In a previous section, components in series were discussed. For a series circuit, the Voltage across each component added up to the supply Voltage. The diagram below shows a simple series circuit containing two Resistors:-
The values on the Voltmeters will add together to give the supply Voltage :-
Vs = V1 + V2
However, the value of each individual Voltage will depend on the Resistance of each Resistor, the larger the Resistance, the larger the Voltage share.
If the two Resistors have the same Resistance, then they will each have an equal share of the Voltage. In the above example, each Voltmeter would therefore show 6 V.
If the two Resistors had different Resistances, then they would share the Voltage following a ratio of their sizes:-
Example 1 -
For the above circuit, what is the Voltages across each Resistor, if the Resistances of the Resistors are as follows:-
R1 = 10 Ω
R2 = 20 Ω
V1 = ( 10 / ( 10 + 20 ) ) x 12
V1 = ( 10 / 30 ) x 12
V1 = 4 V
Series circuit therefore:-
V2 = Vs - V1
V2 = 12 - 4
V2 = 8 V
Note - the Voltage divider formula could have been used a second time to calculate the value of V2, either method is valid.
Applications of Voltage Dividers
In the above circuit, two Voltmeters were attached across the Resistors, showing the Voltage across each. If instead of a Voltmeter, an additional circuit is attached across the Resistor, this Voltage share can be used. The circuit below shows an application of this:-
In the above diagram, two motors have been connected across the Resistors. As the right-hand Resistor has double the Voltage share of the left-hand Resistor, the motor on the right will rotate at twice the speed.