1. Potential Dividers and their application as sensors Electricity Lesson 9

2. Learning Objectives To know that a potential divider is a source of variable potential difference. To derive and know how to use the potential divider equation. To know that the load affects the output of a potential divider To know the applications of potential dividers; to detect temperature or light levels.

3. Question How do we get a variable supply voltage from a fixed one? How can we measure physical changes in position, temperature or light level etc.

4. Answer A potential divider is one way of producing a variable p.d. A combination of a suitable sensor (angle / position sensor, thermistor or LDR) and a potential divider enables measurement / monitoring / control of physical changes.

5. VSVS V1V1 V2V2 R2R2 R1R1

6. Deriving Potential divider equation For an unloaded potential divider the current is the same through both resistors So the voltage is proportional to the resistance

7. Potential divider equation If R1 >> R2 then V1 is more or less the supply voltage If R1 << R2 then V1 is close to 0 V. VS as an input to the potential divider and V1 as an output. The circuit itself provides a way to tap off a voltage between 0 V and VS.

8. V INV V out

9. Alternative formula The potential divider equation can be derived by rearranging the ratios above to give: V output = R1 / (R1+R2)  V input.

10. Using potential dividers Use as a volume, brightness or contrast control. Making and designing a circuit to use as a temperature sensor Making and designing a circuit to use as a light sensor Controlling logic devices

11. Effect of different resistive loads Output voltage is affected by load resistance Connecting a resistor across the output reduces the output voltage Shorting out across a bulb reduces the total resistance of the bulb – the wire is in parallel with the bulb

12. Example

13. Questions A series circuit is connected as shown in the diagram. 1.What is the potential difference between A and B? 2.An additional resistor of 100 W is connected between the 50 W resistor and the cells. What is the potential difference between A and B now? 3.The additional 100 W resistor is now connected in parallel with the first 100 W resistor. What is the potential difference between A and B now?