Physics remains inadequate if you don’t talk about thermistors. Well, these remarkable devices, thermistors, are the invention of Michael Faraday, who brought this concept to light. He invented the first thermistor in the year 1833. A thermistor is a hybrid of a thermal and a resistor. The common thermistor is an RTD which is highly sensitive to temperature. There are some fantastic features of thermistors that we are going to decode the science behind thermistors through this post for you! So wake up that physics nerd inside you and join us to discuss thermistors right now!

Basic Principle of Thermistor Circuit

Thermistors work on a basic principle so that the devices can work at their finest without any disturbance. 

• A thermistor is not a metallic device and works as a resistor and has a similar working phenomenon to a semiconductor with a negative resistance.
• The resistance of thermistors directly depends on the temperature. Higher temperatures mean more resistance.

Parts Of Thermistor Circuit

Thermistors cannot survive without some crucial components of their work:

• A special metallic tube that contains sensors inside a thermistor.
• A fine insulative layer that divides the thermistor from the metal tube.
• A wiring network made from lead that extends out from thermistors.

Different Types of Thermistor Circuits

We are going to explain about common types of thermistors:

Negative temperature Coefficient or (NTC) Thermistor Circuit

• In an NTC thermistor, the resistance and temperature are inverse to each other. 
• These thermistors are highly sensitive to even a slight change in temperature therefore these are reliable for censoring temperature changes in the devices. 
• You can trust a thermistor when you need to record the temperature changes and amounts of currents.
• It keeps the resistance low and lets more current pass through the system until it reaches an optimum level which is useful for the devices like fire censoring equipment, baking ovens, and ACs. 

Positive temperature Coefficient or (PTC) Thermistor Circuit

• A PTC thermistor is directly proportional to the temperature. The resistance increases as you will increase the temperature. 
• These are reliable for guarding the system against overcurrents that can lead to electrical fuses and glitches.
• You align the PTC thermistors in a series circuit that helps to cope with the electrical fuses. 
• There are two types of PTC thermistors, and we are going to have a deep look at these below:

Silistor Temperature sensor:

• It has a silicon structure and follows a linear temperature graph for functioning. The resistance follows the temperature if it increases. 
• These thermistors are not abundantly available in the market, but they are still existing.

Switching type Circuit

• It is another type of PTC thermistor that sometimes acts as an NTC thermistor.
• There is a specific temperature limit after which the nature of this thermistor just changes, like the colors of the sky. Now this temperature limit is known as Curie point. 
• Some substances are more likely to undergo sudden changes in their nature when they cross Curie Point.
• The resistance after crossing the Curie point also improves quickly. 
• The fine applications of PTC thermistors are for small to midrange heaters, electric motors, and thermostats.

Disk & Chip Thermistor Circuit

• Such thermistors have fine metallic contacts.
• These often have lagging responses as compared to other variants.
• The size of these thermistors is larger than the others.
• A copper bar is important for maintaining the sensitivity of this thermistor on all counts.
• It also offers impressive power dissipation, which is important for different devices. 
• Due to the abovementioned features of this thermistor, it offers exceptional performance than bead thermistors. 
• Such thermistors also offer exceptional endurance to tackle high levels of current. 

Epoxy Thermistor Circuit

• You need an industrial-grade epoxy to solder it between the wires made of Teflon and PVC.
• The size of these wires is small.
• Due to their small size, these do not need extensive assembly procedures.
• You can easily change the shape and position of these wires as well. 

Bead Thermistor Circuit

• The wiring for bead thermistors works best if you use platinum.
• You also need to connect them with the ceramic to improve the performance.
• These thermistors are more reliable since they are fast with responses, offer more stability, and can handle extreme temperatures impressively. 
• These thermistors are delicate in nature.
• Since these are delicate and sensitive, a protective glass covering is important to conceal them.
• Now, there is no need to worry since this covering does not create a negative impact on these thermistors at all. 

Glass Encapsulated Thermistor Circuit

• These show efficient performance when you are talking about extreme temperatures. 
• There is a glass layer that encapsulates these thermistors, thus named glass encapsulated thermistors.
• These offer a higher degree of stability and also are reliable for adverse environmental changes. 

Probe Assemblies:

• You use these thermistors probably every day in your life. Shocking, isn’t it? Your geysers and air conditioners use probe assembly thermistors. 

