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In electronics, TO-18 is a designation for a style of transistor metal case. The case is therefore more expensive than the similarly sized plastic TO-92 package. The name is from JEDEC, signifying Transistor Outline Package, Case Style 18.
Construction and orientation
The tab is located 45° from pin 1, which is typically the Emitter. The lead diameter is nominally 0.45 mm, the diameter of the cap is 4.5-4.95mm (0.178-0.195 inches) and the overall width (not including the tab) is 5.4 mm.
Different manufacturers have different tolerances, and the actual form factor may vary slightly, depending on function.
Uses and variants
The 3-lead TO-18 is used for transistors and other devices using no more than three leads. Variants for diodes, photodiodes and LEDs have only two leads. Light-sensitive or light-emitting devices have a transparent window, lens, or parabolic reflectors in the top of the case rather than a sealed, flat top. For example, diode lasers such as those found in CD players may be packaged in TO-18 cases with a lens.
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