Wall sockets deliver AC (alternating current); the electronics inside your phone, laptop, and tablet need DC (direct current). Every charger you own is fundamentally an AC-to-DC converter. This guide covers what AC and DC actually are, why the grid uses one and devices need the other, and what the charger inside that wall brick is doing.
What AC and DC actually are
Direct current (DC)
DC flows in one direction at a constant voltage. A battery is the simplest DC source: the positive terminal is always positive, the negative terminal is always negative, and current flows from positive to negative through a circuit. The voltage stays constant (or decreases slowly as the battery drains).
Common DC sources:
- Batteries (AA, AAA, lithium-ion phone batteries, car batteries)
- Solar panels
- The output of any USB charger
- The output of an automotive 12 V outlet
- The output of any DC power supply
DC is the natural form for electronics because integrated circuits and transistors require constant voltage to operate correctly.
Alternating current (AC)
AC reverses direction periodically. In 60 Hz systems, the current changes direction 60 times per second (or 120 times if counting both directions). In 50 Hz systems, 50 times per second.
The voltage oscillates between positive and negative peaks. For 120 V US mains, the actual voltage at any instant ranges from +170 V to -170 V; the "120 V" is the RMS (root mean square) value, an average that represents the effective power delivery.
Common AC sources:
- Wall outlets in all countries
- Most industrial generators
- Most large electric motors
For more on the frequency side specifically, see our 50 Hz vs 60 Hz explainer.
Why the grid uses AC
The choice goes back to the 1880s and 1890s, the so-called War of Currents between Thomas Edison (advocating DC distribution) and Nikola Tesla and George Westinghouse (advocating AC).
AC's key advantage: it can be transformed easily. A transformer with no moving parts can step AC voltage up for long-distance transmission (reducing the current required for a given power and therefore the line losses) or step it down for safe delivery to homes.
DC couldn't be transformed practically in the 1890s. To deliver high voltage for transmission and then convert to low voltage for use, you'd need a motor-generator pair: a motor driven by high-voltage DC, mechanically coupled to a generator producing low-voltage DC. Wasteful, expensive, and unreliable.
So AC won the grid. By 1900, most cities had AC distribution networks. By 1950, virtually every electrified country was on AC.
The trade-off: AC is more dangerous than DC at the same voltage (the oscillating current can disrupt heart rhythm at lower thresholds than DC), and AC is harder to store than DC (no efficient AC battery exists).
Why electronics need DC
Modern electronics are built from transistors and integrated circuits that switch between "on" and "off" states based on input voltage thresholds. If the input voltage is oscillating like AC, the circuit can't tell whether the signal is in the "on" or "off" state at any given moment.
Imagine a chip that decides "if voltage is above 1 V, it's a 1; if below 0.5 V, it's a 0". With AC at 120 V, the voltage swings from +170 V to -170 V many times per second. The chip would see the same input as oscillating between 1 and 0, completely scrambling its operation.
DC's constant voltage solves this. A 5 V DC supply gives the chip a stable reference: anything around 5 V is a 1, anything around 0 V is a 0. The chip operates predictably.
Beyond logic, DC is required for:
- Battery charging (batteries are inherently DC storage)
- LED lighting (LEDs are diodes that only pass current in one direction)
- Audio amplifiers (need stable supply voltage)
- Most sensors and measurement instruments
What chargers actually do
A modern phone or laptop charger is a small AC-to-DC converter. The internal sequence:
- AC from the wall enters at 100-240 V depending on country
- A rectifier (diode bridge) converts the AC to pulsing DC (one direction only, but with the voltage still swinging up and down)
- A capacitor smooths the pulsing DC to a higher voltage steady DC (typically 300-380 V)
- A switching transistor chops the DC bus into high-frequency pulses (typically 100 kHz or higher)
- A small transformer steps the chopped DC down to the low voltage needed
- A second rectifier and capacitor produce the final smooth DC output (5 V, 9 V, 15 V, or 20 V depending on the USB-C PD negotiation)
The whole process happens inside a small wall brick. The output is DC at whatever voltage your device needs. See our USB-C Power Delivery explainer for the full negotiation protocol.
DC in modern applications
DC is making a partial comeback in specific applications:
USB-C Power Delivery
USB-C cables carry DC at negotiable voltages (5 V, 9 V, 15 V, 20 V, 28 V, 36 V, 48 V) up to 240 W. The USB-C PD standard is essentially a DC distribution micro-grid for personal electronics.
Electric vehicles
EV batteries store DC. The motor drives are inverter-based, converting DC to AC for the motor windings. Charging stations come in two flavors: AC chargers (slower, the car converts internally to DC) and DC fast chargers (bypass the car's AC-to-DC step for faster charging).
Solar power
Solar panels produce DC. Most solar installations use an inverter to convert to AC for grid use, but in off-grid setups and some commercial installations, DC stays DC throughout.
Data centers
Some modern data centers are exploring DC distribution (typically at 380 V DC) to reduce conversion losses. Each server in a traditional AC-fed rack has its own AC-to-DC converter; centralizing the conversion at the rack or building level can save 5-15% in efficiency.
HVDC transmission
High-voltage DC (HVDC) is now used for long-distance and submarine cables (typically 400-800 kV DC). The conversion equipment is expensive but the line losses are lower than AC for very long runs.
What this means for travelers
For travel purposes, the AC/DC distinction shows up in:
Charger compatibility
Wall outlets are AC at the country's mains voltage and frequency. Your charger is designed to accept whatever AC the country provides (if it's dual voltage 100-240 V) and convert to the DC your device needs.
If your charger is single-voltage (rare for modern devices, common for old hair dryers), it can't handle a different country's AC voltage. See our voltage converter explainer.
Power banks
Power banks store DC and output DC via USB. Charging a power bank from a wall outlet runs the same AC-to-DC conversion as charging any device. The power bank just stores the DC for later use.
Car charging
Automotive 12 V outlets (the "cigarette lighter" socket) provide DC. Devices designed for car charging accept 12 V DC directly. A wall charger's USB output (5 V DC) can't be plugged into a car outlet without a step-up converter; a car charger's USB output (5 V DC) can't be plugged into a wall outlet without a step-down converter.
The bottom line
AC won the grid 130 years ago because transformers made high-voltage transmission efficient. DC stayed in electronics because integrated circuits require constant voltage to function. The interface between the two is the charger.
Every USB charger you own is a small AC-to-DC converter that handles whatever AC the country provides and outputs the DC your device needs. For modern dual-voltage chargers, the device on either end of the conversion is identical regardless of which country's mains you're in.