So far we have assumed, that the resistance of components in an electrical circuit does not change. This is ok for resistors, which are designed so that their resistance remains fairly constant, despite changes in temperature. However it is important to realise, that the resistance of some materials does change significantly, with changes in temperature. When current flows through a component, it generates heat and causes the temperature to increase.
The resistance of a metal increases as its temperature increases, (the technical term for this is that: metals have a positive temperature co-efficient of resistance).
This explains why when tungsten filament light bulbs fail, they tend to "blow" when you first switch them on. As the tungsten filament ages, then eventually the heat generated by the current, will cause the element to fail. The resistance of the tungsten element increases significantly and quickly when you first switch it on. This is due to the increase in temperature due to the current flowing through the filament. This means the resistance of the bulb is much lower, when you first switch it on and therefore the current is highest at this time. It is this initial surge of current, that causes the bulb to blow. |
The resistance of a semiconductor decreases as its temperature increases (it has a negative temperature co-efficient of resistance). Semiconductors are used to make components called thermistors which are used as temperature dependant resistors.
As described above, conventional light bulbs eventually blow when you switch them on, due to the current surge that occurs at switch on. For some types of bulbs this current surge would cause more severe degradation from first use onwards. To solve this problem a thermistor is connected in series with the filament. The initial high resistance of the thermistor, limits the current to a safe level at switch on. As the thermistor gets hot its resistance drops. However at the same time the resistance of the light filament increases and so it can now maintain a safe level of current, without the additional high initial resistance, from the thermistor.
The diagrams below show graphs of voltage and current, (i.e. voltage & current characteristics) for a resistor, a metal and a thermistor. The resistance of each device is given by the ratio of voltage to current, (R = V/I (Ohm's Law)). As the resistance of a resistor does not change, the ratio of voltage to current is constant. Therefore the gradient of the graph is also constant, producing a straight line graph. The resistance of a metal increases as the current increases and so the gradient of the graph increases. For the thermistor the gradient of the graph reduces, because the resistance of the thermistor reduces as the current increases.
Note: Often the graphs are shown with the axis reversed, i.e. current on the vertical axis and voltage on the horizontal axis. The shape of the graph of the resistor remains unchanged, but the change in gradient on the metal and thermistor graphs are reversed. (The gradient of the graph would now represent conductance rather than resistance.)