# Does Arduino run AC or DC

Does arduino run AC or DC? Based on my small knowledge on the crystal in an arduino, it seems like AC makes sense. True? No?

• You are correct about the crystal producing an alternating signal, but it is a timing signal only and doesn't power the arduino. In fact, the crystal timing circuit is powered Arduino's DC power. Jul 6 '14 at 23:18
• Thanks. Could you reference me to materials to help me understand amplifier circuits/resonator circuits such as the one that crystals use? I have already read the wikis. Jul 6 '14 at 23:27
• (Edited): Try these two: Wikipedia; Circuits Today. Also this question about how crystals work on Electrical Engineering Stack Exchange. Jul 7 '14 at 0:17
• Lol I asked that question man Jul 7 '14 at 0:18
• (Oops!) Good answers there, though - I learned a bunch, too. Jul 7 '14 at 0:19

As a general rule, any modern electronic device with computational ability has to use DC. The reason is that they use specific voltage levels to indicate binary/logical states (i.e. 0's and 1's).

In theory, an ideal DC supply (Direct Current) should provide a constant voltage. That allows the circuit to effectively turn it on an off to represent binary states. (In reality, there's no such thing as truly "off" though, because voltage is relative.)

However, the voltage from an AC supply (Alternating Current) is constantly varying. If you plot it on a chart, you should see a nice sine wave, flowing smoothly between positive and negative several times per second. Hypothetically, if a digital circuit tried to use AC then it would get very confused, because everything would seem to be turning on and off all the time.

With all that said, it's very common to turn AC into DC through a process called rectification. For example, that's what allows DC devices to get power from an AC outlet (e.g. to run your computer or charge your phone).

• Rectification, filtering, and regulation. Simple rectification leads to switched DC, where the negative part of the sine wave is inverted and you just have a series of positive humps that still drop to zero. Filtering smooths out the power to a flat DC voltage. However, the voltage from a filtered but not regulated supply will vary based on load. The more current you draw from it, the more the voltage will drop. A regulated power supply will adjust the output to keep the voltage constant. Logic circuits run much better with a regulated supply. Some logic circuits like TTL require it. Jul 24 '14 at 0:14

All digital logic circuits run on DC.

Most Arduino boards include a voltage regulator. You can feed them an input ranging from 7 to 12 volts, and they "clean up" that input and lock it to precisely 5 volts, the voltage needed by the Arduino.

They can also run directly off the regulated 5 volts from the USB connector.

Have a look at this: http://arduino.cc/en/uploads/Main/arduino-uno-schematic.pdf and try and have a guess.

I don't want to be nasty, but have you at least googled this? Google: "Arduino Power Source" and the first hit is http://playground.arduino.cc/Learning/WhatAdapter

It runs from USB as well as from an external power source.