Definition of thermistor:
- Thermister is a contraction of the expressions thermal resistor so we can mention that it is built of two terms thermal and resistor.
- The word ‘thermal’ means it is a special type of resistor whose resistance replace 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
- A best chemist and also a physicist named Michael Faraday in 1791 is well known for his work in electromagnetic induction. Because the inconvenient thermistors have the strong ability for manufacture and petition for the technology were confined, commercial counterfeit and make use of thermistors did not set after hundred years.
- During 1940s, Bell Telephone developed techniques to improve for the consistency of the manufacturing process.
- Some of the first display thermistors were disc type and now the thermistor were at quite broad.
- These devices were used fundamentally for regulation, shielding, and temperature recompense.
- As the dependbility of these devices modified during the 1980s, the use of electronic thermometers in the health care industry has grown up.
- The rising rate of sterilization and concerns about cross-infection for low-cost disposable temperature probes, for which chip thermistors were suited. During the year 1980 and 1990, the purpose of thermistors has continued to grow in many different companies like food processing, medical, HVAC, and also in telecommunications comapny.
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 substance in which the temperature in which the resistance grows with the increasing temperature they are assuming to have a positive temperature coefficient.
- On the other hand the substance in which the resistance decline with growing in temperature they are assuming 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.
- Mainly the semiconductors having negative temperature coefficients. 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%.
- The two thermistors are used to for accuracy measurements, control because they are mostly sensitive elements having very high rise 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 various types of thermistors.
- They all work on the same fundamental. A concept of variable resistance depends on temperature.
- There are mains and two important types of thermistors NTC and PTC. These NTC and PTC mainly works.
Negative Temperature Coefficient:
- NTC thermistors are the most frequently type available for use.
- The decided propeties of this thermistor is when its resistance reduces as temperature increases.
- These sensors are obtainable extensive all round the HVAC industry, transportation and home 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 happen at low temperatures where it can vanish much more willingly into the enclosed 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 properties of thermistors is not customary but they do execute a particular function of 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 under high point the opposing 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 metal opposite 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.
Comparsion b/w thermistors with different sensors:
|Temp range||Within 50 celsius||-260 to +850 celsius||-40 to +100 celsius||-20 to +105 celsius|
|Relative cost||Inexpensive||Mostly expensive||Comparatively expensive||Comparatively expensive|
|Time constant||6 to 14sec||1 to7sec||1 to3sec||2to60sec|
6.Best of measuring single point temperature
3.Widest operating temperature
4. Best of measuring of range temperature
2.Limited temperature range
3.Slow response time
|1.Limited temperature range
|1.Slowest response time
2.Limited temperature range
THERMISTOR Work as Controlled System
- The principle of a thermistor is to calculate the temperature of any devices. In a temperature controlled system, the thermistor is a compact but main piece of a big system. A temperature controller detect the temperature of the thermistor. Then it notify us to tells a heater or cooler when wants to turn it on or off for the maintainess of temperature.
- In the diagram, demonstrate an example, there are three main components used to contro the temperature of a device. One is temperature sensor, second is temperature controller, and the third is Peltier device. The sensor forefront is connected to the cooling plate that needs to maintain a particular temperature to cool the device,on the other wires are connected to the temperature controller.
- The temperature controller is also electronically attached to the Peltier device, which heats and also cool the device. The heatdissipation is connected to the Peltier device to help with heat dissolution.
- The task of the temperature sensor is to send the temperature report to the temperature controller. The sensor has a tiny amount of current passing through it, called bias current, which temperature controller is sending. The controller can not lppk through resistance, so it convert resistance changes to voltage changes by the use of current source to apply a bias current passing the thermistor to build a control voltage.
- The temperature controller is the main part of this operation. It takes all the sensor information compares it with the part which wants to cooled called the setpoint and then modified the current through the Peltier device to convert the temperature for equal the setpoint.
- The position of the thermistor in the system influence both the stability and also the precision of the control system. For stability, the thermistor required to placed as closer to thermoelectric as feasible. For accuracy, the thermistor required the position to close to the device essentail temperature control. The thermistor is implant in the device but it can also be connect by using thermally conductive glue. Even if a device is implant air gaps should be remove using thermal glue.
- The figure below shows two thermistors, one connected directly to the device and one with the remote. If the sensor is far from the thermal device the time primarily decrease the perfection of the temperature evalution while adjust the thermistor far from the Peltier device reduce the steadiness.
- The thermistor connected to the device behave rapidly to change in thermal load and also recorded precise temperature. The isloated thermistor also reacted but not rapidly. Basically the readings are coming in a small but more half a degree. This variation can be very remarkable when precise temperatures are necessary.
- Once the arrangement of the sensor. The rest of the system required to be setup. This incoporate condition of the base thermistor resistance. The bias current of the sensor, and the actual value of temperature of the load depending on the temperature controller.
- Thermistors are temperature-depending on resistors which changes with temperature. They are very sensitive and behave a small changes in temperature. They are best for the used when a particullar temperature require and also for maintain and also for monitoring temperatures range 50°C of background.
- Thermistors is the part of a temperature control system and habe best ability to measure and control the heating and cooling of a Peltier devices. Their capacity to balance in minute increase and permit for the greatest system strength. Thermistors can be implanted in the surface-mounted on the device required temperature for monitoring. Determined, they can calaculate the quantity of liquids, gases,and solids.
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
- Vin= Vcc
- R1= Resistor value
- R2= Resistance of thermistor
The value of the resistor should be violently equal to the resistance of your thermistor.
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.
What is a Thermistor