Potential Dividers and their application as sensors

Electricity Lesson 9

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.

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.

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.

V S R 2 V 2 R 1 V 1

Deriving Potential divider equation

 For an unloaded potential divider the current is the same through both resistors

V V

2 1 

IR IR

2 1 

So the voltage is proportional to the resistance V V

2 1 

R R

2 1

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.

V INV V out

Alternative formula



The potential divider equation can be derived by rearranging the ratios above to give:

 V output = R1 / (R1+R2)  V input.

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

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

Example

6 V 50  A 100  B

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?