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Thermistors

A thermistor is a resistor that is very sensitive to temperature.  This means that it is something that is inserted in an electrical circuit, and its resistance can change a lot when there is a small change in temperature.  Changes in resistance in a circuit can change the current and voltage in different parts of the circuit.  This effect can be used to switch electrical devices on or off.  For example, we may like the fire alarm to go off automatically when there is a fire.  We can use a thermistor to do this.   So we say that a thermistor can be used as a temperature sensor.
The filament in a simple light bulb also changes resistance when temperature changes.  When a metal wire or filament gets hot, it resistance normally increases.  So why can't we just use a light bulb as a temperature sensor?  Actually, we can.  The main difference is that thermistor changes resistance a lot more than a filament in a light bulb because a thermistor is made from a semiconductor.  This big change in resistance makes it a lot easier to design an electrical circuit to switch things (like fire alarms) on or off.

Just to get an idea on how big this difference is, the filament in a light bulb has a resistance of about 10 W at room temperature.  When the bulb lights up, its temperature is 2000 to 3000 ^oC.  Its resistance goes up to about 100 W.  On the other hand, if temperature changes from 0 to 100 ^oC, the resistance of a themistor could drop from 100,000 W to 1 W.  (Yes, thermistor resistance falls with temperature.)  There is clearly a huge difference in sensitivity.


So why does a bigger change in resistance make a difference?  To understand this, we need to look at how resistances in an electrical circuit affect voltages.  This diagram shows a battery connected to two resistances.  Suppose that R₁ is a normal resistance and R₂ is the thermistor.  The voltage from the battery is split into two parts by these resistances.  This means that each resistance can in turn act like a battery itself.  So R₁ can act like a battery with voltage V₁, and R₂ like another battery with voltage V₂.  But the total voltage must remain equal to the voltage of the actual battery on top.
Lets put in some numbers now.  Suppose that the battery is 10 V.  Suppose that R₁ is a fixed resistance of 1000 Ohms.  Suppose that R₂ the thermistor is 100,000 Ohm at room temperature.  The ratio is 1:100 and the 10 V is split according to this.  So voltage V₁ across resistor R₁ is about 0.1 V.  

Then suppose there is a fire and the temperature at the thermistor goes up to over 100 degrees Celsius.  Its resistance then drops to about 1 Ohm.  The ratio of resistances is now 1000:1.  The 10 V from the battery is then split according to this and gives about 9.99 V across resistor R₁.  Imagine that a fire alarm is connected across R₁ - to points A and B.  This fire alarm will start ringing.