Resistors
Resistors control the amount of CURRENT that flows in a circuit.
+Voltage
In this circuit, the
resistor controls
the amount of
current
flowing
through the LED
(Light Emitting
Diode).
Imagine a hose pipe spraying water on the garden. If you stand
on the pipe the water slows down or stops. This is like using a
large value of resistor.
The larger the RESISTOR, the more the flow of CURRENT
is reduced.
0 Voltage
Circuit symbol for a Resistor
Resistors come in many different values. The majority of resistors that you use will all be the same size, no
matter what value they have. So, how do you tell what a resistors value is?
Look at a resistor. You will notice that they all have coloured bands around them. There are four coloured
bands, one of which is usually a metallic colour like GOLD or SILVER.
So, what do these colours mean?
Each colour represents a number. So, for example, the colour Yellow represents the number 4.
The rest of the colours and their values are shown in the table below. You do not have to remember them
as in the exam you will have a formula sheet with them written on.
Colour
Number
To find out the value of a resistor, follow these stages.
BLACK
ZERO
1. Hold the resistor with the three coloured bands on the left.
BROWN
ONE
RED
TWO
ORANGE
THREE
Yellow = 4
YELLOW
FOUR
GREEN
FIVE
3. Look up the second colour in the table, and write down the
number.
BLUE
SIX
VIOLET
SEVEN
GRAY
EIGHT
WHITE
NINE
2. Look up the first colour in the table, and write down the number.
Red = 2
4. With the third colour band the colour relates to the number of
zeros that you need to add.
Brown = 0
So, this resistor has a value of 420 Ohms
Page 1
So, what about that metallic coloured band, what does that mean?
Resistors are not totally accurate, that is to say that our 420 Ohm resistor may well have a value of 415
Ohms, or 423 Ohms. This metallic coloured band lets us know how accurate the resistor is.
A SILVER band means that the resistor is accurate to ± 10%
A GOLD band means that the resistor is accurate to ± 5%
This band is known as the TOLERANCE
So, the resistor in the example is accurate to ± 10%.
To find out what RANGE of values your resistor can have, you first need to work out 10% of the value.
Calculation
10% of 420
=
10 x 420
100
= 42
Resistor Value Range = 420 ± 42
Resistor Value Range = 378 Ohms to 462 Ohms
If the resistor had a GOLD band, you would first need to work out 5% of the value.
Calculation
5% of 420
=
5 x 420
100
= 21
Resistor Value Range = 420 ± 21
Resistor Value Range = 399 Ohms to 441 Ohms
The manufacturers of resistors supply resistors in two main ranges, E12 and E24. For the E12 range, so
called because there are 12 main values, resistors are available in multiples or sub-multiples of;
10
12
15
18
22
27
33
39
47
56
68
82
for example 1, 100, 1000, 10000 and so on
for example 180, 1800, 18000 and so on
for example 390, 3900, 39000 and so on
for example 820, 8200, 82000 and so on
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E12 Value
Tolerance (10%)
Range of Values
10
1
9 to 11 ohms
12
1.2
10.8 to 13.2 ohms
15
1.5
13.5 to 16.5 ohms
18
1.8
16.2 to 19.8 ohms
22
2.2
19.8 to 24.2 ohms
27
2.7
24.3 to 29.7 ohms
33
3.3
29.7 to 36.3 ohms
39
3.9
35.1 to 42.9 ohms
47
4.7
42.3 to 51.7 ohms
56
5.6
51.4 to 61.6 ohms
68
6.8
61.2 to 74.8 ohms
82
8.2
73.8 to 90.2 ohms
100
10
90 to 110 ohms
As you can see by looking at the table, the
tolerance of the resistors has meant that in
the E12 series (so called because there are
12 values) we have all possible values
available to us.
So, if we wanted a 17.5 Ohm resistor, the
nearest value is 18 Ohms. But this resistor
has a range of 16.2 Ohms to 19.8 Ohms,
which includes the value we want.
