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The AC adapter, AC/DC adapter or AC/DC converter is a type of external power supply
Power supply
A power supply is a device that supplies electrical energy to one or more electric loads. The term is most commonly applied to devices that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy to electrical energy...

, 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
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...

-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.

Modes of operation

Originally, most AC to DC adapters were linear power supplies
Power supply
A power supply is a device that supplies electrical energy to one or more electric loads. The term is most commonly applied to devices that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy to electrical energy...

, containing a transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

 to convert the mains electricity
Mains electricity
Mains is the general-purpose alternating current electric power supply. In the US, electric power is referred to by several names including household power, household electricity, powerline, domestic power, wall power, line power, AC power, city power, street power, and grid power...

The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...

age to a lower voltage, a rectifier
A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...

 to convert it to pulsating DC
Pulsating direct current
A pulsating direct current is a direct current that changes in value at regular or irregular intervals.A pulsating direct current may change in value, i.e., be always present but at different levels, or it may be a current that is interrupted completely at regular or irregular intervals, but when...

, 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
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

. Ratings over a few watts made the devices too large and heavy to be physically supported by a wall outlet. The output voltage of these adapters varied with load; for equipment requiring a more stable voltage, linear
Linear circuit
A linear circuit is an electronic circuit in which, for a sinusoidal input voltage of frequency f, any output of the circuit is also sinusoidal with frequency f...

 voltage regulator
Voltage regulator
A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level. A voltage regulator may be a simple "feed-forward" design or may include negative feedback control loops. It may use an electromechanical mechanism, or electronic components...

 circuitry was added. Losses in the transformer and the linear regulator were considerable; efficiency was relatively low, and significant power dissipated as heat even when not driving a load.

In the early years of the twenty-first century, switched-mode power supplies
Switched-mode power supply
A switched-mode power supply is an electronic power supply that incorporates a switching regulator in order to be highly efficient in the conversion of electrical power...

 (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. Safety is ensured, as in the older linear circuit, because there is still a transformer which electrically isolates the output from the mains.

A linear circuit must be designed for a specific, narrow range of input voltages (e.g., 220–240VAC) and must use a transformer appropriate for the frequency (usually 50 or 60 Hz), but a SMPS can easily be designed to work efficiently over a very wide range of voltages and frequencies; a single 100–240VAC unit will handle almost any mains supply in the world.

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
Transient (oscillation)
A transient event is a short-lived burst of energy in a system caused by a sudden change of state.The source of the transient energy may be an internal event or a nearby event...

 ones, which can come from lightning
Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...

, 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 capacitor
Electrolytic capacitor
An electrolytic capacitor is a type of capacitor that uses an electrolyte, an ionic conducting liquid, as one of its plates, to achieve a larger capacitance per unit volume than other types. They are often referred to in electronics usage simply as "electrolytics"...

s whose equivalent series resistance
Equivalent series resistance
Practical capacitors and inductors as used in electric circuits are not ideal components with only capacitance or inductance. However, they can be treated to a very good approximation as ideal capacitors and inductors in series with a resistance; this resistance is defined to be the equivalent...

 (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. Sometimes after a designer designs an SMPS, due to financial considerations lower cost components are used which don't meet the specified ESR. The cost-reduced power supply will initially work just as well, but may suffer a high premature failure rate.


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
    Coaxial power connector
    A coaxial power connector is a miniature power connector used for attaching extra-low voltage devices such as consumer electronics to external electricity...

    ), 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
    Safety standards
    Safety standards are standards designed to ensure the safety of products, activities or processes, etc. They may be advisory or compulsory and are normally laid down by an advisory or regulatory body that may be either voluntary or statutory...

     such as Underwriters Laboratories
    Underwriters Laboratories
    Underwriters Laboratories Inc. is an independent product safety certification organization. Established in 1894, the company has its headquarters in Northbrook, Illinois. UL develops standards and test procedures for products, materials, components, assemblies, tools and equipment, chiefly dealing...

     or Technischer Überwachungsverein
    Technischer Überwachungsverein
    TÜVs are German organizations that work to validate the safety of products of all kinds to protect humans and the environment against hazards. As independent consultants, they examine plants, motor vehicles, energy installations, devices and products TÜVs (short for Technischer...