Surface mount Thermistor Circuit

• Such thermistors have important applications in the motherboards of consumer electronics like laptops and PCs. 

Composition and Structure of Thermistor Circuit

The shapes & designs of thermistors know no boundaries. Consumers use different types of thermistors for their personal use. In fact, industrial producers also rely on various sizes & shapes of thermistors as per the projects. These types include different materials for production as well. Iron, cobalt, and nickel are recommended materials that come in handy for the production of such thermistors. You need to grind the metallic oxides and change them into a powder-like form. 

After this, you will compact this powder which is called centering. It is this powder that lets you make different shapes of thermistors as per your requirements. Tools like probes are for shaping these thermistors into rod-like or bead-like thermistors. You also need to determine the temperature range of the application that you are making a thermistor. Different metals have different temperature ranges, which is why you need to make sure that your thermistor is the best fit for this application. 

Amazing Benefits of Thermistor Circuit

Thermistors give you multiple advantages in daily life. You are going to be grateful for this marvel of physics after reading these benefits below:

1. Incubators used in the labs consist of thermistors for monitoring the temperatures of the samples.
   1. Thermistor is an affordable option to go for instead of other pricy temperature regulatory devices and sensors.
   1. Thermistors are easy and affordable to shape, which is why these are much more convenient to include in consumer electronics.
   1. Water cannot harm a thermistor at all, which is why it is safe to use around water and moisture.
   1. Lesser amount of current can prevent the thermistor from overheating.
   1. The properly developed thermistor, like the bead thermistor, has satisfactory strength.

Working & Testing of Thermistor Circuits

There are only two factors that hugely influence the performance of a thermistor. One is resistance, and the other is temperature. The resistance and temperature also influence each other and can change according to the situation. It is important to check the thermistors to ensure their fine performance. Some tools like an ohmmeter and a multimeter can help you with testing the thermistors. 

If you are using a PTC thermistor, just join its ends with the multimeter. Now heat up the sensor, and you will see that the resistor with resistance will start to increase. That proves you are testing a PTC thermistor since the temperature is directly proportional to the resistance in the case of a PTC thermistor. For NTC thermistors, the resistance will alleviate due to an inverse relation. 

Different Projects of Thermistor Circuits

Automatic fan Controller with Temperature Control

If you increase the temperature, it will turn the fan on.

You need the following components to successfully complete this project:

• A Thermistor 
• An industrial-grade resistor
• A fan that is highly important for this project
• A transistor 
• A battery of 9V

Working

You need to construct a circuit that contains 10K ohm thermistors which are joined with 10K ohm resistors. We are using an NTC-type thermistor for this project. It means when you will increase the temperature, it will decrease the resistance. Now the thermistor includes a 10k ohm resistance, as we know. 

Now connect the fan with a transistor. Start heating the thermistor, and surely it will increase the temperature. When this happens, it will drive the resistance to decrease, which will turn on the fan. The contrary will happen, and it will switch off the fan when the temperature decreases. 

Specified Temperature Thermistor Circuit

We are now using an abThermistor & an amplifier (operational) for this project. Now this OP-AMP will act as a voltage comparator. Here is what you need to remember. Every amplifier (operational) can be a voltage changer, but every voltage comparator cannot be an OP-AMP. When we take LED1 and LED2, we will notice that the thermistor will record the resistance as per the temperature changes. More temperature means less resistance, and vice versa. 

The following components are important for this project:

• Battery
• Light emitting diode 
• Lm-393 
• Thermistor
• Resistor

So we are working with two dividers for this project. You need to fix one divider and leave the other one as it is without fixing it. 

The fixed divider also has a fixed amount of resistance, and you can consider it as a reference voltage. The non-fixed divider has a thermistor that has a variable resistance. 

The OP-AMP will record the inputs of both these dividers. The job of the comparator is to keep comparing these inputs and minus one input from the different. 

Remember, we have not used any feedback loop for this project. The amplifier (operational) will follow saturation, and the output will remain lesser than 9V, which is the actual voltage of the battery that we are using in this circuit. Once you see the voltage approaching 9V, it will turn on LED 2, and when it approaches 0V, it will switch on LED 1. The LEDs will be not too bright since the amplifier (operational) does not interpret 0V because it is an ideal situation.