Online Shopping New And Used AC Power Supply Adapters

How To Shop For A AC Power Supply Adapter

When choosing one of our adapters you need to know:

1) Output Voltage, Current and Wattage.

Volts and current (mA) are printed on your original adaptor, and often also on the device that uses the adaptor.

Choose a voltage equal to the original voltage and of the same type (AC or DC).

Choose current (mA) equal or higher than the original amount, NEVER lower.

Choose a wattage (w) equal or higher than the original amount, NEVER lower.

2) Output Polarity

AC to DC adaptors will have a negative center or a positive center.

NEVER choose an adaptor with the wrong polarity.

AC to AC adaptors have no polarity requirements.

3) Output Plug Size

There are a number of different output plugs.

Output plug size is most often measured and expressed in milli-meters (mm)

A plug size of 5.5mm OD x 2.1mm ID means that the outer diameter is 5.5mm and the inner diameter is 2.1mm.


What is an AC adapter?

Basically, it converts your mains power AC 110-240V AC into a smaller usually DC voltage that your small device needs to operate. Often these are required for charging or powering battery operated devices.

A look at your typical device

By far the easiest way is to check if your device has a specific power adapter already available, this takes the guess work out of getting your own third party power adapter. If you can’t find one then this quick guide covers the basic things to look for.

The first thing you want to do is find out what your device needs to operate so inspect the device and look for any labels or symbols that indicate either the voltage or mAh/A. If you can not find anything have a look in the user manual or try a quick Google search.

Laptop label

This laptop has

Once you have this key data you are ready to start looking for an adapter.

Power adapter Sticker

Choosing the right adapter specs.

The general rule is to match the V and equal or beat the mAh, so if you are choosing an AC adapter to make sure you check the following,

Usually, all of this info is on the adapter sticker, remember you are always looking for the output V and mAh or A.

Closeup of adapter


A typical ac adapter sticker with all the essential info.  Output is 8V 2.5A.

If that is all you needed and feel confident start hunting for your adapter now, if you still want a deeper breakdown of the adapter features we have a deeper explanation below;


Checking the (V)olts


AC and DC

These symbols show the type of output of the adapter; usually, it is labeled as AC/DC or with the symbols above.

It is important to use the correct volts when choosing the adapter as a device can only run on the rated voltage. The most common are 18V, 12V, 9V, 6V, 5V and 3V.


Checking the (A)mps

Once you have confirmed the voltage of the device we need to find the amount of current that it uses. Current tells us how much power the device needs to operate.

Your device is unlikely to operate if you have an adapter with a lower A or mAh output then the device, and if it does operate the adapter is likely to get very hot and could be dangerous.

Amps and volts what about Watts?

Voltage carries the power and the Amps are the measurement of how much power it consumes, think of it like a river, the V is how wide the river is and the A is how fast the water is flowing, so a low volt high amp circuit is a small river flowing fast.

If we look at the river we could calculate how much water is flowing in the river per hour. This would be the Watts. There is a direct relationship between these three variables.

A 12V 2A adapter (12m river flowing at 2m per hour) would have the watts of 12 X 2 = 24W.

The basic equations are here

W = V X A

A = W/V

V = W/A

Most devices will list A or mAh and not list the Watts, if you have one but need the other use the basic equation to convert.

Reading the polarity of the connector

Once you have the V and A down then comes the tricky part, most appliances will have a circular connector with the inside hole carrying one pole and the outside carrying the other.

The most common are center positive (+) connectors.

Positive center

There is also the less popular center negative (-) connector.

Negative Center

The adapter and the device must match so always double check the symbol on the AC adapter and the device show the same polarity.

negative example

A center negative connector.

Choosing the plug

Hopefully you have the size of the devices plug and can just check the adapters measurements, there are two measurements for these types of connectors.


Typical low voltage adapter sizes.

A 5 mm OD x 2 mm ID says the outer diameter is 5mm and the inner is 2mm. If you go into a shop you can compare and test the connector size, or if you are technical minded you can measure the size of the port and pin yourself.

If you are measuring you need the diameter of the center pin for the (ID) and the inner diameter of the port (OD).

Inner and outer diameter

ID an OD measurements for both the female and male connector must match to connect properley.

The universal power adapter

If you are not sure on the pin type or are looking to use it for a few devices then make it easy with a universal adapter, they allow you to have different voltages, polarities and connectors.

As USB slowly takes over, the charging system is becoming universal, USB is now becoming available in lower power devices and as chargers become more powerful their will likely be more bigger and powerful devices running natively on USB.  Until then remember to be safe with all electronics and double check everything as incorrect voltage or polarity can damage your device.