Transistor Case Packages

Transistor Package TO-257 Case style, with Metal Tab

2-Terminal TO-257 Diode Package
TO-257 Two Terminal Package

TO-257 Outline
TO-257 transistor is a through-hole device, with two leads and a tab [2-pin Isolated]. Which means that the metal tab is isolated from either of the pins, but still functions as a heat-sink.

Another variant uses semi-flexible leads with the origins [near the case] being rigid; however that version is even less common than the normal two pin variant.

Dimensions are provided in inches and millimeters.

A TO-257 is also supplied with 3 leads, refer to the 3-Terminal TO-257 Package.
However other packages are also available in 2 lead packages [3-terminal are shown].
Both the TO-220 Package and TO-247 Package come as 2-lead or 3-lead cases.

The most common application of this package is with diodes;
TO-257 Diode Terminals: Lead 1, Cathode. Lead 2, Anode.
However a power resistor might also use this package, or the TO220 or TO-247 packages.
Common power dissipation in these packages range from 20, 30 35, 50 & 100 watt resistors.

Tab mount. This package contains a metal tab which provides a method of readily attaching that surface of the semiconductor to a heat-sink to achieve thermal management of the case temperature. If required a metal washer and thermal grease could also be used. Heat Sink Manufacturers. The heat sink may be a bolt-on or clip-on, and of varying size. In addition to bolting to the metal tab, a number of heat sinks will also have fingers to attach to the printed wiring board. Some heat sinks attach to the PCB to stabilize the device, so there is not a large component hanging un-attached just above the circuit board, which would be poor design practice in high vibration environments. Many pages in the transistor package section show different heat sink styles that would conform to a TO257 package style.

Types of Semiconductor cases; Transistor Package styles, Diode Package styles.

Devices that use the 2-Terminal TO-257 package include;
Ultrafast Power Rectifier Diode, 1N6774 through 1N6777, 1N6778 and 1N6779
Power Rectifier, Schottky Diode, 1N6781
Of course there are many more devices, these are just examples.
Note that these devices may or may not be in production, offered only as examples.

TO-257 Dimensions
TO-257 Diode Package Dimensions

Component Manufacturers;
FETs & BJTs Vendors
Diode Manufacturers

PC motherboard

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Modified 1/11/12
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