Power Supplies

Power Supplies
When it comes to thinking about how to supply our circuits with voltage and current there are many
possibilities. We may connect the circuit directly to the mains supply, or we may use batteries. To begin
with we will take a closer look at battery power. Although you may think that a battery is a battery, there
are in fact many different types, each with their own uses, advantages and disadvantages. The picture below
shows just some of the Sanyo range of batteries.
For our purposes we need to consider four main types of battery.
Dry Cells
This type of battery is the one that we are most familiar with. They are the disposable alkaline batteries that
we use extensively in electronic products. They are cheap, easy to use and readily available in shops. They
are available in five sizes.
1.5V cell which has a capacity of 1.2Ampere Hours (Ah)
1.5V cell which has a capacity of 2.7Ah
1.5V cell which has a capacity of 7.8Ah
1.5V cell which has a capacity of 18Ah
As you can see these all have a voltage of 1.5V and are called cells. A cell is always 1.5V and a battery
consists of a number of cells joined together. For example a 6V battery is four 1.5V cells joined together in
series. The only difference is in their capacity which tells us how long a battery will last. The AAA cells
can supply 1.2Amps of current in 1 hour before they need to be replaced, however a D type cell can supply
18Amps of current in 1 hour before it needs replacing - in other words it would last 15 times as long.
So why don’t we always use D cells? It is quite simple really, they are physically much larger in size and
weigh a lot more. Not what is required in a small light weight portable music player for example.
We said that there are five sizes, the fifth is a PP3 battery.
9.0V battery which has a capacity of 0.6Ah
These are the familiar rectangular batteries, for example, as used in many smoke alarms. As you can see
they have a very low capacity, which means that they should only be used in products that require very little
current when working. However, they are much smaller than the equivalent of joining 6 cells together, and
cheaper too.
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Ni-Cad (Nickel Cadmium)
These batteries are rechargeable and come in exactly the same sizes as the dry cells, namely AAA, AA, C,
D and PP3.
1.2V cell which has a capacity of 0.3Ah
1.2V cell which has a capacity of 0.6Ah
1.2V cell which has a capacity of 2.4Ah
1.2V cell which has a capacity of 4Ah
8.4V battery which has a capacity of 0.1Ah
Can you think why the PP3 battery is rated at 8.4Volts?
The picture below shows a 7.2Volt Ni-Cad Sanyo racing pack commonly used in radio controlled cars. You
should be able to work out how many cells it is made up from, and the type of cell given the name ‘RC-2400’
where the 2400 refers to the capacity in mAh (milli Ampere hours).
Of course the main advantage is that they are rechargeable, so although they are more expensive to buy
initially and you need to buy a charger, in products where the batteries will drain quickly, such as in a radio
controlled car, they are more cost effective in the long run.
Another type of rechargeable battery is also available called a Ni-Mh cell or Nickel Metal hydride. These
are superior to Ni-Cad cells in that they have higher capacities.
Lead Acid Batteries
These batteries are commonly
rated at 12Volts but are also
available rated at 6Volts. They
have very large capacities, as
high as 70Ah, so are well suited
to applications which require
large amounts of current, such as
a car battery.
They are
rechargeable but have the
disadvantage that they are heavy
and bulky.
Lead acid batteries can leak acid
which is potentially dangerous
unless Sealed Lead Acid (SLA)
batteries are used which cannot
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Lithium cells are rated at 3.7Volts and have very large capacities for their physical size.
3.7V cell which has a capacity of 2Ah
3.7V cell which has a capacity of 8Ah
3.7V cell which has a capacity of 14.5Ah
The picture below shows a range of lithium cells and batteries supplied by Tadiran Batteries.
One of the uses of this type of battery is in marine applications, such as marine buoys as shown below.
Lithium cells are well suited to the switching on and off (called pulsing) of the
beacon on top of the buoy. Other advantages of using lithium batteries are.
Long life
High energy density (cell capacity compared to physical size)
Withstands extremes of temperature
Withstands harsh environments
Reduced size in comparison with alternatives
Reduced weight in comparison with alternatives
So as you can see Lithium batteries are well suited to the demands of powering
a beacon on a marine buoy. You may also notice that the buoy has solar panels
on it to provide another source of power. The buoy not only acts as a warning,
hence the beacon, but also measures, records and sends back (via the antenna on
the top) data about the sea.
So, as you can see when you talk about needing batteries to power your electronic product you have many
different types available to choose from, each with their distinct advantages and disadvantages. The choice
of which battery or cell to use depends on you defining, amongst others, exactly what voltage and current
your circuit requires, weight and size considerations and cost constraints. Only then can you select the type
of battery or cell that best meets the needs of your circuit.
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Mains Power Supplies
A disadvantage of all batteries is that they run down and have to either be disposed of and replaced or
recharged. Mains power supplies offer the advantage of never running out (except in a power cut) and
unless they are damaged never needing to be replaced. You are probably familiar with two types of mains
power supply for electronic products, pictures of which are shown below.
This is a bench power supply which
can provide a variable voltage to
suit a wide range of circuits.
This is a fixed power supply that is often used
to power printers, scanners, speakers etc. It
provides a fixed output voltage.
Both of these power supplies are plugged into the mains and convert an ac (alternating current) signal into
a dc (direct current) signal to power electronic products. But what do we mean by ac and dc?
This is an an ac (alternating current)
signal or waveform. This is because it
alternates or changes between positive
voltage (above the line) and negative
voltage (below the line).
This is a dc (direct current) signal or
waveform. This is because it is a fixed
value which does not change. The dc
voltage can either be positive (above the
line) or negative (below the line).
Mains voltage is an ac waveform which
has a maximum voltage of 240V in the
United Kingdom (different in other
Battery voltage is a dc waveform, for
example 1.5V or 9V. But unlike a
battery this voltage does not reduce over
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