The 12v power supply is one of the most common types of power source for consumer electronics that need an external power supply. Logically then, if you have 240v mains voltage and you need a 12v power supply, a 240v 12v transformer (as they are sometimes called) is probably involved. If you specifically want the transformer, I would guess you are either actually looking for a lighting transformer, or you are someone who dabbles in electronics. Most people are really looking for a 12v power supply though. This is also called a 12v adapter, but is is the same thing.
No matter what you are trying to power, you need to know a lot about its exact power requirements. This includes knowing what voltage it needs, how much current it draws (electrical power) and if there are special considerations about the polarity of the power input. This last bit is relevant if the device needs dc voltage - in which case the the internal circuitry will be very sensitive to the polarity of the voltage. If ac voltage is specified, it simply means to circuitry to rectify ac to dc is present inside the device instead of in the external power supply. In principle, ac plugs can be inserted any way you want, but the presence of a ground prong in the plug often prevents this. Electric shavers with two-pin power sockets which are connected straight to mains voltage, have power plugs with no polarity.
Other things to consider before buying 240v 12v transformers include things like fuses. The device you want to power may be protected from overloading by means of an internal fuse. This can sometimes accessed from the back of the enclosure, but often you have to open it to see the fuse. Thankfully, most electronics manufacturers inform you of any fuses, either on a label on the device or in the user manual. Should no fuse be present (one usually is), you could get a 12v adapter with a built in fuse. A 240v to 12v dc transformer often includes a fuse in addition to the rectifying circuitry, and it is a good idea to have a fuse somewhere in the chain.
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