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THE LIGHT DEPENDENT RESISTOR

This is a resistor which has a resistance that changes with the amount of LIGHT that falls on it. The light energy produces more free electrons which increases the current for a certain voltage across the LDR which means a drop in resistance.

In the DARK its resistance is LARGE (millions of ohms), in the LIGHT its resistance is SMALL (tens of ohms). Very little current will flow through it in the dark.

A graph for the LDR of resistance against light intensity is shown in Figure 1.

LDRs are used as light sensors to switch on devices such as street lamps when it gets dark and to operate curtains at night time. They can also be part of a fire alarm that detects the flame of a fire.

 

THE THERMISTOR

This is a type of resistor which has a resistance that changes with TEMPERATURE. The increasing temperature produces more free electrons and so the resistance falls.

At LOW TEMPERATURE its resistance is LARGE (thousands of ohms), at HIGH TEMPERATURE its resistance is SMALL (tens of ohms). Very little current will flow through it when it is cold. This means that its resistance increases as the temperature falls and this type of thermistor is called a negative temperature coefficient thermistor or ntc thermistor.

A graph for the thermistor of resistance against its temperature is shown in Figure 2.

You can investigate this property practically by doing the experiment described in the 14-16 Experiment guides called Thermistor resistance. (See: Thermistor resistance variation)


Thermistors are used as temperature sensors in thermostats in ovens and irons, in fire alarms and on the wing of a plane to detect when the temperature falls low enough for ice to form. They are also in use in premature baby units to detect when a baby may have stopped breathing.

 

A VERSION IN WORD IS AVAILABLE ON THE SCHOOLPHYSICS USB
 
 
 
© Keith Gibbs 2020