    . 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 strip
    Power strip
    A power strip is a block of electrical sockets that attaches to the end of a flexible cable and allows multiple electrical devices to be plugged in. As such it can be considered a type of trailing socket though that term is more often used for single and double cable mounted sockets...


  • 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.

A survey of consumers showed widespread dissatisfaction with the cost, inconvenience, and wastefulness of the profusion of power adaptors used by electronic devices. The science fiction
Science fiction
Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...

 author and satirist Douglas Adams
Douglas Adams
Douglas Noel Adams was an English writer and dramatist. He is best known as the author of The Hitchhiker's Guide to the Galaxy, which started life in 1978 as a BBC radio comedy before developing into a "trilogy" of five books that sold over 15 million copies in his lifetime, a television...

 once wrote an essay bemoaning the profusion and confusion of power adaptors, and calling for more standardization.


The issue of inefficiency of some power supplies has become more well known, with US president George W. Bush
George W. Bush
George Walker Bush is an American politician who served as the 43rd President of the United States, from 2001 to 2009. Before that, he was the 46th Governor of Texas, having served from 1995 to 2000....

 referring in 2001 to such devices as "Energy Vampires". Legislation is being enacted in the EU and a number of states, including California, to reduce the level of energy wasted by some of these devices. See standby power
Standby power
Standby power, also called vampire power, vampire draw, phantom load, or leaking electricity , refers to the electric power consumed by electronic and electrical appliances while they are switched off Standby power, also called vampire power, vampire draw, phantom load, or leaking electricity...

 and the One Watt Initiative
One Watt Initiative
The One Watt Initiative is an energy-saving initiative by the International Energy Agency to reduce standby power-use by any appliance to not more than one watt in 2010, and 0.5 watts in 2013, which has given rise to regulations in many countries and regions.-Standby power:Standby power,...

 for more details.

But others have argued that these inefficient devices are low powered, e.g., devices that are used for small battery charger
Battery charger
A battery charger is a device used to put energy into a secondary cell or rechargeable battery by forcing an electric current through it.The charge current depends upon the technology and capacity of the battery being charged...

s, so even if they have a low efficiency, the amount of energy they waste is less than 1% of household consumption of electric energy.

Considering the total efficiency of power supplies for small electronic equipment, the older mains-frequency linear transformer-based power supply was found in a 2002 report to have efficiencies from 20–75%, and have considerable energy loss even when powered up but not supplying power. Switched-mode power supplies
Switched-mode power supply
A switched-mode power supply is an electronic power supply that incorporates a switching regulator in order to be highly efficient in the conversion of electrical power...

 (SMPSs) are much more efficient; a good design can be 80–90% efficient, and is also much smaller and lighter. In 2002 most external plug-in "wall wart" power adapters commonly used for low-power consumer electronics
Consumer electronics
Consumer electronics are electronic equipment intended for everyday use, most often in entertainment, communications and office productivity. Radio broadcasting in the early 20th century brought the first major consumer product, the broadcast receiver...

 devices were of linear design, as well as supplies built into some equipment. External supplies are usually left plugged in even when not in use, and consume from a few watts to 35 watts of power in that state. The report concluded that about 32 billion kilowatt-hours (kWh) per year, about 1% of total electrical energy consumption, could be saved in the United States by replacing all linear power supplies (average efficiency 40–50%) with advanced switching designs (efficiency 80–90%), by replacing older switching supplies (efficiencies of less than 70%) with advanced designs (efficiency of at least 80%), and by reducing standby consumption of supplies to not more than 1 watt.

Since the report was published, SMPSs have indeed replaced linear supplies to a great extent, even in wall warts. The 2002 report estimated that 6% of electrical energy used in the US "flows through" power supplies (not counting only the wall warts). The website where the report was published said in 2010 that despite the spread of SMPSs, "today's power supplies consume at least 2% of all U.S. electricity production. More efficient power supply designs could cut that usage in half"

Since wasted electrical energy is released as heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...

, an inefficient power supply is hot to the touch, as is one that wastes power without an electrical load. This waste heat is itself a problem in warm weather, since it may require additional air conditioning to prevent overheating, and even to remove the unwanted heat from large supplies.


AC adapters are often reused on other appliances, but there are 5 parameters which all must suit the appliance:
  • Voltage
    Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

  • Current
    Electric current
    Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...

