TO3

TO-3, short for Transistor Outline 3, is a vintage metal-can package used for power transistors and some voltage regulators. It was widely adopted in the 1960s and 1970s for devices requiring solid heat dissipation and robust mechanical construction. The package is larger than later methods, reflecting its purpose for higher current operation.

Physically, the TO-3 consists of a circular metal case with a mounting hole for a heatsink and two external leads at the bottom edge. The transistor’s collector is connected to the case, while the two leads provide the base and emitter connections. Because the case is part of the electrical circuit, mounting to a heatsink also serves as the primary heat path. If isolation from the heatsink or system potential is required, an insulating washer and appropriate interface are used.

Electrical characteristics and usage have historically centered on high-power applications. The case’s large surface area enables effective heat removal, enabling higher current capabilities than many later plastic-packaged transistors. Classic devices in TO-3 include parts like the 2N3055 and its complementary 2N2955, among others. As electronics design shifted toward smaller, cheaper, and more mass-manufacturable packages (such as TO-220 and surface-mount options), the TO-3 fell out of common use for new designs, though it remains of interest for vintage equipment repair, hobby projects, and specialized applications requiring its rugged, thermo-mechanical properties.

Standardization and variations are documented in historical JEDEC references and vendor datasheets. Compatibility depends on attention to case connection, lead pinout, and proper heatsinking.