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

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

3. 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

4. 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

5. 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

6. 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.

7. 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.

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

9. 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.

10. Applications V2 R1
thermistor V