Transcript Uses of a Potential Divider

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.
6V
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
Another Example
R2
V2
VS
R1
V1
Supplying a fixed pd

The total resistance of the circuit = R1 + R2
The current through the resistors
= V0/(R1 + R2)
V

V 1  IR1
V2  IR2
R1
V1 
R1  R2
0
V0 R2
V2 
R1  R2
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
V IR

V IR
1
1
2
2
V R

V R
1
1
2
2
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.