Electronics – Using a Thermistor

Background:

Last lesson we examined the operation of fixed resistors (ie resistors that have a constant resistance to the flow of electricity). We found that increasing the resistance reduced the current flow in the circuit.

Resistance can be found from the rule:

Ohm’s Law is the same rule re-arranged:

Thermistor:

A thermistor is a type of variable resistor. Its resistance changes as its temperature changes. It can be used as a sensitive temperature measuring device (eg. if the surface temperature of a plane’s wings falls below freezing point, a thermistor circuit connected to a buzzer could warn the pilot in time to take action to stop the wings from icing up).

Aim:

To investigate the connection between a thermistor’s temperature and its resistance.

The Circuit:

  1. Use Crocodile Clips to create this circuit of a power supply, resistor, thermistor, ammeter and voltmeter.

Set the power supply to 10 V.

Set the fixed resistor to 100 

Set the thermistor to a resistance of

15 ohms at 250C by clicking on the

number beside the symbol and

completing the resulting table as

shown on the left.

  1. Label this circuit: Thermistor Circuit by opening the Add menu and choosing Text. Type your label then drag it into place. Double click to alter the text

Name:……………………………………

  1. Open an Excel spreadsheet and set up a table so you can record the following measurements:

Temperature of Thermistor
T (0C) / Resistance of Thermistor
R (ohms) / Current Through Thermistor
I (amps) / Voltage across Thermistor
V (volts) /
40
30
20
10
0
-10
-20
  1. Copy your circuit [Edit  Copy Design] then paste it beside your Excel table.

[First remove the tick beside the Copy with Green Fill choice in the Options menu of Crocodile Clips if you do not want the green fill in ammeters and voltmeters].

  1. Note: the voltmeter and ammeter automatically change units at times so make sure you convert all measurements back to volts and amps before recording V and I in the table.

eg. 96.8 mA should be recorded as 0.0968A but 1.96V should not be changed

  1. Use a formula of the type: = D4/D3 to complete the last column.
  2. Use Excel to produce two graphs. Include correctly labelled axes and these headings:
  • Effect of Temperature on the Resistance of a Thermistor(plot R on the vertical scale and T on the horizontal)
  • V-I Characteristics of a Thermistor (plot V on the vertical scale and I on the horizontal).
  1. Finish by writing a summary of your findings. Include answers to the following:
  • As temperature increases/decreases, what happens to the resistance of the thermistor?
  • As resistance increases/decreases, what happens to the current and voltage?
  • Does the rule R= V/I still apply?

Assessment:

Staple the completed instruction sheet to a printout containing the circuit diagram, the table, the line graphs and your summary (could be typed at bottom of graphs). Make sure all sections are fully labelled, and your name is on all sheets.

Maureen Trotter – Monbulk College - Electronics – Characteristics of a Thermistor
Maureen Trotter