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Pnp npn transistors

  1. Jun 27, 2011 #1

    I just bought lot of pnp and npn transistors. They are not labeled. How do i tell them apart?!
  2. jcsd
  3. Jun 27, 2011 #2
    Are they distinct in any way at all? A lot of times, they make one lead longer than the others, or one side of the body flat.
  4. Jun 27, 2011 #3
    well the leads are weird at one set. they are spread. Does that mean anything?
  5. Jun 27, 2011 #4
    not usually - assuming a standard TO-92 package, you can get most transistors with different lead forming (p or n channel).

    Is there no marking (screening or laser etching) on the body of the components?
  6. Jun 27, 2011 #5
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  7. Jun 27, 2011 #6
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  9. Jun 27, 2011 #8
    figured it out. But really, 2 types of bjt and they both look same to the eye? This is flaw of EE community i think. They should have come up with some specific label or something .
  10. Jun 27, 2011 #9
    You might find this gadget interesting.

    Attached Files:

  11. Jun 27, 2011 #10


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    Gold Member

    Sure you can use an ohmeter to check the base emitter junction. Now tell me how you will tell it from the base collector junction. This method will tell you the difference between NPN and PNP but it won't likely tell you which is the collector and which is the emitter.
  12. Jun 27, 2011 #11


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    Staff: Mentor

    Yeah, you really need something that can check the AC gain, to distinguish between the collector and emitter leads. I like the find by Studiot:

  13. Jun 27, 2011 #12


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    Sencore (others possibly as well) made a transistor checker that just hooked 3 leads to the device under test. You rotated a permutator switch until you got a reading on a meter. This narrowed it down to 2 possible positions. Then hit the gain switch to determine which was which. Read out the position on the switch which lead was connected to emitter, base, and collector. This was way before the digital revolution. Berkeman, the link you provided looks kinda cool at a glance anyway.
  14. Jun 27, 2011 #13


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    Staff: Mentor

    Bassalisk -- do you have an idea for a simple circuit you could make with a signal generator and DVM that would let you figure out the forward and reverse gains of your transistors? Might be a good learning project for you.
  15. Jun 28, 2011 #14
    Do you really not have these in the States?
    Peak do a range of little automatic component testers in the same format.
  16. Jun 28, 2011 #15
    I used DIODE tester. It lets current in one direction and measures how much current is getting back. If no current is getting back at the meter then I reverse biased the junction.

    I have diode tester in my multimeter.
  17. Jun 28, 2011 #16
    Are you aware how and why the diode test function in your multimeter works?
  18. Jun 28, 2011 #17
    Well I tested it with an actual diode. I hook red wire(positive) on the anode, and the black wire(ground/negative) to the cathode. It gave out a reading. Its says here its measuring continuity voltage, I believe that is voltage in conducting state?

    But when i hook it the other way around, i get a zero. Pretty straightforward.

    I used it on the transistor, emitter-base junction because I know that works in forward bias.
  19. Jun 28, 2011 #18
  20. Jun 28, 2011 #19
    But it is not always that straightforward and an electonics/electrical major should known the pitfalls.

    Yes, I seem to remember you have recently bought or borrowed a digital multimeter.

    Older type Analog meters, with pointers respond to current not voltage.
    We make a voltmeter out of them by putting a high value resistor in series with them.

    We make an ohmmeter by putting a ressitor either in series or paralle with the unknown and measuring the current through the resistor. This yields in a highly non linear scale.
    In order to measure higher value resistors we need a relatively high supply voltage (my old meter had 22.5 volts.)

    This high voltage was more than enough to turn on (bias on in the forward direction or sometimes breakdown in the reverse) semiconductor junctions.

    Further the colours of the terminals and leads of these older type meters were reversed ie the black lead produced a positive voltage on the black lead.

    So you cannot use this type of meter to measure in resistors incircuit as you will switch or destroy junctions. Electronic ohmeters were introduced that applied about 0,2 volts to overcome this.

    Modern digital meters respond to voltage
    So we make current meters by measuring the voltage across a known resistor with them.
    We measure resistance by either passing a constant current through the resistor and measuring the voltage or using a feedback amplifier or by other means.

    Whichever we do, the red lead should now be positive and around 0.2 volts only applied to the unknown resistor.

    This means that we can no longer use an ohmmeter to test for forward and reverse resistance on a junction as we simply will not switch it on.

    So modern meters have a special setting which applies sufficient current limited voltage to turn on the junction and presents the voltage across it as a reading in the forward direction.
  21. Jun 28, 2011 #20
    Yes but I didn't use an Ohmmeter to test my junction. I understand now that the ohmmeter uses external voltage supply to drive current through resistor and measure the resistance.

    But I have special feature on my multimeter, called diode test, designed specially for diodes. I don't think it sends large currents through test leads because its designed for diodes.
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