However, our 18 Ohm resistor could have
a value of 16.5 Ohms or 19 Ohms. If we
wanted to assure ourselves that our
resistor was 17.5 Ohms we would have to
get a bag of E12 18 Ohm resistors and
measure the value of each one until we
found one that was 17.5 Ohms.
In practice we rarely need to be this
precise, and a 18 Ohm resistor would be
fine to use.
These 12 values are called PREFERRED VALUES
Common Resistors and What They Look Like
Metal Film Resistors. The most Power Resistors. Housed in a
common type of resistor you metal case (aluminium), they are
used in high power applications.
will use.
SIL Resistor Network.
Resistors of the same
value housed together
to reduce space. SIL
stands for Single In
Line (one line of legs)
Power Resistors. Housed in a
ceramic case, they are used in
high power applications. They
are also flameproof.
Surface Mount Device
(SMD)
Resistors.
Extremely
small
resistors, as small as
1.6mm long, 0.85mm
wide and 0.3mm high.
Used to reduce space.
Page 3
Variable Resistors
On occasions we may need to vary the resistance in a circuit, for this variable resistors are used. Whilst a
fixed resistor, like all the ones we have looked at already, have only one value, variable resistors can have
many values.
There are three main types of variable resistors.
Rheostats
Potentiometers
Presets
Rheostats and Potentiometers commonly have a spindle, or shaft, which is rotated. When rotated the
resistance changes. It is easy to change the value, but they can be changed accidentally.
An example of using a Rheostat would be in a light dimmer circuit. When the spindle is rotated, the
amount of current would change, and the brightness of the light would change.
The symbol for a rheostat might look like the one below, but they work in exactly the same way.
Alternative symbol for a Rheostat.
Potentiometers control the amount of Potential or Voltage produced from a circuit.
Presets do not have a spindle, instead you need to use a screwdriver or a special tool which is put into the
preset and rotated. When it is rotated the resistance changes.
Presets are used when you do not want to change the resistance once it has been set. They are also very
accurate as it often takes many complete rotations to change their value.
Common Variable Resistors and What They Look Like
Two different types of
potentiometer. The top
one has a spindle which
you rotate.
The bottom one is
called
a
slide
potentiometer as you
move a slider to change
the resistance. These
can be found on audio
mixing desks.
Three
different
types of preset. Top
left is an open type
(it has no cover)
whilst the other two
are closed types
(they have covers).
On all three pictures
you can see where
the screwdriver or
tool needs to be put
to rotate it.
Page 4
Calculations Involving Resistors
Sometimes we need to use a value of resistor that just is not available. In these cases we combine two
resistors which together will have the correct value.
There are two ways in which resistors can be connected, in SERIES or in PARALLEL. The diagrams
below show these two different connections together with how we work out the combined resistance.
Connecting Resistors in Series
Total Resistance (Rt) = R1 + R2
R2
R1
So, for example, if we had two resistors one of which had a value of 120 Ohms and the other a value of
150 Ohms then by connecting them in SERIES we would have a combined resistance of 270 Ohms.
Calculation
Total Resistance (Rt) =
=
=
R1 + R2
120 + 150
270 Ohms
Connecting Resistors in Parallel
R1
Total Resistance (Rt) = R1 x R2
R1 + R2
R2
So, for example, if we had two resistors one of which had a value of 100 Ohms and the other a value of
50 Ohms then by connecting them in PARALLEL we would have a combined resistance of 33.3 Ohms.
Calculation
Total Resistance (Rt) =
R1 x R2
R1 + R2
=
100 x 50
100 + 50
=
5000
150
=
33.3 Ohms
Page 5
Potential Dividers
One type of Variable Resistor that we have looked at is called a Potentiometer. These control the amount
of Potential or Voltage produced by the circuit. A potential (or voltage) divider always contains two
resistive elements. If we draw the circuit diagram for a potential divider it would look like the diagram
shown below.
+Voltage
R1
Voltage Out
R2
0 Voltage
As its name suggests the potential divider will divide (or split) the +Voltage into two parts.
+10V
Do you notice that the 10V supply voltage
has been divided into two parts?
