Editing AC adapter (section)

==Modes of operation==
[[Image:Wall wart opened.JPG|thumb|A disassembled AC adapter showing a simple, unregulated linear DC supply circuit: a transformer, four diodes in a [[diode bridge|bridge rectifier]], and a single [[electrolytic capacitor]] to smooth the waveform]]

Originally, most AC/DC adapters were [[Power supply|linear power supplies]], containing a [[transformer]] to convert the [[mains electricity]] voltage to a lower voltage, a [[rectifier]] to convert it to [[Pulsating direct current|pulsating DC]], and a filter to smooth the pulsating waveform to DC, with residual [[ripple (electrical)|ripple]] variations small enough to leave the powered device unaffected. Size and weight of the device was largely determined by the transformer, which in turn was determined by the power output and [[utility frequency|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 circuit|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.

Early in the twenty-first century, [[switched-mode power supply|switched-mode power supplies]] (SMPSs) became almost ubiquitous for this purpose due to their compact size and light weight relative to their power output ability. Mains voltage is rectified to a high direct voltage driving a switching circuit, which contains 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 a transformer still provides [[galvanic isolation]].

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

Many inexpensive switched-mode AC adapters do not implement adequate filtering and/or shielding for [[electromagnetic interference]] that they generate. The nature of these high speed, high-energy switching designs is such that when these preventative measures are not implemented, relatively high energy harmonics can be generated, and radiated, well into the radio portion of the spectrum. The amount of RF energy typically decreases with frequency; so, for instance, interference in the medium wave (US AM) broadcast band in the one megahertz region may be strong, while interference with the FM broadcast band around 100 megahertz may be considerably less. Distance is a factor; the closer the interference is to a radio receiver, the more intense it will be. Even WiFi reception in the gigahertz range can be degraded if the receiving antennae are very close to a radiating AC adapter. A determination of if interference is coming from a specific AC adapter can be made simply by unplugging the suspect adapter while observing the amount of interference received in the problem radio band. In a modern household or business environment, there may be multiple AC adapters in use; in such a case, unplug them all, then plug them back in one by one until the culprit or culprits is found.

Traditionally, wall adapters provided a constant voltage. For USB-powered devices, it is 5 volts. Later, [[Charge controller#Charging protocols|battery charging protocol]]s such as [[Quick Charge]] by Qualcomm and [[USB Power Delivery]] started allowing charged devices to request different voltages suited for their needs, usually higher voltages to increase power without adding heat to the copper wires of the USB cable. In the past, "SuperCharge" by Huawei and "Dash Charge" by OnePlus did the opposite, requesting a slightly lowered voltage to directly match the battery voltage inside the smartphone so no change of voltage has to take place inside the phone, leading to less heating up. This required special USB cables with thicker copper wires.<ref>{{cite web |last1=Todorov |first1=Nick |title=How it works: Dash Charge fast charging on the OnePlus 3 |url=https://www.phonearena.com/news/How-it-works-Dash-Charge-fast-charging-on-the-OnePlus-3_id82646 |website=PhoneArena |access-date=27 February 2025 |date=8 August 2016 |quote=Both Dash Charge and Quick Charge carry up to 15-20-ish watts of power, so why does the latter need a special cable? Well, it is because in an electric circuit, greater current requires thicker wires. Greater voltage – not as much. This is why Qualcomm Quick Charge will work with an ordinary USB cable, while Dash Charge requires the thicker cable bundled with the OnePlus 3.}}</ref><ref>{{cite web |last1=Liu |first1=Jessica |title=What is the fast charging protocol - IETCHARGER - China Leading Charger Manufacturer |url=https://www.ietcharger.com/what-is-the-fast-charging-protocol/ |website=www.ietcharger.com |date=24 February 2025}}</ref><ref>{{cite web |title=Huawei Unveils the Mate 10 and Mate 10 Pro - Huawei Press Center |url=https://www.huawei.com/en/news/2017/10/HUAWEI-Mate10-Mate10Pro |website=huawei |language=en |date=2017-10-16}}</ref><ref>{{cite web |title=How Does Huawei SuperCharge Revolutionize Charging? - Global Batteries |url=https://www.global-batteries.com/how-does-huawei-supercharge-revolutionize-charging/ |access-date=27 February 2025 |date=23 February 2025 |quote=Huawei SuperCharge uses a proprietary charge pump system to split high-voltage inputs (e.g., 10V/4A) into lower-voltage, higher-current outputs. This minimizes energy loss as heat while enabling faster electron transfer.}}</ref>

In the early 2020s, the use of [[gallium nitride]] instead of silicon in switching wall adapters bumped up their output power at the same physical size, making compact wall warts able to power even some laptops, not only smartphones and tablet computers. The creation of Gallium Nitride chargers was delayed owing to their excessive costs and that fast charging technology was less in demand than it has become after 2020.<ref>{{cite web |last1=published |first1=Jerry Hildenbrand |title=GaN charging explained: why this next-gen charging tech is so powerful |url=https://www.androidcentral.com/what-makes-gan-chargers-something-you-should-care-about |website=Android Central |access-date=27 February 2025 |language=en |date=11 June 2021 |quote=What held back the use of GaN in cheap, disposable consumer goods — and yes, to the global giants that make things like phone chargers they are just cheap and disposable — was the cost. [...] }}</ref>