Principle of Three-Terminal Voltage Regulator
A three-terminal voltage regulator is a semiconductor device that maintains high resistance until it reaches the critical reverse breakdown voltage. During reverse breakdown, within a certain current range (or within a certain power loss range), the terminal voltage remains almost constant, exhibiting voltage regulation characteristics. Therefore, it is widely used in regulated power supplies and limiting circuits.

Classification of Three-Terminal Voltage Regulators
There are mainly two types of three-terminal voltage regulators: one has a fixed output voltage, known as a fixed output three-terminal voltage regulator, and the other has an adjustable output voltage, known as an adjustable output three-terminal voltage regulator.

Function of Three-Terminal Voltage Regulator
The function of a three-terminal voltage regulator block is to perform step-down processing on the voltage and output it as a stable, fixed value. For example, the 7805 regulator block can step down a voltage of less than 35V to a stable 5V output voltage. It is much better than a circuit that uses only a single Zener diode for regulation, and the cost is not very high, so it is widely used. Common three-terminal regulator blocks can be divided into positive voltage regulator blocks and negative voltage regulator blocks. The 78 series is for positive voltage, and the 79 series is for negative voltage. These two series are not interchangeable, so do not confuse them when selecting.

Internal Mechanism of Three-Terminal Voltage Regulator
When the input voltage decreases or the load resistance decreases, causing the output voltage to drop, the sampling voltage decreases accordingly, and the base potential drops. However, since the emitter potential (maintained by the Zener diode) remains constant, the emitter voltage decreases, leading to a decrease in collector current and an increase in collector potential. Because the collector of the amplifier tube and the base of the adjustment tube are connected, the base potential rises, causing the collector current to increase and the tube voltage drop to decrease. Since the amplifier tube is in series, the output voltage remains basically unchanged. During the operation of a series-type regulated power supply circuit, the adjustment tube is required to always remain in an amplified state. The current through the adjustment tube equals the load current; therefore, a suitable high-power tube must be selected as the adjustment tube and installed with a heat dissipation device according to regulations. To prevent short circuits or long-term overloads from burning out the adjustment tube, DC voltage regulators are generally equipped with short-circuit protection and overload protection circuits.