Tuesday, 13 May 2014

Resistance of a Thermistor

This experiment gives a way to find the resistance of a thermistor at a fixed temperature.

Equipment

Water bath held at 20 degrees C
Jug of cold water
Thermometer
Thermistor
12V d.c. power supply
Variable resistor
Connecting leads
Ammeter
Voltmeter

Method

Place the thermistor in the water bath with the thermometer to allow you to monitor the temperature throughout.

Connect the power supply, thermistor and variable resistor and ammeter in series. Then connect the voltmeter in parallel across the thermistor.

Adjust the variable resistor so it is allowing the least current to flow through the circuit and note the current. Then adjust the variable resistor to find the maximum value for the current flow. This is the range of your independent variable. Use these values to decide on a suitable interval for your current values to ensure that you get at least 5 different values of the current

Starting at the lowest current value record the current and voltage. Increase the current using the variable resistor and again record the current and voltage. Repeat this increase and measurement until you reach the maximum current.

Monitor the temperature of the water to ensure that the thermistor is not heating the water during operation.

Take another two sets of readings for the same values of current you used on the first run though. Then calculate an average of these values.

Safety

Be careful that the water from the water bath does not splash onto any of the electrical equipment being used. Also be aware that they thermistor may be warm after use - the temperature should be 20 degrees however if you are not carefully maintaining this the thermistor may get warm.

Analysis

Plot a graph of current on the x axis and voltage on the y axis. The gradient of this graph will be the resistance of the thermistor