Potential Divider Aims What is a potential divider How can we supply a variable pd from a battery How can we design sensor circuits

A potential divider normally consists of two resistors in series with a fixed pd across them. See diagram I R1 R2 Vin Vout

What is the potential divider used for? To supply a pd which is fixed at any value between zero and the source pd (VIN) To supply a variable pd To supply a pd which varies with physical conditions such light intensity or temperature

in the same ratio a the resistors 1. To supply a pd which is fixed at any value between zero and the source pd (VIN). A B C I R1 R2 If NO current is drawn through the output, then by Ohm’s Law Vin Vout and Vout = I x R2 so 2 1 R V in out + ´ = The voltage is divided in the same ratio a the resistors

Volts k R V 100 5 10 150 = + ´ Example 150V A Vout B So RAB = 5 Ohms (i) Find Vout Volts k R V in out 100 5 10 150 2 1 = + ´ 150V 5 k A 10 k Vout (ii) Find Vout if another 10 k resistor is connected across AB B So RAB = 5 Ohms Vout = 75 V

2. To supply a variable pd Sliding contact The source pd (VIN) is connected to a fixed length of uniform resistance wire. A sliding contact on the wire can be moved along the wire. The VOUT varies as the sliding contact is moves along the wire. This set up can be used as dimmer switch to vary the brightness of a bulb or vary the loudness of a loudspeaker.

Replacing R2 with a Light dependent resistor (LDR) 3. To supply a pd which varies with physical conditions such light intensity Replacing R2 with a Light dependent resistor (LDR) The LDR is a component that has a resistance that changes when light falls on it. As the intensity of the light is increased so the resistance of the LDR falls. If the LDR is connected as part of a potential divider as shown in the diagram on the next slide then as the light level is increased its resistance falls and the proportion of the input voltage dropped across it will also fall. So in the light V2 is low and in the dark V2 is high.

3. To supply a pd which varies with physical conditions such light intensity LDR V Applications

Replacing R2 with a thermistor Applications Replacing R2 with a thermistor Something very similar happens if R2 is replaced by a thermistor. As the temperature of the thermistor rises its resistance falls and so the voltage dropped across it falls. When the thermistor is hot V2 is low and when the thermistor is cold V2 is high.

Applications V2 R1 thermistor V