  • Polarity
    Polarity symbols
    Many electrical devices use direct current power, often provided by a AC to DC adapter, also known as a wall wart or power brick...

     (not relevant for AC
    AC power
    Power in an electric circuit is the rate of flow of energy past a given point of the circuit. In alternating current circuits, energy storage elements such as inductance and capacitance may result in periodic reversals of the direction of energy flow...

  • Voltage regulation
    Voltage regulation
    In electrical engineering, particularly power engineering, voltage regulation is the ability of a system to provide near constant voltage over a wide range of load conditions.-Electrical power systems:...

     (or stabilization)
  • Connector type

"Universal" adapters are available on which the user can adjust these parameters, except for voltage regulation (which is a fixed characteristic of a given power supply design).

Universal power adapters

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.

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. Other replacement power supplies have arrangements for changing the power connector, with from four to nine different alternatives available when purchased in a set. RadioShack
RadioShack Corporation   is an American franchise of electronics retail stores in the United States, as well as parts of Europe, South America and Africa. As of 2008, RadioShack reported net sales and operating revenues of $4.81 billion. The headquarters of RadioShack is located in Downtown...

 sells universal AC adapters of various capacities, branded as "Enercell Adaptaplug", and fitted with 2-pin female sockets compatible with their Adaptaplug connector lineup. This allows many different configurations of AC adapters to be put together, without requiring soldering. Philmore and other competing brands offer similar AC adapters with interchangeable connectors.

A suitable power supply for a particular use must have the matching plug dimensions, the matching DC (or AC) voltage and polarity, and the ability to supply at least the required current. The input voltage must match the wall socket (120/240VAC at 60/50 Hz) or other power source, such as 12VDC automotive battery power.

But the label on a power supply may not be a reliable guide to the actual voltage it supplies under varying conditions. Most low-cost power supplies are "unregulated", in that their voltage can change appreciably with load. If they are lightly loaded, they may put out much more than the nominal "name plate" voltage, which could damage the load. If they are heavily loaded, the output voltage may droop appreciably, in some cases well below the nominal label voltage even within the nominal rated current, causing the equipment being supplied to malfunction or be damaged. Cheap external power supplies of traditional linear design with undersized transformers tend to have poor regulation, whether originally supplied or replacement units.

In general, more modern high-quality switched-mode power supplies
Switched-mode power supply
A switched-mode power supply is an electronic power supply that incorporates a switching regulator in order to be highly efficient in the conversion of electrical power...

 (SMPSs) are smaller, more efficient, and put out a much more constant voltage even as the input voltage and the load current may vary. Configurable switched-mode power supplies have come down considerably in price, and they are especially convenient for use when traveling because of their decreased weight and size.

Auto-sensing adapters

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. Different tips may use the same connector, but automatically supply different power; it is of course essential to use the right tip for the apparatus being powered, but no switch needs to be set correctly by the user. The advent of switch-mode power supplies has allowed adapters to work from any AC mains supply from 100 to 240V with an appropriate plug; operation from standard 12V DC vehicle and aircraft supplies can also be supported. With the appropriate adapter, accessories, and tips a huge variety of equipment can be powered from almost any source of power.

A "Green Plug" system has been proposed, based on USB
Universal Serial Bus
USB is an industry standard developed in the mid-1990s that defines the cables, connectors and protocols used in a bus for connection, communication and power supply between computers and electronic devices....

 technology, by which the consuming device would tell the external power supply what kind of power is needed.

Use of USB

The USB connector (and voltage) has emerged as a de-facto standard in low-power AC adapters for many portable devices. Although the original purpose of the USB (Universal Serial Bus) was to enable serial digital data exchange, the USB standard also provided the option of providing 5V DC power, up to 500mA. This standard became sufficiently widespread that numerous accessory gadgets ("USB decoration
USB decoration
A USB decoration is a decorative device that uses the Universal Serial Bus connector for electrical power, and sometimes the protocol, on a computer or other host. In addition, some lightweight devices A USB decoration is a decorative device that uses the Universal Serial Bus (USB) connector for...

