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Identifying the legs of a transistor.

Discussion in 'Electronic Basics' started by Joe Norton, Mar 4, 2005.

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  1. Joe Norton

    Joe Norton Guest


    Very basic question. Could someone remind me which leg is which on a
    transistor, starting from the tag on the case.

  2. Chris

    Chris Guest

    Hi, Joe. Thousands of transistors out there. How about describing it,
    and giving any numbers printed on it?

  3. If there is a tag it is indicating the emitter.

    Tagged transistors are very uncommon today, so you will have to rely on
    other methods. If we are talking about bipolar transistors they can be
    described as two diodes and you can easily find these diodes with the
    help of a diode tester (on a modern digital nultimeter).

    When you have found the two diodes and how they are joined you know which
    pin is the base.

    To find out which of the other pins are emitter and collector you can
    test both possibilities in the transistor tester. The combination which
    gives the highest hFE is very likely to be the right combination.
  4. Joe Norton

    Joe Norton Guest

    Hi, Joe. Thousands of transistors out there. How about describing it,
    I have two transistors. One is a 2N2219 A and the other is a 2N2905 A.
    Both have round metal cases with a tag at the edge. One pin seems to be
    connected to the case. Does this help? Please excuse my ignorance! :)

  5. Chris

    Chris Guest

    Hello again, Joe. If I need a transistor pinout, I'll go to the NTE
    Semiconductor cross-reference page first. They sell equivalents for
    television/radio repair shops and other service techs.$$Search?OpenForm

    Your 2N2219 crosses to an NTE123

    and your 2N2905 crosses to an NTE128

    The pinout on both is the same (view in fixed font or M$ Notepad):

    Emitter(o o) Collector/Case

    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    The NTE website is really good for quickly finding transistor pinouts.
    They're also handy if you need a repair or engineering part today.
    They have retail outlets in many repair shops and depots, which means
    that frequently you can just drive down the road and get your part
    within an hour. Great stuff.

    Good luck
  6. PeteS

    PeteS Guest

    Then do what we do and head for a manufacturer's site.

    For example, has both of them (go to the site
    and put in the part number in the part number search box) and
    eventually you'll get a pretty picture or pdf you can download showing
    you the pin description,

    Note that I chose microsemi at random - those transistors used to be
    made by everyone+dog. Philips, OnSemi (nee Motorola SPS) will have them
    to name but another two.

  7. Joe Norton

    Joe Norton Guest

    Thanks, that gave me the info I needed. I've got my circuit working.
    Thanks to you all.

  8. John Fields

    John Fields Guest

    With the leads pointing at you, the lead closest to the tab is the
    emitter then, moving clockwise , the next lead is the base and the
    next the collector.

    I believe the collector is supposed to be internally connected to the
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