I'm having a problem finding the right circuit/device for a circuit I need to make (I'm quite a noob). It doesn't matter what the input voltage is, but I need an output voltage which I can digitally change from 0 (or at least the lowest possible) to 12 volts. It needs to be able to handle 5 amps and sudden changes in the output voltage. Since I don't know how such a circuit is called, I have no idea if it has been asked before. Please tell me how I can make this or at least how this sort of thing is called.
That sort of thing is called a power supply and typically uses a voltage regulator to stabilize the output. Without fancy converter circuitry you would need the input to be higher than 12V, say 15V for example.
There's the LM138/338 voltage regulator which regulates a voltage according to the values of two resistors:
The output voltage is determined by the formula on page 7 of the datasheet.
You could use a digital resistor or digital potentiometer for R2 and thus make the regulation digital.
There's one problem: this IC needs a reference voltage (VREF) of 1.25V to work properly, which means that the minimum output voltage is 1.25V. You will therefore not be able to regulate the output from 0-12V but from 1.25-12V.
You want a switching regulator with an enable pin. A step up or boost regulator, or a step down buck regulator, depending on your input voltage supply. Most will have 80% or higher efficiency, but you still need to make sure your input power supply can meet or exceed that. 12v 5A is 60W, so your input should be a 72W power supply. I'd go with a 6A regulator too because you don't want to push a 5A one to the limit.
The enable pin allows you to turn on and off the supply with digital logic.