s") are designed to connect to USB only for DC power, and not for data interchange. Recognizing this market trend / need, and the USB standard's relatively low current limit of 500mA, the USB Implementers Forum
USB Implementers Forum
The USB Implementers Forum is a non-profit organisation to promote and support the Universal Serial Bus. Its main activities are the promotion and marketing of USB, Wireless USB, USB On-The-Go, and the maintenance of the specifications, as well as a compliance program.It was formed in 1995 by the...

 in March, 2007 released the USB Battery Charging Specification which defines, "...limits as well as detection, control and reporting mechanisms to permit devices to draw current in excess of the USB 2.0 specification for charging ..." Electric fans, lamps, alarms, coffee warmers, battery chargers, and even toys have been designed to tap power from a USB connector. Plug-in adapters equipped with USB receptacles are widely available to convert 120VAC or 240VAC power or 12VDC automotive power to 5V DC USB power (see photo at right).

Portable "USB chargers" are available which convert energy from an internal battery to deliver DC power via a standard USB connnector, which can be used to power a variety of portable electronics (e.g. MP3 player, cellphone). There is even a popular DIY Open Hardware USB charger which can be built from a kit.
Also, pocket-sized portable "USB battery packs" are available which can charge from a powered USB port, and can later in turn provide power from their own USB port.

The trend towards more-compact electronic devices has driven a shift towards the micro-USB connector, which is backward compatible
Backward compatibility
In the context of telecommunications and computing, a device or technology is said to be backward or downward compatible if it can work with input generated by an older device...

 in function to the original USB connector but physically smaller. In 2009, the International Telecommunication Union
International Telecommunication Union
The International Telecommunication Union is the specialized agency of the United Nations which is responsible for information and communication technologies...

 (ITU) announced support of the Open Mobile Terminal Platform
Open mobile terminal platform
The Open Mobile Terminal Platform was a forum created by mobile network operators to discuss standards with manufacturers of cell phones and other mobile devices. During its lifetime, the OMTP included manufacturers such as Huawei, LG Electronics, Motorola, Nokia, Samsung and Sony...

's (OMTP) "Common Charging and Local Data Connectivity" standard which specifies the use of micro-USB receptacles on mobile phones and standard USB receptacles on (common / interchangeable) chargers. The hope is to markedly reduce the profusion of non-interchangeable power adapters needed for each year's new crop of mobile phone models.


The European Union
European Union
The European Union is an economic and political union of 27 independent member states which are located primarily in Europe. The EU traces its origins from the European Coal and Steel Community and the European Economic Community , formed by six countries in 1958...

 has defined a Common External Power Supply
Common External Power Supply
In 2009 several government, consumer and industry initiatives resulted in the European Union's definition of a common External Power Supply for use with data-enabled mobile phones sold in the EU. An external power supply is the AC power adapter that converts household AC voltages to the much...

 intended to replace the many incompatible proprietary power supplies to eliminate waste by reducing the total number of supplies manufactured.

Larry Page
Larry Page
Lawrence "Larry" Page is an American computer scientist and internet entrepreneur who, with Sergey Brin, is best known as the co-founder of Google. As of April 4, 2011, he is also the chief executive of Google, as announced on January 20, 2011...

, a founder of Google
Google Inc. is an American multinational public corporation invested in Internet search, cloud computing, and advertising technologies. Google hosts and develops a number of Internet-based services and products, and generates profit primarily from advertising through its AdWords program...

, has proposed a 12V 15A standard for almost all equipment requiring an external converter. New buildings would also have 12V DC wiring, so that in effect the AC to DC adapter circuitry would be built into the wall.

See also

  • AC power plugs and sockets describes the usual sources of power for an AC adapter
  • Coaxial power connector
    Coaxial power connector
    A coaxial power connector is a miniature power connector used for attaching extra-low voltage devices such as consumer electronics to external electricity...

     has extensive coverage of the profusion of coaxial DC power connectors in use.
  • Common External Power Supply
    Common External Power Supply
    In 2009 several government, consumer and industry initiatives resulted in the European Union's definition of a common External Power Supply for use with data-enabled mobile phones sold in the EU. An external power supply is the AC power adapter that converts household AC voltages to the much...

     describes standards for mobile phone battery chargers.
  • Power supply
    Power supply
    A power supply is a device that supplies electrical energy to one or more electric loads. The term is most commonly applied to devices that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy to electrical energy...

     describes other types of power supplies.
  • USB Power describes the USB standards for DC power delivery to connected devices.

External links