Does alternating current mean any variable voltage or only specific case of it when polarity (e.g. current flow direction) gets reversed?
I want to understand what AC really is, and if PWM or AC passed through a rectifier (which is essentially \$\lvert sin(x)\rvert\$ if \$sin(x)\$ was passed on input) is an AC voltage?
If not – what is it?
Best Answer
DC means that the voltage and/or current (practically) don't change over time. Example: a 5 V USB adapter that we use to charge a phone.
AC means that the voltage and/or current do change over time and that is relevant for the application. Example: mains AC is a voltage that changes over time, it is a 50 Hz or 60 Hz sinusoidal shaped voltage.
Any other voltage or current that changes over time can be called AC so a PWM signal is AC as long as it isn't a 0 % (exactly) or 100% (exactly) PWM signal because those signals are constant and thus they're DC.
Do note that a signal can have an AC and a DC component. A 50% PWM signal which is 5 V 50% of the time (and 0 V the other 50% of the time) has an average value of 2.5 V. You get 2.5 V if you filter it! That means it has a DC voltage component of 2.5 V. I'm talking about a 50% Duty Cycle signal like shown here:
Source
So on top of that 2.5 V DC there's a 2.5 V peak AC signal. The amplitude of the AC component is 2.5 V. The AC signal varies between -2.5 V and + 2.5 V relative to the +2.5 V DC.