Transistor Case Styles

JEDEC

• In the late 1950s, several electronic manufacturers formed a trade association called Joint Electron Devices Engineering Council (JEDEC). The group recognized that standards for device numbers and case styles worked to their mutual benefit; they created numbering systems and physical specifications for transistors, diodes and other electronic devices. Part numbering systems such as the use of the "2N" prefix for transistors came from this group. They also devised a series of transistor cases carrying the "TO" prefix.

TO-92

• Used for small-signal transistors such as the 2N2712, the TO-92 case is made of black plastic epoxy resin. The case has a half-cylinder shape; the flat front side indicates the orientation and location of the transistor's three leads, which come out of the bottom of the package. Small plastic cases such as the TO-92 are suitable only for low-power devices of no more than a few watts.

TO-3

• High-power transistors use the metal TO-3 case style. Roughly hat-shaped, the TO-3 has two vertical pins protruding from the bottom of the device's center. The case acts as the third terminal needed for transistors. TO-3 transistors handle up to 200 watts; the metal case has bolt holes for securely fastening it to a metal chassis or heat sink. A strong metal-metal connection efficiently conducts heat away from the component.

TO-220

• Transistors of intermediate power use the TO-220 case. It is flat and rectangular, mostly plastic with a metal tab on its back. The tab has a hole for hardware to bolt the transistor to a metal chassis. While not quite at the power level of the all-metal TO-3 case, TO-220 transistors handle in excess of 100 watts.