Simple Thermistor Circuits

thermistor



In this article, we go over how to build simple thermistor circuits.

A thermistor is a specialized resistor which changes resistance value depending on the amount of heat which it is exposed to. Its main characteristic is that it is thermally sensitive; in response to the heat it is exposed to, it alters its electrical resistances to changes in temperature. It can be used to measure temperature, or to sense temperature changes and act accordingly for the temperature changes, depending on its designed use for the circuit.

There are 2 main types of thermistors; negative temperature coefficient (NTC) thermistors and positive temperature coefficient (PTC) thermistors. NTC thermistors are thermistors whose resistance decreases when the temperature they are exposed to decreases. PTC thermistors are thermistors whose resistance increases when the temperature they are exposed to increases.

In this article, we do examples with both NTC and PTC thermistors.

Thermistor Circuits

NTC Thermistor Circuit
PTC Thermistor Circuit


NTC Thermistor Circuit

Now we will build a simple NTC thermistor circuit.

The parts we will need to do is:

Parts

  • 100KΩ NTC Thermistor
  • 100KΩ Resistor
  • 12V DC Fan
  • 6V DC Motor
  • DC Power Supply (to give 15V)


The circuit we will build is shown below:

NTC thermistor circuit

You can see the thermistor is below the resistor. The thermistor is a resistor with an arrow going through with the letter 'T' printed beside it.

This circuit is a voltage divider circuit. The amount of voltage which feeds the entire circuit is 15 volts DC. This 15 volts is divided between the resistor and the thermistor. When the thermistor is at room temperature, the voltage is divided up almost equally between the 100KΩ resistor and the 100KΩ thermistor. In this setup, the resistor and the thermistor get about 7.5V each of the 15V DC power. When they both get 7.5V, the DC motor supply runs. This is because it is a 6V DC motor. The 7.5V is sufficient to power it, and, thus, the motor spins. This is what occurs when the thermistor is exposed to room temperature or below.

Now apply a great enough amount of heat directly to the thermistor. With a NTC thermistor being exposed to high heat, its resistance will drop signficantly. It may even lower to near 0Ω of resistance. With this setup, the majority of the 15V powering the circuit will drop across the 100KΩ resistor. Remember that voltage drops across the greater resistance. Being that the thermistor's resistance drops significantly to almost 0Ω, it receives very little voltage now in the voltage divider. So most of the voltage drops across the resistor, over 14V. With this setup, the motor no longer receives sufficient voltage to power on. At the same time, being that over 12V falls across the 100KΩ resistor, it now receives sufficient voltage to power on the 12V fan.

So without any heat applied to the thermistor, the DC motor runs; the fan does not. When sufficient heat is applied to the thermistor, the motor shuts off and the fan powers on.

This is a classic example of a NTC thermistor circuit.



PTC Thermistor Circuit

Now we will build a simple PTC thermistor circuit.

The parts we will need to do is:

Parts

  • 10KΩ PTC Thermistor
  • 1KΩ Resistor
  • 12V DC Fan
  • 6V DC Motor
  • DC Power Supply (to give 15V)


The circuit we will build is shown below:

PTC thermistor circuit

This circuit, just like the one before, is a voltage divider circuit. The voltage supplied to the circuit is 15VDC. This 15V gets divided up between the 1KΩ resistor and the 10KΩ PTC thermistor.

When the thermistor is exposed to cold or low temperatures, its resistance is very low. Remember that PTC thermistors are the opposite of NTCs. When temperatures are low, the resistance it offers is low. When temperatures are high, the resistance it offers is high. So when the thermistor is exposed to low temperatures, the thermistor offers very little resistance. So in this circuit, the thermistor may offer 100Ω or less of resistance. So the majority of the voltage goes to the 1KΩ resistor. In this setup, the 6V DC motor receives sufficient voltage to be powered on, while the 12V fan does not. Therefore, the motor operates and run, while the fan does not. This is when the thermistor is exposed to low temperatures.

Now if a good deal amount of heat is applied to the thermistor, its resistance increases dramatically to near its rated value. Thus, the resistance may rise to near 10KΩ. When this is the case, the majority of the voltage falls across the thermistor. In this setup, the fan has sufficient voltage to be powered on, while the DC motor does not. So when a great deal of heat is applied to the thermistor, the fan turns on, while the motor shuts off.



These simple circuits help to show how NTC and PTC thermistors work. While NTC thermistor's resistance value decreases while the temperature it is exposed to increases, PTC thermistor's resistance value increases when the temperature it is exposed to decreases. Thus, in circuits, circuit designers can exploit these principles to accomplish the needed task in a circuit to compensate for temperature fluctuations which may exist.

Related Resources

How to Test a Thermistor

Thermistor Resistance- Explained



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