Potential Divider Circuits
National 5 Extension
In a series circuit the supply voltage is divided up between the components in the circuit
i.e.
V
V
=
V +V
s
s
1
where
R1
Vs = supply voltage
V1 = voltage across R1
V2 = voltage across R2
R2
V1
2
V2
From Ohm’s law we know that since current is constant in a series circuit, the higher the
resistance of a component the greater the potential difference across it.
This idea is used in the following example to calculate the potential difference across
components in a ‘potential divider’ i.e. series circuit.
24V
Example
8
4
Use the fact that the voltage ‘split’ across each component is in the same ratio as the
resistance of each component.
V1
=
=
=
R1 x Vs
Rt
4 x 24
12
8V
V2
=
=
=
R2 x Vs
Rt
8 x 24
12
16 V
where Rt = total resistance
( Remember to check your answer e.g. does V1 + V2 = Vs )
Lastly!
Circuit problems in electronics are usually drawn slightly differently than you are used to
seeing.
e.g.
+ 24 V
24 V
8
would be
drawn as
4
8
4
0V
Find the potential difference across each resistor in the following circuits:
(1)
+ 12 V
(2) + 24 V
10 
6
3
10 
6
3
0V
(4)
+ 12 V
0V
0V
(5) + 10 V
(6) +36 V
2
4
20 
10 
6
16 
0V
(7)
+ 24 V
0V
0V
(8) + 24 V
(9) + 24 V
10 
8
36 
5
4
18 
0V
(10)
0V
(11)
+ 36 V
0V
(3) + 36 V
0V
(12)
+ 36 V
+36 V
9
12 
21 
3
4
7
0V
0V
(13)
(14) + 10 V
+ 12 V
(15) + 240 V
6
40 
36 
3
120 
144 
0V
0V
(16)
0V
(17)
+5V
(18)
+ 36 V
+ 24 V
2 k
15 
36 
3 k
25 
180 
0V
0V
(19)
0V
(20)
+ 240 V
0V
+ 20 V
62 
22 
434 
100 
0V
Helpful Hint
LDR’s and thermistors often make up part of a potential divider circuit in electronic systems.
It is important to remember that the resistance of these components varies with external
conditions.
Use the following data to answer questions 21 – 25.
The tables below show how the resistances of a certain LDR and thermistor vary with
external conditions.
LDR
Thermistor
light condition
dark
light
bright
resistance ()
10 000
2 500
20
temperature (oC)
10
40
100
resistance ()
4 000
1 980
200
21. The following circuit is part of the input to an electronic frost alarm.
+ 230 V
Calculate the potential difference across the
thermistor when it is
100 
(a)
(b)
10 o C
40 o C
0V
22. The following circuit could be part of a light meter for a camera.
+ 12 V
Use the information above to find the potential
difference across the LDR when it is:
(a)
(b)
1 k
0V
dark
light
23. Calculate the potential difference across the resistor in the following circuit when the
temperature is:
+ 24 V
(a) 100 o C
(b) 40 o C
500 
0V
24. A young engineer designs part of an electronic system to trigger an alarm when it gets
too bright.
Determine the ‘trigger voltage’ across the resistor in the following system when the light
level becomes ‘bright’.
+ 36 V
1·5k
0V
25. Determine the temperature at which the following voltmeters will show identical
readings.
+ 230 V
V1
200 
0V
V2