The AC adapter, AC/DC adapter or AC/DC converter is a type of external power supply, often enclosed in what looks like an over-sized AC plug. Other names include plug pack, plug-in adapter, adapter block, domestic mains adapter, line power adapter, or power adapter. Informal terms include wall wart, wall cube and power brick. AC adapters are used with electrical devices that require power but do not contain internal components to derive the required voltage and power from mains power. The internal circuitry of an external power supply is very similar to the design that would be used for a built-in or internal supply.

External power supplies are used both with equipment with no other source of power, and with battery-powered equipment, where the supply both charges the battery and powers the equipment when plugged in.

Use of an external, rather than internal, power supply allows battery-powered equipment to be used portably without the added dead weight and bulk of internal power components, and makes it unnecessary to produce equipment for use with a specified power source.

Originally, most AC to DC adapters were linear power supplies, containing a transformer to convert the mains electricity voltage to a lower voltage, a rectifier to convert it to pulsating DC, and a filter to smooth the pulsating waveform to DC, with residual ripple variations small enough not to affect the device being powered. Size and weight of the device was largely determined by the transformer, which in turn was determined by the power output and mains frequency.

In the early years of the twenty-first century, switched-mode power supplies (SMPSs) became almost ubiquitous for this purpose. Mains voltage is rectified to a high direct voltage driving a switching circuit, which has a transformer operating at a high frequency and outputs direct current at the desired voltage. The high-frequency ripple is more easily filtered out than mains-frequency. The high frequency allows the transformer to be small, which reduces its losses; and the switching regulator can be much more efficient than a linear regulator. The result is a much more efficient, smaller, and lighter device.

However, unless very carefully designed and using suitable components, switching adapters can be more likely to fail than the older type, due in part to complex circuitry and the use of semiconductors. Unless designed well, these adapters may be easily damaged by overloads, even transient ones, which can come from lightning, brief mains overvoltage (sometimes caused by an incandescent light on the same power circuit failing), component degradation, etc. A very common mode of failure is due to the use of electrolytic capacitors whose equivalent series resistance (ESR) increases with age; switching regulators are very sensitive to high ESR (the older linear circuit also used electrolytic capacitors, but the effect of degradation is much less dramatic). Well-designed circuits pay attention to the ESR, ripple current rating, pulse operation, and temperature rating of capacitors

External AC adapters are widely used to power small or portable electronic devices, because of a number of advantages:

* Safety — External power adapters can free product designers from worrying about some safety issues. Much equipment uses only voltages low enough not to be a safety hazard internally, although the power supply must out of necessity use dangerous mains voltage. If an external power supply is used (usually via a power connector, often of coaxial type), the equipment need not be designed with concern for hazardous voltages inside the enclosure. This is particularly relevant for equipment with lightweight cases which may break and expose internal electrical parts.

* Heat reduction — Heat reduces reliability and longevity of electronic components, and can cause sensitive circuits to become inaccurate or malfunction. A separate power supply removes a source of heat from the apparatus.

* Electrical noise reduction — Because radiated electrical noise falls off with the square of the distance, it is advantageous to convert potentially electrically noisy AC line power or automotive power to “clean”, filtered DC in an external adapter, at a safe distance from noise-sensitive circuitry.

* Weight and size reduction — When most power adapters were simple mains-frequency transformer-based designs, they were quite heavy and large, and added considerably to the weight and bulk of devices designed to be light-weight. Even modern power supplies are relatively heavy compared to other circuitry. Removing power components from equipment powered by rechargeable batteries reduces the weight and size which must be carried.

* Ease of replacement — Power supplies are more prone to failure than much other circuitry due to their exposure to power spikes and their internal generation of waste heat. External power supplies can be replaced quickly by a user without the need to have the powered device repaired.

* Configuration versatility — Externally powered electronic products can be used with different power sources as needed (e.g. 120VAC, 240VAC, 12VDC, or external battery pack), for convenient use in the field, or when traveling.

* Simplified product inventory, distribution, and certification — An electronic product that is sold and used internationally must be powered from a wide range of power sources, and must meet product safety regulations in many jurisdictions, usually requiring expensive certification by national or regional safety agencies such as Underwriters Laboratories or Technischer Überwachungsverein. A single version of a device may be used in many markets, with the different power requirements met by different external power supplies, so that only one version of the device need be manufactured, stocked, and tested. If the design of the device is modified over time (a frequent occurrence), the power supply design itself need not be retested (and vice versa).

While useful for many purposes, some external AC adapters have attracted criticism. Problems with this type of power supply may include:

* Size — Power supplies which plug into the mains directly without using a plug on a cable (true wall warts) are bulkier than bare plugs; sometimes they are too large to plug into power sockets with restricted space, or into adjacent sockets on power strips.

* Weight — Some AC adapters can be heavy, exerting excess weight on the plug socket (this depends on the socket design of the country in question). Some external power supplies are “power bricks” having a short AC cord so they can lie on the floor, thus relieving strain, at the expense of clutter. Other wall-hanging types are made long and thin, minimizing the leverage of their weight vector that pulls the plug out, at the expense of exacerbating the size problem. The weight for equipment that must be carried (e.g., for travelling) is not a disadvantage of external supplies, as the alternative is an equally heavy internal supply; in many cases a single universal supply can replace several proprietary ones.

* Inefficiency — Some idling power is wasted if the power supply is left running after being disconnected from the equipment.

* Confusion — External power supplies are often generic and not clearly marked to identify the equipment they are designed to power. It is very easy to separate power supply and equipment, and difficult to re-match the many power supplies and devices that users often had.

* Compatibility problems — There is no standardisation of connectors; the same connector is often used for different voltages, and for both DC supplies and AC-to-AC transformers. This easily leads to using the wrong power supply, which can destroy equipment.

One inherent disadvantage of external power adapters is that they can get separated from the product they are intended to power. Consequently, there is a market for replacement adapters. In addition, failed power supplies must be replaced. Not only must the replacement match voltage, current, and polarity requirements, but it must also match the connector. Many electrical products are poorly labeled with information about the power supply they require, so it is prudent to record the specifications of the original power supply in advance, to ease replacement if the original is later lost. Careful labeling of power adapters can also reduce the likelihood of a disastrous mixup which could cause equipment damage.

AC adapters are often reused on other appliances, but there are 5 parameters which all must suit the appliance:

* Voltage
* Current capacity
* Polarity (not relevant for AC)
* Voltage regulation (or stabilization)
* Connector type

Some so-called “universal” replacement power supplies allow the voltage and polarity to be switched, which can ease the matching problem. In addition, the power connector must be matched.

Four-way X connectors or six-way star connectors, also known as spider connectors, with multiple plug sizes and types are common on generic power supplies.

Some universal adapters, for example those by iGo, automatically set their output voltage and maximum current according to which of a range of interchangeable tips is fitted; tips are available to fit and supply appropriate power to many notebook computers and mobile devices.

A de facto standard has emerged in low-power AC adapters, namely the USB connector.

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