Common three-terminal voltage regulator integrated circuits in electronic products include the 78×× series for positive voltage output and the 79×× series for negative voltage output. As the name suggests, these three-terminal ICs feature only three pins: input, ground, and output. Their appearance resembles that of a standard transistor, typically housed in a TO-220 package, though some adopt the TO-92 package resembling the 9013.

Using 78/79 series three-terminal voltage regulator ICs to construct a regulated power supply requires minimal external components. The internal circuitry incorporates protection against overcurrent, overheating, and regulator tube failure, ensuring reliable and convenient operation at an affordable price. Within this series of integrated voltage regulator ICs, the digits following “78” or “79” denote the output voltage of the three-terminal integrated regulator circuit. For instance, the 7806 indicates a positive 6V output, while the 7909 signifies a negative 9V output.

The 78/79 series of three-terminal voltage regulator ICs have been manufactured by numerous electronics companies since the 1980s. Typically, the prefix denotes the manufacturer's code; for instance, TA7805 is a Toshiba product, while AN7909 is manufactured by Panasonic. (Click here to view information on identifying integrated circuits by prefix.)

Occasionally, an “M” or “L” suffix follows the numerical designation (e.g., 78M12 or 79L24) to denote output current rating or package type. The 78L series offers a maximum output current of 100mA, the 78M series 1A, and the standard 78 series 1.5A. Multiple package options exist, as detailed in the diagram. Plastic-encapsulated voltage regulator circuits offer advantages such as ease of installation and low cost, making them widely adopted. The 79 series shares identical naming conventions and physical form with the 78 series, differing only in negative output voltage and pin configuration.

Owing to their user-friendly operation, three-terminal fixed integrated voltage regulator circuits are frequently employed in electronic projects. They can be used to retrofit discrete component voltage regulators and commonly serve as operating power supplies for electronic equipment. The circuit diagram is shown in the figure.

Note that the input, output, and ground terminals of the three-terminal integrated voltage regulator circuit must never be connected incorrectly, as this can easily cause burnout. Typically, the minimum input-output voltage difference for a three-terminal integrated voltage regulator circuit is approximately 2V; otherwise, it cannot output a stable voltage. Generally, the voltage difference should be maintained at 4-5V, meaning the voltage after transformer step-down, diode rectification, and capacitor filtering should be slightly higher than the regulated voltage value.

In practical applications, a sufficiently large heat sink should be fitted to the three-terminal integrated voltage regulator circuit (though this is unnecessary for low-power conditions). When the regulator diode overheats, its voltage regulation performance deteriorates and it may even be damaged.

When constructing a regulated power supply requiring an output current exceeding 1.5A, multiple three-terminal regulator circuits are typically connected in parallel to achieve a maximum output current of N × 1.5A. However, the following precautions must be observed: Integrated regulator circuits used in parallel must be sourced from the same manufacturer and batch to ensure parameter consistency. Additionally, a margin should be maintained in the output current rating to prevent cascading failure and subsequent burnout of other circuits should an individual integrated regulator fail.