How AC Adapters and Chargers Work: From Wall Outlet to 5 V

In everyday life: from heavy boxes to palm-sized chargers

Phone chargers, laptop power supplies, and the power adapters for Wi-Fi routers and game consoles. A home is full of small boxes that plug into the wall. Each one turns the electricity from the outlet into a form that suits the device.

Not long ago, the adapter that came with a home appliance was a heavy, square box. A modern phone charger can deliver about the same power from a much smaller and lighter box. That is because the way it works inside has changed completely.

In this article, we look at what happens inside a modern charger, in five stages.

Why devices need low-voltage DC

The electricity from an outlet is AC. Its direction swings back and forth 50 or 60 times a second. On 100-volt mains, the voltage reaches about 141 volts at each peak (see "AC vs DC Explained" for details). The circuits inside phones and computers, however, run on DC, which flows in one constant direction.

They also need much lower voltages. A phone's lithium-ion battery is about 3.6 volts per unit, or "cell" (Battery University). In DDR5, a standard for computer memory, the supply voltage is 1.1 volts (JEDEC, 2020). The tiny transistors inside chips cannot withstand high voltages.

So the AC adapter comes first. It turns the 100-volt AC into DC, usually somewhere around 5 to 20 volts. Inside the device, smaller power circuits then split this into the exact voltage each chip needs ("What Chips Are Inside a Smartphone?"). The AC adapter handles the first and biggest step on this long path.

Old adapters: a big transformer lowers the voltage directly

Old adapters were very simple inside. The AC from the outlet went straight into a transformer, which lowered the voltage. Next, diodes, which let current flow in only one direction, lined up its direction. A capacitor then smoothed it into DC ("What Is a Diode?").

The weight came from this transformer. Handling a slow change of 50 or 60 times a second takes a thick iron core and many turns of copper wire. The core is made of many thin iron sheets stacked together (the OpenStax physics textbook).

This design had other weak points too. The output voltage is set by the ratio of the coils' turns. So in a country where the outlet voltage is twice as high, the output is nearly twice as high too. Using the adapter there as-is could damage the device. When the connected device drew little power, the voltage could also be higher than the label said. For example, one transformer-based product is labeled 9 volts AC. With nothing connected, its output can rise to nearly 11.6 volts (Triad Magnetics).

It also wasted a lot of energy. In a document from the U.S. ENERGY STAR program, transformer-based power supplies measured 20–75% efficiency. Efficiency is the share of the power taken in that gets delivered. The rest became heat and warmed the box.

How it works 1: The overall flow inside a modern charger

Most chargers today use a method called a switching power supply. The name comes from the way it rapidly "switches" the electricity on and off. Electricity passes through the stages in the order shown below.

At first glance, this looks like a detour. The AC is turned into DC on purpose, then chopped up again and sent through a transformer. But this detour is what lets the transformer fit in the palm of your hand. Let's go through it step by step.

How it works 2: First, turn it into DC and store it at high voltage

In the first stage, a circuit of four diodes makes the outlet's AC flow in one direction. This is called rectification. Then a large capacitor stores charge and releases it bit by bit, smoothing out the ripples.

The voltage has not been lowered yet. A 100-volt outlet gives about 140 volts of DC, close to the height of the peaks (ROHM). In a 240-volt country, it is about 340 volts (our own calculation).

This capacitor must withstand high voltage (ROHM). Its job is to carry the circuit through the troughs in the outlet's wave, which come 100 or 120 times a second. Its size is based on how long these troughs last and on times when the outlet voltage runs low (a document from TI, a U.S. company). It has nothing to do with the switching speed. So the "faster switching" trick we will see later does not make it smaller.

How it works 3: A power transistor switches on and off fast

Next, the DC is chopped finely by a switch that is controlled by an electrical signal. The switch is a transistor built to handle large currents and high voltages. It can switch between passing and blocking current tens of thousands of times a second ("What Is a Power Semiconductor?").

The switching speed varies by product. One example is about 50,000 times a second, or 50 kilohertz (ROHM). That is fast enough to switch on and off a thousand times during one cycle of 50-hertz AC from the outlet.

The chopping is needed because a transformer works only with "changing current." Steady DC produces no voltage on the other side. Switching on and off over and over looks, to the transformer, just like very fast AC.

How it works 4: A small transformer lowers the voltage, and it becomes DC again

The chopped electricity passes through a small transformer, which lowers the voltage. Small chargers often use a method that stores energy in the transformer as magnetism while the switch is on. Then, while the switch is off, that energy goes to the output side. This is called the flyback method. It is widely used in power supplies up to about 100 watts (ROHM).

What comes out on the output side still pulses in step with the switching. So it is rectified again here and smoothed with a capacitor. Only then do you get steady DC, such as 5 volts.

The rectifying part here uses a clever trick. A diode uses up a little voltage every time current passes through it. Even a Schottky diode, which uses less, takes about 0.4–0.7 volts (Toshiba). Losing 0.5 volts while making 5 volts is a 10% loss on its own.

So today's chargers widely use a transistor in place of the diode (Monolithic Power Systems). It is switched on only at the moments when current should flow. This is called synchronous rectification.

How it works 5: Watch the output and correct the on-time

When a phone suddenly draws more power, the output voltage drops a little. The outlet voltage also varies. The charger still holds 5 volts because it watches the output all the time and corrects any drift.

