# How do DC voltage regulators work

## How does an LM7805 voltage regulator work?

How do most voltage regulator ICs work? Is this the same as hooking up a variable resistor and voltmeter and turning the knob until the desired voltage is reached?

Voltage regulators achieve "stiffness" through 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 contain a control loop (historically analog ... some of the newer switchers use digital control loops) to set certain parameters of the circuit so that the output voltage remains constant with changes in load current and input voltage.

In a linear regulator, the circuit parameter is the pass transistor driver circuit (which generates a base current for an NPN / PNP power transistor, a gate voltage for a MOSFET).

In a switching regulator, the switching parameter is the duty cycle of the switching element (s).

So there are really two areas that you need to understand in order to understand how regulators work:

• Topology design (reaching the required current / voltage limits, etc.)
• Control loop tuning + stability

Voltage regulators have a transistor that can conduct more or less in a control loop as required. So this is a bit like a variable resistor.
This diagram shows the basic principle on which most linear regulators are based:

The Zener diode is a 6.2V version, so the node labeled "Feedback" needs about 6.8V for Q1 to conduct. R1 + R2 divide the output voltage by 2 so that the output is 13.6V.
If the output voltage increased, Q1 would begin to conduct, pulling the base of Q2 down so that Q2 is delivering less current to the output and its voltage decreases again.
When the output voltage falls below the set voltage of 13.6 V, Q1 switches off and the input voltage Q2 supplies sufficient current via R3 so that the output voltage rises again.
So Q1 will make sure the output stays at 13.6V.

This is a very simple setup, and stability and line regulation are not optimal. Integrated voltage regulators add additional components to increase (temperature) stability, current limitation and overheating protection.

This is an excellent way to understand the theory. A linear regulator uses a transistor to reduce the voltage as an in-line resistor (the transistor can be modeled as a variable resistor), with the feedback changing its resistance to give a very reliable output voltage. This method is very low-noise, but generally not energy efficient.

The Wikipedia page is not a bad place to learn about it. Switching regulators employ a method that can be more than a charge pump and take advantage of inductors that change voltage to provide a continuous current.

Basically yes. There is a pass transistor, the resistance of which changes to keep the output voltage constant. It's like a variable resistor, not a potentiometer:

(Source: techitoutuk.com)

The level 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?

simulate this circuit - circuit diagram created with CircuitLab

The special features of the built-in components are basically those mentioned above and are published in the data sheets. If you can't see any common circuitry in the current schematic of the 7805 and you can figure out the details of the complex internal circuitry, then I think it's way too complicated to go into detail here.