CHAPTER 31 – ELECTRONIC CIRCUITS Thermistors


A thermistor is a variable resistor that changes resistance depending on the temperature it is exposed to. As temperature increases, the resistance of a thermistor decreases.


Thermistors are used in circuits when temperature needs to be monitored. Any appliance that needs to operate at a certain temperature uses a thermistor, for example ovens and fridges. The thermistor constantly monitors and activates the necessary equipment to keep the temperature at the correct levels.


CONCENTRATE! To remember what a thermistor is, split its name into two parts. ‘Therm’ means heat, which tells us that this is the variable factor that affects it, and ‘istor’ reminds us that it is a type of resistor.


:Fig. 31.20 Air conditioning units contain thermistors ? TEST TUBE 9. Phone chargers contain thermistors. Why do you think that is? 10. Research common uses of thermistors and list three appliances they are used in. EXPERIMENTAL INVESTIGATION 31.4 How does the resistance of a thermistor change at different temperatures?


Equipment Wires Switch


Method


1. Connect the switch, buzzer and thermistor to the battery in series, as shown in Fig. 31.21.


2. Close the switch and record your observations.


3. Place your hand on the thermistor to increase its temperature.


4. Record your observations.


5. Move your hand away and allow the thermistor to cool.


6. Record your observations.


:Fig. 31.21 To investigate how a thermistor operates at different temperatures


Thermistor j Go to page 101 of your Experimental Investigations Log to write up this investigation. Elements of Science 311 Switch Buzzer


SAFETY • Be careful of sharp ends on the wires.


• Do not set the power supply too high (check with your teacher before turning circuit on).


Power supply Buzzer Thermistor


Battery Crocodile clips (to connect wires to components if necessary)


;Fig. 31.19 A thermistor


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