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Does the Arduino Uno supply AC or DC power when connected to the computer via USB?
Can we specify is we want AC or DC?
I am trying to build a circuit that has a transistor but I don't currently have an external battery power supply for the DC current (see pic) and thus was wondering if I could just use the Arduino for that... (I am already using the Arduino for the AC power supply because up until now I assumed it supplies AC current...)
The circuit you are trying to build is a simple motor driver circuit.
The Arduino supplies, uses, and thinks in DC. There is nothing AC about the Arduino*.
The "AC" current that you have labelled is supposed to be a DC switched signal. Typically this is a pulse width modulated DC signal which switches on and off rapidly. The ratio of on to off periods sets the speed of the motor.
The "DC" supply is just plain old DC which the Arduino could supply, depending on the power consumption of the motor.
USB is unable to supply more than 100mA normally, or 500mA if it has been negotiated (it's up to the Arduino to do this - I don't know if it does it or not).
I would suggest it is better to power the motor from a separate supply unless it is a very very small low powered motor.
*true, the clock signals, etc, are alternating signals, but not AC in the style of mains etc.
