How to Select the Correct Power Supply

The information below provides a brief summary of each specification that relates to Power Supplies and an overview of which details must match when seeking a replacement unit. Following these guidelines will ensure that the correct replacement is selected.


Voltage is usually denoted by the SI derived unit V.

Voltage is provided (pushed) by the power supply. When selecting a replacement power supply, the voltage should match that of the unit being replaced. While some devices may be tolerant of voltage variations, other devices may not be. Considering this, it is best to match voltage.

Amperage (Amps)

This is the strength of an electric current in amperes and is usually denoted by the SI unit symbol A.

Amperage is taken (pulled) by the device being powered. The device will draw as much current as it requires from the power supply, therefore it is important to select a power supply that is capable of providing equal to or more current (amps) than the unit that is being replaced.

The replacement power supply should not provide less amps than the unit being replaced as this may result in damage. However, it is perfectly safe to select a power supply with a higher amperage rating.


Most power supplies will contain two wires, one positive and one negative. One will be wired to the centre connector and the other to the outer ring. For example, a positive centre means that the positive wire is connected to the centre.

Polarity must match for power supplies with fixed tips. The polarity will be indicated on the labelling of each power supply.

For power supplies with interchangeable tips the polarity can be reversed, therefore selecting the correct polarity is not required. Instead, you will need to ensure that the plug tip is fitted correctly to provide the desired polarity.

Tip Size

The tip size is in reference to the connector that inserts into the electrical device. This must match the unit being replaced.

If two figures are provided for the plug tip size, the first figure refers to the outer diameter of the plug tip and the second figure refers to the internal diameter (5.5mm x 2.1mm = 5.5mm outer diameter and 2.1mm internal diameter).

Some manufacturers may only provide a single figure for the tip size. This usually refers to the internal diameter of the plug tip. For example, a plug tip specified as 2.1mm would be the same as the above example denoted as 5.5mm x 2.1mm.

Another factor to consider is the tip length. Although not as critical as most devices will have a fair amount of tolerance in this respect, there are some instances where a shorter plug tip may not make sufficient contact or a tip that is too long may not provide the desired stability.

Australian Power Supply with Interchangeable Plug Tip
Australian Power Supply with Interchangeable Plug Tip