transistor

Discussion in 'General Electronics Chat' started by isha, Nov 25, 2005.

  1. isha

    Thread Starter Active Member

    Sep 16, 2005
    36
    0
    when a transistor is in black box only three terminal are outside the box

    and do not know that mid terminal is base

    how can we label the three terminal with out measuring the voltge by multi meter across the terminal
     
  2. Brandon

    Senior Member

    Dec 14, 2004
    306
    0
    Go online, look up the xtr.
    Use a ohmeter and mesure the res between the terminals. Remember ur Rpi, Rce and other values and you can determine that way.
    Continuity tester will give you the direction of the xtr but not which leg is what.
    And most newer meters have a xtr slot, u plug it into the slots and when u plug it in correctly, the meter will give you a internal gain value or something similar and then you know which is what.

    Anyone else got any ideas for doing this?
     
  3. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    hi

    if you are using a japanese analog multimeter the negative post has a positive output.

    check 1st to find out which legs are the E & C. if you get infinite resistance between them, even after reversing your probe, then that is your E & C. that leaves your B leg. :D this applies to both the NPN & PNP.

    if you get low resistance readings on all legs tested, then you have a defective transistor. :eek:
     
  4. richbrune

    Senior Member

    Oct 28, 2005
    106
    0
    Both Brandon and Moz are correct. If you are having trouble with both of those tests, you might consider that the device is a small signal mosfet or other device. On bipolar transistors the middle terminal is almost always the base. On small signal mosfets, however, the gate (similar to base) is usually one of the terminals that is not the center. Harbor Freight and Radio Shack both have inexpensive digital testers with plug in points for testing transistors, and that will tell all for bipolar transistors. Mosfets and J-fets will usually give a result similar to a bipolar transistor, unless you have recently touched the gate terminal with your finger, in which case they will show a low resistance between the other two leads.
    In summary, you should buy a transistor checker.
     
  5. richbrune

    Senior Member

    Oct 28, 2005
    106
    0
    Oh yeah, when I said that the middle lead is almost always the base, I was referring to the small black transistors, TO-92 case. If the black box is around 5/8" inch square with a metal tab across the top, it's a TO-220 or possibly TO-246 or something like that, where the base is usually NOTthe center. If you can't read the lettering on the device as Brandon said, it gets tricky fast. A meter with a transistor checker might help.
     
  6. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    hi,

    i would like to add a little, for TO-220 casing the bjt Base is usually pin 1 assuming the transistor is facing you. but that is not the general rule of Base pin placement.

    i would like to comment on the TO-92 silicon transistors. why can't the manufacturers agree on the specific placement of the pins. like having the base pin always at the center, the emitter at the left and the collector pin at the right. by my experience majority are placed like that, but there others who just loves to jumble those pin placement <_<

    moz
     
  7. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    Off-topic.

    I worked at one of those manufacturers a few years back. The reason the pinouts are different is mainly power dissipation. The chip needs to be tacked on to a piece of metal, which would invariably be the collector. The bigger this metal is the larger the contact area and the larger the maximum chip area that can be tacked on and the faster the heat can be dissipated from the chip. There are only two possibilities here, either the collector is in the middle or on the outside. Most standard small signal TO92s go with collectors on the outside, but the metal area is smaller in this configuration compared to when the collector is in the middle.
     
  8. isha

    Thread Starter Active Member

    Sep 16, 2005
    36
    0
    i want this

    thanks all of you
     
  9. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    hello,

    i might as well add this info on identifying pins on TO-92 transistor.

    sometimes we are confronted with a problem of identifying whether the transistor is an NPN type or PNP. and another problem of knowing w/c is the collector or emitter.

    in consonance of what i have already mentioned on nov. 26, since we already know w/c pin is the base but we specifically do not know w/c pin is the real collector or emitter. we must remember that the resistance between base and collector is higher than the base and emitter resistance when forward biased. this applies to both NPN & PNP.

    now in identifying w/c is the NPN or PNP.

    we now assume that we have already identified the pins correctly.

    1. connect the black probe to the base pin and the red probe to the collector or emitter.

    2. result of step 1 is: no deflection

    3. conclusion: the transistor is reverse biased by the ohmeter battery.

    4. reverse the procedure of step 1.

    5. result of step 4: there is deflection to the right indicating low resistance

    6. conclusion: the transistor is now forward biased. the transistor is an NPN type.

    in case the results of step 1 to 5 is the opposite condition, we can now conclude that the transistor is PNP type.

    hope this helps.

    moz
     
  10. kurios

    Member

    Sep 29, 2005
    14
    0
    yes mozik luv i do agree with u it's the procedure i have carried out in my practical just to determine the type of transistor and here we mostly use NPN transistors, and second thing is that most of the transistors have their base leg in the centre now this thing is so much common that it has become a convention, but remember exceptions are there.
     
Loading...