Capacitors
Where have you come across the word CAPACITY
before?
The chances are it is at a football match, or a pop concert
or a baseball game. These venues have a maximum
CAPACITY which tells us the number of people the
venue can hold. When this maximum capacity has been
reached, the venue is full and no more people are
admitted.
Photo of the Skydome in Toronto
Like stadiums capacitors also have a maximum capacity, although instead of holding people, capacitors
hold CHARGE.
There are many different types of capacitor, but they all fall into either of two categories, POLARISED
and NON-POLARISED capacitors. Their symbols are shown below.
+
Non-Polarised
Polarised means that the capacitor has a
positive and a negative lead. These MUST
be connected the correct way around in the
circuit in order to work properly.
Polarised
Polarised capacitors are also known as ELECTROLYTIC capacitors. But what does the word electrolytic
mean? It comes from the word ELECTROLYTE which means a solution which is capable of conducting
an electric current.
The pictures below show what a range of capacitors looks like in reality.
Polyester Capacitor
Tantalum Capacitor
Non-Polarised
Polarised
General purpose use
where small size
gives a high packing
density.
General purpose use
where small values
and small physical
size are required.
Ceramic Capacitor
Electrolytic Capacitor
Non-Polarised
Polarised
Sometimes called disc General purpose use
capacitors due to their where higher values are
shape.
Used in required.
decoupling.
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Most of the capacitors we saw had both of their leads coming out of one end of the component. These are
called RADIAL capacitors. If the leads come out of both ends they are called AXIAL capacitors.
Radial Capacitor
Axial Capacitor
But why have two different types? Well, radial capacitors take up less area on a circuit board than axial
capacitors, but they stand much higher off the circuit board. Having said that, the majority of capacitors
you use will be radial capacitors.
In the pictures, we have seen capacitors that are broadly rectangular in shape and capacitors that are
cylindrical. So, how do these shapes come about? It all depends on how they are made.
Capacitors are made up of plates which hold the charge separated by a insulating layer called a dielectric.
One of the main factors in determining how much charge a capacitor can store is the physical size of the
plates. For small value capacitors, they are made like this.
Dielectric
As you can see the plates are separated by
the dielectric.
The overall shape is
rectangular and so is the capacitor.
Plate
Plate
A good example of this type of
construction is in the polyester capacitor.
Radial Leads
When the value of a capacitor increases so does the physical size of the plates. To prevent the capacitor
from becoming physically too large it is rolled up like a tube, which gives it its cylindrical shape.
Axial Leads
The plates are still separated by the
dielectric, but because they have been
rolled to reduce the size of the capacitor its
shape has become cylindrical.
Plate
Dielectric
Plate
A good example of this type of
construction is in the electrolytic capacitor.
Note that these capacitors could equally
have radial leads.
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Units of Capacitance
Capacitors are measured in FARADS (F), named after Michael Faraday. In use the Farad is a very large
unit indeed and so sub-multiples are used in capacitor values, for example.
1000mF (1000 milli Farads)
=
1F (1 Farad)
1000uF (1000 micro Farads)
=
1mF (1 milli Farad)
1000nF (1000 nano Farads)
=
1uF (1 micro Farad)
1000pF (1000 pico Farads)
=
1nF (1 nano Farad)
Typical values range from 1pF (or 0.000000000001 F) to 4700uF (or 0.0047 F). However, capacitors are
available in sizes as large as 10F but these are rarely used.
Working Voltage
As well as having its value printed on it (electrolytic capacitors have the value written as above, such as
220uF, but other capacitors often use a code to represent the value) a capacitor will also have its working
voltage written on it.
Value, 4700uF
Working Voltage, 6.3V
If this voltage is exceeded the dielectric will break down and the capacitor will short circuit. This will not
only destroy the capacitor but potentially other components in the circuit as well.
As a guide, it is best to make sure that the working voltage is at least twice as high as the maximum voltage
you are going to apply to the capacitor. In this way the capacitor should always be operating well within
its voltage limit.
Uses of Capacitors
The first use we will look at concerns DECOUPLING
power supplies. This is done when we have integrated
circuits (IC’s) in our circuit.
+ Voltage
These IC’s can often have their operation impaired by
fluctuations in the power supply. The capacitor passes
ripples or pulses of current to ground, which effectively
‘smooths’ the supply.
0.01uF
(10nF)
IC
The diagram on the right shows how to decouple an IC.
The capacitor should ideally be of the CERAMIC type and
placed as close as possible to the IC whilst trying to make
0 Voltage
its leads as short as possible.
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Uses of Capacitors
+ Voltage
The third and final use we shall look at concerns using capacitors in
TIMING circuits.
To create a time delay requires only two components, a capacitor and
a resistor. These are connected together in series between the positive
supply voltage and zero volts.
R
The length of time taken for the capacitor to charge up is referred to as
the TIME CONSTANT and can be worked out using a simple formula.
T = C x R
+
C
In this formula T stands for Time (Measured in Seconds)
C stands for Capacitance (Measured in Farads)
R stands for Resistance (Measured in Ohms)
0 Voltage
For example, suppose we have a resistor valued at 4.7 KOhms (4700 Ohms) and a capacitor valued at
1000uF (1 milli Farad or 0.001 Farad). What would the Time Constant be?
Calculation
T = CxR
T = 0.001 x 4700
T = 4.7 seconds
This time represents the time taken for the capacitor to reach 63% of its final voltage. So, for example,
suppose the circuit was connected to a 10V supply voltage then it would take 4.7 seconds for the capacitor
to charge to a voltage of 6.3Volts.
We can show how a capacitor takes time to charge (fill) and discharge (empty) by using a graph.
Voltage (V)
Supply Voltage (10V)
6.3V
CHARGING
4.7 Seconds
DISCHARGING
Time (seconds)
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