= Resistor value
R2= Resistance of thermistor
The value of the resistor should be violently equal to the resistance of your thermistor. In this occasion, the resistance of thermistor is 100k so resistor is also 100k .
The producer of the thermistor powee tell you it is resistance, you can use a multimeter to find out. If you do not have a multimeter, you can make an Ohm meter with your Arduino. You only require to know the magnitude of your thermistor.
Application of thermistor:
There are eight type of application as follow
- Measurement of temperature:
As you in the picture when thermistor is connected to battery and a micro ammeter ir van be used for the measrument of temperature as the temperature increases its resistance is going to decrease so as the resistance decreases the current increases so when it is connected in this circuit temperature is increased current is going to increase and this current is going to be indicated by the micro ammeter. So this micro ammeter directly calibraed so that the changein current directly gives the value of the change in temperature. So in this way thermistor is used as a measure of temperature.
- Control of temperature:
Thermistors also used for the control in temperature and also used along with a relay. So whenever the temperature is increasing beyond a certain range the relay is going to activated and it is going to cuttoff the supply ofcurent to the other instruments or other devices connected in the circuit.
- Temperature compensation:
The thermistor having negative temperature coefficeient which is present in the semiconductor materials so for the compensation of temperature they can also used for this type of thermistors.
- Measurment of power at high frequency
- Measurment of thermal conductivity
- Measurment of level, flow, pressure
- Vaccuum measurement
- Providing time relays
- Compact, rugged and inexpensive.
- Good stability, highly sensitivity in NTC region
- Possibility of point measurements.
- Response time is fast over narrow temperature range.
- Cost is low.
- It is small in size.
- Not affected by environmental conditions.
- Not suitable for large temperature range. Maximum range 400 degree celsius.
- To avoid self heating.
- Needs fiters, shielded power lines etc due to high resistance.
- Temperature VS resistance characteristic Non-linear.
- Needs external DC power supply for its operation.
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