Choosing a Power Supply

1.9 Power Sources

Function, application and advantages/disadvantages of the following power sources used in electronic products:
  • batteries, including rechargeable types
  • mains power
  • solar power
Once a customer has purchased an electronic product they will have the additional running cost of the power supply.
An important design consideration of any electronic product, is the specification of the voltage, size, and capacity of the power supply
Power Supplies
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Batteries provide a very safe source of electrical energy and are the most likely power source to be used in your electronic products.

  • Calculate the energy used by the circuit.
  • What current is drawn by the circuit?
  • Is the size or shape of cells an important factor in your product design? Is space a problem?
  • What voltage is required? How many cells will be needed to make the right voltage?
  • Is there a manufactured battery which has the correct voltage?
  • How long will it need to last for, when being used normally?
  • What kind of battery holder and connector will you need?
  • How will the customer get access to replace the battery?

The Costs - Value for Money

making comparisons between different batteries and cells to calculate costs is difficult. This is because the amount of time a battery or cell can deliver a particular current or voltage depends on a number of factors:
  • age of the battery/cell
  • design and quality of manufacture
  • physical size
  • number of time each day it is used
  • temperature
  • current delivered with each operation
For example: Button cells are very expensive way of buying electricity, but their small size makes them very important in some applications such as watches and hearing aids. Rechargeable cells and batteries are initially expensive to purchase but can over a period of time be a much cheaper option. for miniaturised circuits with a low current consumption such as a timer, a mercury oxide button cell will be suitable, but is unsuitable for a circuit with a solenoid or transducer or even to light a normal LED for any length of time, an AA battery is more suitable.

To find out more about batteries go to:

Mains Current AC/DC

Electrical current can transfer energy from an energy source to a device in two ways. We are used to a cell providing current in one direction only. We call this direct current or DC.
However, it is possible to transfer energy using a current that changes direction all the time. By moving electrons one way, then back, then repeating, energy can be transferred. This is called alternating current or AC.


Alternating Current is the movement of electrons in a wire backwards then forwards repeatedly. In Europe this change repeats 50 times per second (or 50 Hz). In the USA, the frequency is 60 Hz.
AC is remarkably useful because it allows us to change electricity very easily using transformers which cannot work with DC.
If an oscilloscope (a voltmeter which shows a graph of how voltage varies with time )is connected to an AC power supply, the result is a graph like this:

Alternating Current (AC) is current whose direction changes from one way then back, repeatedly.


Direct Current is the movement of electrons in a wire in a constant direction. If a DC power supply is connected to an oscilloscope, the result is a constant line like this:
This shows an unchanging voltage, such as that provided by a cell or battery.
Direct Current (DC) is current whose direction is constant and unchanging.
AC and DC voltages can be analysed with an oscilloscope, which shows graphically how voltage changes with time.

This graph shows the result of changing the frequency of the alternating voltage from an AC power supply:
The height of a graph on an oscilloscope is a measure of its voltage.
The width of an AC graph on an oscilloscope is the time for the cycle to repeat.
The shorter the time, the higher the frequency.

Solar Power

Solar power turns energy from sunlight into electricity using photo-voltaic cells.
The most common material used in photovoltaics is silicon; whent exposed to light it releases electrons that move into an electrical circuit.
The electricity generated can charge batteries or can even be converted to an alternating current to power standard household or office equipment.

Solar power can also be used in solar water heating systems. Infra red radiation from the sun is collected to heat water for domestic (baths, washing, etc.) or commercial use, such as swimming pools. This can dramatically reduce electricity or gas bills.
Solar power provides energy without any emissions of greenhouse gases and its operation is silent. Lack of moving parts means they require minimal maintenance.
Solar panels can also look aesthetically pleasing, keeping the environmental impact of this technology low. The price of solar energy is constantly reducing as the technology becomes more advanced and economic to produce.

Solar powered stadium built in Taiwan for the 2009 World Games

Alternative power supplies

Using USBs to power everyday electronic products, e.g. charging your iPhone, powering other appliances e.g. scanner, or less serious this Zen Garden

Compliant USB ports supply up to 500mA @ 5v (2.5W) each!

Dynamo power for examples the Trevor Baylis radio and torch