The AC adapter, AC/DC adapter or AC/DC converter[1] 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.
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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. 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 voltage regulator 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 (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 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.[2] 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. The advantages include:
While useful for many purposes, some external AC adapters have attracted criticism. Problems with this type of power supply may include:
A survey of consumers showed widespread dissatisfaction with the cost, inconvenience, and wastefulness of the profusion of power adaptors used by electronic devices.[3] The science fiction author and satirist Douglas Adams once wrote an essay bemoaning the profusion and confusion of power adaptors, and calling for more standardization.[4]
The issue of inefficiency of some power supplies has become more well known, with US president George W. Bush referring in 2001 to such devices as "Energy Vampires".[5] 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 and the One Watt Initiative for more details.
But others have argued that these inefficient devices are low powered, e.g., devices that are used for small battery chargers, 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 (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 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.[6]
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"[7]
Since wasted electrical energy is released as heat, 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:
"Universal" adapters are available[8] on which the user can adjust these parameters, except for voltage regulation (which is a fixed characteristic of a given power supply design).
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 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.[9] 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 (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.
Some universal adapters, for example those by iGo,[10] 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 technology, by which the consuming device would tell the external power supply what kind of power is needed.[11]
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 decorations") 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 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 ..."[12] 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.[13] 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 in function to the original USB connector but physically smaller. In 2009, the International Telecommunication Union (ITU) announced support of the Open Mobile Terminal Platform'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.[14] 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 has defined a Common External Power Supply intended to replace the many incompatible proprietary power supplies to eliminate waste by reducing the total number of supplies manufactured.
Larry Page, a founder of Google, 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.[15][16]