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How do i see the polaity on a npn transistor

Discussion in 'Electronic Basics' started by ragnar_123, Oct 2, 2006.

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  1. ragnar_123

    ragnar_123 Guest

    How do i see the polaity on a npn transistor?
    i got a 2n2222 npn, and i am not sure how the polarity is.

    sry, but i am only 14 year old so please explain what you are saying -
    and my english isn't soo good either.
    btw i am building a relay circuit, which is controlled by the
    computer's parallel port.
    thanks
     
  2. An NPN transistor acts as a switch, when its collector is more
    positive than its emitter and current passes between base and emitter,
    with the base more positive than the emitter.
    See:
    http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/npnce.html
    http://www.phys.ualberta.ca/~gingrich/phys395/notes/node74.html
     
  3. ragnar_123

    ragnar_123 Guest

    thanks, but how do I see which pin is base, collector or emitter?
     
  4. If you have an ohmmeter, measure between the terminals of the
    transistor. There will be one combination of leads and terminals when
    you have the positive meter lead (usually the red one) on the base of
    the transistor, and you can read a low resistance to either of the
    other terminals. The one with the slightly higher resistance reading
    from the base is the emitter, and the other one the collector. Another
    way is, if you know the manufacturer and the exact part number, google
    it to get a data sheet, which will show the pin connections.
    If it's a plastic device with a flat on one side, and you hold it
    facing the flat with the legs pointing down, the terminals will usually
    be in the order E-B-C.
     
  5. Guest

  6. ragnar_123

    ragnar_123 Guest

    thanks guys, it is working now

    cool regards
    Ragnar J
     
  7. Jamie

    Jamie Guest

    most components are looked at from the bottom side, so lets look
    at the bottom, i don't know if you have the plastic type package or
    the metal type ?
    lets assume the metal package here, when looking at the bottom, turn
    the package so that the center leg is pointing to your left.
    in this order, you will have COLLECTOR on top, BASE on the middle
    pointing to the left and EMITTER on the bottom.
    look below at that spec sheet.

    http://www.ee.washington.edu/stores/DataSheets/transistors/2n2222.pdf#search="2N2222"
    ---
    now lets assume you have a plastic case package.
    look below and you can find plastic packages with their
    pin outs.

    http://www.fairchildsemi.com/ds/PN/PN2222A.pdf

    ---
    now to determine functionality of the transistor, using a DMM or ohm
    meter in Diode test mode. place the Red Lead (+) on the the base and
    use the black Lead (-) to test from Collector to Emitter. these two
    points should test like a diode. now reverse the leads and perform the
    same test and you should not get any reading..
    P.S.
    since your using a relay, make sure you have a Diode of 1A or more
    across the relay coil to absorb returned Energy when the relay is turned
    off, other wise you will short your 2N2222 and maybe even the computers
    port. when placing the diode, make sure the Cathode side is on the side
    where the + voltage is being fed too. this is the side with the LINE on it.

    and you want to make sure you get full voltage to the coil and not
    allow the transistor to float between low and high, this will cause
    heating on the transistor more so then should be.
    using the emitter to common and collector to the side of the coil
    that as the anode of the diode connected to it, while the other side of
    the coil which has the Cathode of the diode connected to it along with
    the main source of supply voltage is the base way.. this away you can
    vary the relay's needs for its coil voltage requirements with out
    depending on the computers signal line voltage levels.
    doing this, you should use between a 470 to 680 ohm resistor from the
    signal line of the port to the BASE of the transistor.

    these are my views... i am sure others will comment greatly.
     
  8. Johnny Boy

    Johnny Boy Guest

    I'm pleased to see that there are still guys of your age interested in this
    stuff. Good luck with wherever it takes you, and never be afraid to post
    here. Apart from a few, most of us will be more than happy to help when we
    can.
    P.S. It was a good idea to post your age because you managed to avoid some
    of the characters that might have replied rudely.
    .... Johnny
     
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