AC Power & DC Power Comparison

AC means Alternating Current and DC means Direct Current.

AC and DC are also used when referring to voltages and electrical signals which are not currents! For example: a 12V AC power supply has an alternating voltage (which will make an alternating current flow). An electrical signal is a voltage or current which conveys information, usually it means a voltage. The term can be used for any voltage or current in a circuit.

Alternating Current (AC)

Alternating Current (AC) flows one way, then the other way, continually reversing direction.

An AC voltage is continually changing between positive (+) and negative (-).

The rate of changing direction is called the frequency of the AC and it is measured in hertz (Hz) which is the number of forwards-backwards cycles per second.

Mains electricity in the USA has a frequency of 60Hz.

An AC power supply is suitable for powering some devices such as lamps and heaters but almost all electronic circuits require a steady DC power supply. 

AC from a power supply
This shape is called a sine wave




This triangular signal is AC because it changes
between positive (+) and negative (-).

Direct Current (DC)

Direct Current (DC) always flows in the same direction, but it may increase and decrease.

A DC voltage is always positive (or always negative), but it may increase and decrease.

Electronic circuits normally require a steady DC supply which is constant at one value or a smooth DC supply which has a small variation called ripple.

Cells, batteries and regulated power supplies provide steady DC which is ideal for electronic circuits.

Steady DC
from a battery or regulated power supply,
this is ideal for electronic circuits.





Smooth DC
from a smoothed power supply,
this is suitable for some electronics.




Varying DC
from a power supply without smoothing,
this is not suitable for electronics.



The power plugs on your office and home wall are usually AC power form.

AC to DC power adapters convert AC power to DC power.

DC to DC power adapters are usually used for changing DC power voltages and currents.

DC to AC power adapters are usually called inverters. They convert DC power to AC power.