The Potential Divider

Electricity Lesson 10

Learning Objectives

To know what a potential divider is.

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.

Definition

What is potential divider?

A potential divider consists of two or more resistors connected in series with each other and a source of fixed potential difference.

Draw a circuit diagram for this arrangement.

Circuit Diagram

What is the pd across each resistor? We can use potential difference

between points A and B as a supply for another component.6 V

50 100

A B

Uses of a Potential Divider

To supply a fixed pd between any value between zero and the source pd.

To supply a variable pd from a fixed one.

To supply a pd that varies with physical changes e.g. position, temperature or light level

VS

V1

V2R2

R1

Another Example

Supplying a fixed pd

The total resistance of the circuit = R1 + R2

The current through the resistors = V0/(R1 + R2)11 IRV

22 IRV

21

101

RR

RVV

21

202 RR

RVV

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

2

1

2

1

IRIR

VV

2

1

2

1

RR

VV

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.

V0 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 V0.

Confused?

So what’s the difference between a variable resistor and a potential divider?

Confused?

When a variable resistor is used the total resistance of the circuit is being altered.

Also, the pd is not being taken across the variable resistor.

VINV

V out

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

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?

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.

Alternative formula

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

V output = R1 / (R1+R2) V input.