How to Find the Legs on a 2N3055 NPN Transistor

Written by j.t. barett
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How to Find the Legs on a 2N3055 NPN Transistor
Transistors have standard configurations based on case style. (four gold transistors on blue image by Eugene Tokarev from

The 2N3055 NPN power transistor comes in a standard metal case, developed by the Joint Electron Device Engineering Council (JEDEC), called the TO-3. The roughly hat-shaped case has two short pins coming vertically through the bottom and two holes for mounting securely against a metal heat sink. The pins are slightly offset toward one of the two holes, making identifying the transistor's legs easier. The two pins are two of the transistor's terminals, with the case serving as the third.

Skill level:

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Things you need

  • 2N3055 Data Sheet

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  1. 1

    Ensure that the 2N3055 transistor sits so its two legs point down.

  2. 2

    Turn the transistor horizontally until one of the mounting holes is nearer to you, and the legs are closer to you than the other mounting hole. The legs are not the same distance from the mounting holes; they are closer to one hole than the other.

  3. 3

    Identify the transistor's legs. The left one is the base, the right one is the emitter and the case is the collector.

Tips and warnings

  • The 2N3055 NPN has a PNP complement, the MJ2955. It has the same physical base-collector-emitter configuration.
  • This is a common configuration for bipolar transistors in the TO-3 package. If you find similar transistors having a TO-3 case, they will likely have the same layout.

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