On the output side, a part compares the present output voltage with a fixed reference voltage (TI). If they differ, it reports by how much to the control chip on the outlet side. The chip then changes the fraction of time the switch stays on, which raises or lowers the power sent. This fraction is called the duty cycle.

A system that looks at the result and corrects its action is called feedback. Feedback is also why the output stays the same whether the input is 100 volts or 240 volts.

Under USB PD, a fast-charging standard, a device may ask for 9 volts or 20 volts. A chip on the output side then changes the target voltage. Some products send this instruction to the outlet side through the same optocoupler shown in the figure (Renesas product material; "What Is USB?").

Why faster switching makes parts smaller

In a transformer, one coil creates a magnetic field inside the core, measured as magnetic flux. Changes in that flux create a voltage in the other coil. There is a limit to how much flux the core can hold, set by the core's material and thickness. Beyond that limit, the transformer stops working well.

At the same voltage, the shorter the time the voltage is applied, the less flux builds up in the core. With 50-hertz AC, each half-cycle lasts 1/100 of a second before the direction flips. With 50,000 switches a second, each on-period lasts about 1/50,000 of a second at most. If less flux builds up, the core can be thinner and the coils can have fewer turns (a power electronics textbook by Erickson and others).

But switching a thousand times faster does not make the transformer a thousand times smaller. With fast changes, more energy is lost as heat in the core and wires. So the core material also changes, from iron sheets to ferrite, which loses less even at high frequencies. Ferrite is made mainly of iron oxide, fired until it is hard. "What Is GaN (Gallium Nitride)?" explains this with a different analogy.

Efficiency and heat: the trade-off with speed

Today's chargers are much more efficient than old ones. Say a charger with 90% efficiency delivers 20 watts. It takes in about 22.2 watts, and about 2.2 watts becomes heat (our own calculation).

The heat comes from four main sources. They are the small resistance of the transistor while it is on, the rectification on the output side, the transformer's core and wires, and losses during switching. The last one arises in the brief instant when the switch flips between on and off, when voltage and current are both present. This is called switching loss.

Here is the trade-off. Faster switching makes parts smaller, but each switch adds switching loss, so the heat goes up. A small box also has little surface area, so it sheds heat poorly. Chargers can get smaller only when there are parts that "switch fast without adding much heat."

There are also rules for the power a charger uses with nothing connected (as of 2026). In the EU, since April 2020, external power supplies of 49 watts or less that turn AC into DC must use 0.10 watts or less (EU Regulation 2019/1782). In the U.S., since 2016, the limit is 0.100 watts for a similar range (U.S. Department of Energy). In the EU, a new and stricter regulation will replace this rule from December 2028 (EU Regulation 2025/2052).

Why GaN chargers got smaller

More and more small chargers now use transistors made of a material called GaN (gallium nitride) for the switch. A GaN transistor loses less electricity per switch than a silicon one. So it can switch faster without adding much heat, and the transformer and the output-side capacitors can be smaller.

But only the parts whose size depends on the switching speed get smaller. The large input capacitor from How it works 2 is matched to the outlet's wave, so it barely shrinks. Better control chips and tighter packing of parts also helped bring chargers to their current size.

Isolation for safety: no wire connects the two sides

Look above and below the dashed line in Figure 1. The outlet side carries electricity at 100 volts or more. On the output side, people's fingers touch the USB connector directly. So the two sides are built with no wire between them. Separating parts with a material that does not conduct electricity, or with a gap, is called isolation.

Then how does energy get across? Power crosses as magnetic flux in the transformer's core. The two coils are not connected electrically. The monitor's information is often sent across as light (Figure 2). Some designs instead wind an extra coil on the transformer and estimate the output voltage from it (ROHM).

A part that sends signals with light is called an optocoupler. An LED at its input lights up, and a part at its output receives the light and turns it back into an electrical signal. The input and output are electrically separate. So signals can pass even when the voltages on the two sides are very different (Toshiba).

Rules for this kind of isolation are set in international safety standards. For computers, AV equipment and their power supplies, a standard called IEC 62368-1 is used (COSEL).

Charger labels often show a mark of two squares, one inside the other. This symbol stands for "Class II" equipment, which is protected by double insulation or a similar method (IEC 60417-5172).

In Japan, AC adapters are "specified electrical appliances" under national rules, a category with especially strict safety checks. They must show the diamond-shaped PSE mark (Ministry of Economy, Trade and Industry). In Japan, avoid products without this mark. And never take a charger apart. The large capacitor inside can hold a high voltage for a while, even after you unplug it.

Common myths

By the numbers

At 50,000 switches a second, one on/off cycle lasts 1/50,000 of a second (20 microseconds). In each of these short intervals, the transformer takes in energy and passes it on to the output side.

Learn more

• See the differences in a table: LDO vs switching regulator (two ways to lower a voltage) / Converter vs inverter / PN junction vs Schottky diode / SiC vs GaN
• Next terms: transformer / smoothing capacitor / synchronous rectification / switching loss / duty cycle. Also DC-DC converter (a circuit that changes voltages inside a device) / soft switching (a way to cut switching losses)
• Try the numbers: DC-DC converter calculator (on-time fraction and output voltage) / junction temperature calculator (losses and part temperature)
• Related articles: "What Is an Inverter?" (a device that turns DC into AC) / "What Is a Capacitor?" / "How Wireless Charging Works" / "How Lithium-Ion Batteries Work"