In the world of electronics there are many different devices that help or entertain us.
Whether it be a calculator to solve equations, or a gaming console to take us away from reality.
Every electronic device contains a circuit which contains vital components that contribute to the overall functionality of that device.
A Thermistor is a component which serves a specific purpose in many different circuits.
So, what does a thermistor do in a circuit?
The main purpose of a thermistor is to give a circuit the ability to sense changes in temperature. A thermistor is a component which has the ability to change its resistance as temperature changes.
Closer look at a thermistor
Let’s take a closer at the thermistor which will help you better understand what it actually does in a circuit.
The thermistor is essentially a resistor which has the ability to change its resistance.
This change in resistance is caused by an external change in ambient temperature. Think of it as a thermometer whose temperature is related to its resistance.
What this means is that a change in ambient temperature will cause a change in resistance.
The word thermistor is a combination of the words ‘thermal’ and ‘resistor’.
They are constructed using metal oxides which can be encapsulated into a bead, disk, or cylindrical form.
Below is the common circuit symbol of a thermistor;
Working principle of a thermistor
If we opened up a thermistor we would find a semiconducting material.
A semiconducting material is chosen as they have a greater resistance than conducting materials, while having a lower resistance than insulating materials.
Choosing the right materials is crucial as this directly affects the relationship between temperature and resistance (this can be viewed better by the graph curve of Resistance vs. Temperature)
They contain metal oxide semiconductors, binders and stabilizers pressed into wafers which are then cut into a particular form depending on their packaging they will be used in.
So, a change in ambient temperature causes a change in resistance within the semiconductor material. The key is that the initial resistance of the thermistor is known.
There are two types of thermistors, each having different working principles. However, their purpose stays the same which is to alter their resistance to changes in temperature.
The first is known as a Positive Temperature Coefficient (PTC) thermistor.
The resistance of a PTC thermistor increases as temperature increases and decreases as temperature decreases.
The relationship between resistance and temperature in a PTC thermistor is directly proportional.
Negative Temperature Coefficient (NTC) thermistors have reverse functionality.
Their resistance decreases with an increase in temperature and increase with a decrease in temperature.
The relationship between resistance and temperature in a NTC thermistor is inversely proportional.
Main purpose of a thermistor in a circuit
Just like how we have five senses (sight, smell, hearing, touch and taste) that help us interpret and navigate the physical world, a thermistor gives a circuit the ability to sense the physical world as well.
It does this by allowing it to sense changes in temperature by varying its resistance.
Because of this the thermistor is used in many applications where temperature sensing is a crucial component of the system.
Below are some common household devices that you will be familiar with that will use a thermistor as part of their circuit;
- Fire alarms
- Ovens
- Refrigerators
- Heaters
Other applications of a thermistor in a circuit
While their main purpose in a circuit is to sense temperature, the working principle of a thermistor enables it to be used for other applications too.
Inrush current limiting
In every electrical and electronic circuit you have a Steady-state current which is the nominal safe value of current seen by the circuit.
However, there are times when the current exceeds this steady-state value which can damage components of the circuit.
This current is known as Inrush current.
Inrush current is the maximum draw of current and occurs when the circuit is switched ‘ON’. It can last for a few cycles of the input waveform.
A thermistor can be used in a circuit to protect it from inrush currents.
When the circuit is off and no current is flowing, the thermistor has a high resistance. When the circuit is switched ‘ON’ the thermistor opposes the inrush current due to its high resistance.
As the flow of current warms the thermistor, its resistance drops thus allowing the current to flow at a much steadier rate.
Over-current protection
Thermistors can be used in circuits to protect components from Overcurrents.
Similar to inrush currents, overcurrent is an excess in current not nominal to the steady-state value which can be caused by overloading the circuit, a short circuit, a ground fault, or an arc fault.
A motor is a component that can be regularly subject to overcurrents.
If the motor is overloaded or its rotation has locked, an overcurrent scenario can occur causing high currents to flow through the motor.
High currents cause high temperatures which can place a thermal stress on the internal coils and windings of the motor.
PTC thermistors are used to reduce this thermal stress.
The overcurrents cause the thermistor to heat up which then increases its resistance thereby reducing and limiting the high currents.
Is a thermistor the same as a thermocouple?
A Thermocouple is a device whose main purpose is the same as a thermistor, which is to measure temperature.
However, while they both measure temperature, they go about doing it two different ways.
A thermistor does so by varying its resistance according to temperature.
On the other hand, the thermocouple varies its output voltage based on the connection of two dissimilar metals.
Is a thermistor better than a thermocouple?
If both do the same thing, which is better?
Each of them have their own advantages in different scenarios and choosing between a thermistor and thermocouple in a circuit comes down to the needs of the application.
Below is a table of some important characteristics of both the temperature sensing devices which will help you decide which is best suited for your application.
| Thermocouple | Thermistor | |
| Accuracy | High | Low |
| Temperature range (°C) | -50 to 250 | -200 to 1250 |
| Average response time (seconds) | 0.12 – 10 | 0.2 – 10 |
| Characteristic curve | Non-linear for negative temperature coefficients | Linear |
| Cost | Expensive | Cheap |
In general, thermistors are chosen when ruggedness, reliability and stability are of importance. This makes them great for applications and environments where conditions are extreme and electronic noise is present.
