AC (alternating current) and DC (direct current) wall transformers fall under the category of a Linear Wall Transformer which is a type of power supply unit. An AC power supply takes current from a power outlet and then converts it to the desired voltage. DC, on the other hand, uses a rectifier to convert alternating voltage to a pulsating direct voltage. Because power supplies in different regions of the world can either be in AC or DC form, linear wall transformers are needed so that we can safely operate electronics and electrical devices.

In the past people used transformers to convert different types of electricity to operate electronic devices. These transformers were usually very heavy and bulky. For example, during the 1980s a TV set bought from Japan and imported to the United Kingdom needed a transformer to convert the 220V-240V wall socket current to the 100V-120V needed to operate the TV set. Without it, plugging the TV directly to the power outlet will cause the device to short out and possibly catch on fire, or worst, explode.

Nowadays, buying electronic devices not intended for your country's mains (or AC power) is more convenient because of the power supply included with it. The power supply units today have become smaller, cheaper and lighter. The transformer of yesterday is now replaced by a switching power supply that can convert AC or DC current to the desired voltage needed.

There are many kinds of linear wall transformers. Basically they are used to convert AC to DC and vice-versa. One of the most common kinds we see are chargers for mobile phones and laptops. Since these types of equipment are usually imported from factories abroad, manufacturers have come up with an easy way for consumers to safely charge their electronic device. These types of chargers have small transformers with a few diodes making it inexpensive, lightweight and very effective for its purpose.

The alternative to bringing either AC or DC wall transformers, or both, should you want to travel anywhere on the planet, is to buy a Switching Mode Power Supply, or SMPS for short. This type of power supply has a switching regulator to efficiently convert power. In other words, it works both ways which makes it perfect for travel, because carrying two types of power supply, however small they may be, can still be very inconvenient. Another good option is to buy a programmable power supply. This allow users to control output voltage through an analogue input or via a computer program. Other variables, such as frequency and voltage, can also be customized for a specific need. This type of power supply is usually used for equipment testing, crystal growth monitoring or differential thermal analysis.

For simple applications and common household devices an AC or DC wall transformer is good enough. These transformers are becoming more common as technology improves and our demand for the latest gadgets grow. Unknowingly, we are surrounded by linear wall transformers everywhere we go. Many of the power supply units for our digital/video cameras, computers/laptops, or cell phone chargers, to name a few, already double as linear wall transformers.