How do most voltage regulator ICs work? Are they the same as hooking up a variable resistor and a voltmeter and turning the knob until you get the desired voltage?
Voltage regulators achieve "stiffness" via a feedback control loop, where "stiffness" means that a large change in load current causes a small change in voltage.
Both switching and linear regulators include a control loop (historically analog... some of the newer switchers use digital control loops) to adjust some parameter of the circuit so that the output voltage remains constant in the presence of load current changes and input voltage changes.
In a linear regulator the circuit parameter is the pass transistor drive circuit (which produces base current for an NPN/PNP power transistor, gate voltage for a MOSFET).
In a switching regulator the circuit parameter is the duty cycle of the switching element(s).
So there's really two areas you need to understand if you want to get into the details of how regulators work:
- topology design (achieve required limits of current/voltage/etc)
- control loop tuning + stability
Voltage regulators have a transistor which in a control loop can conduct more or less, according to demand, so that's a bit like a variable resistor.
This schematic shows the basic principle upon which most linear regulators are built:
The zener diode is a 6.2V version, so the node marked "feedback" needs about 6.8V to make Q1 conduct. R1+R2 divide the output voltage by 2, so that makes the output 13.6V.
If the output voltage would rise, Q1 would start conducting and pull the base of Q2 down, so that Q2 supplies less current to the output and its voltage decreases again.
If the output voltage will go below the set voltage of 13.6V, Q1 switches off and via R3 the input voltage will give Q2 sufficient current so that the output voltage rises again.
So Q1 will make sure that the output remains at 13.6V.
This is a very basic setup, and stability and line regulation are not optimal. Integrated voltage regulators will add extra components for increased (temperature) stability, current limiting and overheating protection.
This is an excellent way to understand the theory. A linear regulator will use a transistor to step the voltage down as an inline resistor(the transistor can be modeled as a variable resistance) with feedback changing its resistance to get a very dependable output voltage. This method is very low noise but not power efficient in general.
The wikipedia page is not half bad to learn about them. Switching regulators use a method that can be though of more as a charge pump, taking advantage of inductors changing voltage to push a continuous current.
Essentially, yes. There is a pass transistor that changes in resistance so that the output voltage stays constant. It's like a variable resistor, though, not a potentiometer:
The amount of resistance is controlled by a feedback amplifier. It adjusts the resistance so that the voltage at the output is constant, regardless of changes in the source voltage or the load resistance.
Does this simplified circuit help?
The specifics of the internals is basically the above and is published in the data-sheets. If you can't recognize common circuits in the actual 7805 schematic, and figure out the details of the complex internal circuitry, then I am afraid it is far too complicated to detail here.
There are numerous links already given in the other answers and comments, that should get you well on your way though.
Bitrex gave a description of the internal function of LM7805. I think it is far away from reality. If one would learn how it works I recommend to read http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=snva512&fileType=pdf by Robert Widlar. And you will find the Voltage Reference in the green box, identify the red box as starting circuit and thermal shutdown, the Zdiode in the violet box as SOA protection etc. Best Regards KPK