6V
R1
Voltage Out = 4Volts
4V
You should also notice that the total of the
smaller voltages equal the supply voltage.
6V + 4V = 10V
R2
This is ALWAYS the case.
0V
Usually we are not told what the individual voltages are, we have to work them out. To do this we need
to use a formula.
Voltage Out =
R2
x Supply Voltage
R1 + R2
For example.
+9V
Voltage Out =
R1
100 Ohms
R2
200 Ohms
200
x 9
100 + 200
Voltage Out = 200 x 9
300
Voltage Out
Voltage Out = 6 Volts
0V
Page 6
Current Limiting Resistors used with LED’s
One common use of a resistor is to protect a LED from having too much current flowing through it. If this
happens the LED will be destroyed (it will shine very brightly before going out).
To work out what value of resistor to use, we need to make use of a formula called OHMS LAW.
V = I x R
In this formula V stands for Voltage (Measured in Volts)
I stands for Current (Measured in Amps)
R stands for Resistance (Measured in Ohms)
So, for example, if we had a LED in our circuit we would need to have a resistor in SERIES with it, this
is shown in the diagram below.
+9V
To work an LED typically has,
7V
a maximum current of 20mA (20 milli Amps or 0.02 Amps)
a voltage drop across it of 2Volts
2V
0V
Notice that the voltage across the resistor is 7Volts. This is because we have a 9Volt supply and the LED
has a voltage drop of 2Volts across it, which leaves 7Volts across the resistor.
Using this information we can now work out the value of resistor to protect the LED from damage.
Calculation
V=IxR
so
R=V/I
R = 7 / 0.02
R = 350 Ohms
However, if we look back at the work on Preferred Values we see that we can only obtain 330 Ohm or 390
Ohm resistors. To make sure that the current value never exceeds the maximum of 20mA we choose the
nearest HIGHER value. This is because a higher value of resistance will lower the amount of current
flowing through the LED. The LED will be slightly dimmer, but it will not be destroyed.
So, we choose a preferred value resistor of 390 Ohms for use in series with the LED.
Page 7
Resistors that Respond to Changes in the Environment
Whilst Variable Resistors need human intervention to change their resistance, there are two components
which change their resistance in the presence of LIGHT or HEAT.
A Light Dependent Resistor, or LDR, changes its resistance when exposed to light.
As the light level increases (it gets brighter) the resistance of the LDR decreases
Low
Resistance
High
This is often shown by using a graph, which is below, together with the symbol for an LDR and what some
LDR’s look like in reality.
Low (Dark)
High (Bright)
Light Intensity
A Thermistor (a shortened form of Thermo Resistor) changes its resistance when exposed to heat.
As the heat level increases (it gets hotter) the resistance of the Thermistor decreases
High
Again, this is often shown by using a graph, which is below, together with the symbol for a Thermistor
and what some Thermistor’s look like in reality.
Low
Resistance
-tº
High (Hot)
Low (Cold)
Heat Intensity
Page 8
Questions on Resistors
1. A fixed resistor has coloured bands as follows; Brown, Black, Red, Silver. Work out the value of this
resistor and its minimum and maximum values.
2. A fixed resistor has coloured bands as follows; Yellow, Violet, Orange, Gold. Work out the value of
this resistor and its maximum and minimum values.
3. Students are building an alarm circuit and want to use an LED to show that the alarm has been switched
on. They have drawn the following circuit diagram for you.
+12V
0V
i)
ii)
iii)
Work out the value of resistor that the students should use.
State the correct preferred value that they should use from the E12 range giving a reason for your
choice.
The students now want to use two LED’s in series instead of one so that they will be easier to
see. Work out the new value of resistor that they should use, and state the correct value from the
E12 range.
4. The diagrams below show resistors in series and parallel. For each work out the total value of
resistance of the combination.
560
1000
270
560
For the example using resistors in parallel, what do you notice about the total value when compared to the
value of one of the resistors?
Write down a statement about the total value of two resistors in parallel when each resistor is the same.
Page 9