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How to identify different transistors?

  1. Nov 28, 2009 #1

    I bought some large "grab-bags" full of electrical components (I was looking for a good deal). But while trying some of the transistors out, I noticed that they're not all the same.

    They ALL look like this: http://quality-deal.com/osCommerce/images/MPSA06%20NPN%2080V%20500mA%20General%20Purpose%20Transistor.jpg [Broken]
    but with different codes on it, of course. In fact, about half don't even have anything on the face.

    I then built a small circuit that I could test them out with. About 1/3 of them proved to be NPNs. I could find no PNPs though. And no SCRs (not even sure if SCRs can look like that).

    The rest are a mystery. About half of them release nothing, not with a charged or uncharged base. And the few rest seem to just act like bridges that emit the collection and base charge, again regardless of the bases charge.

    The questions are: How do I know identify which ones are what? And what useful function do the other 2/3 even provide??

    I'm really new to this, so take it easy please. ;)
    Thanks for any help you be able to provide!
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Nov 28, 2009 #2


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    Not all transistors have the same connections as the one in the photo.

    Did you check with a multimeter before you tested them?
    The base wire on a NPN transistor should behave like the anodes of two diodes joined together.
    So if you make this wire positive it should conduct to the other wires which are more negative.
    A PNP will test the opposite way. The base has to be negative relative to the other wires.

    Digital multimeters that have a "diode" function on them are best for this because the normal "ohms" range can have too little voltage on it to turn on diode junctions. Write down the readings you get like this and it may make more sense.

    However, there are lots of devices that come in a TO-92 package. There are even integrated circuits that come like that.

    There could also be FETs, UJTs Darlington pairs, voltage regulators, temperature sensors, Hall probes.
    Who knows? You can't hope to find out with a simple transistor tester.
  4. Nov 30, 2009 #3
    Thanks for your response.
    I didn't know different 'packages' could hold such a variety of things. (Also nice to know their some of their names now)
    Yes, the multimeter's diode mode works just as you said it would for my NPNs.

    I took one my TO-92's that I said was acting like a simple bridge and tested it with the multimeter from all angles. Here are the results from the multimeter diode reading:

    > Current runs in the base, and out the emitter, at 300
    > Current runs in the emitter, and out the base, at 300
    > Current runs in the base, and out the collector, at 1000
    > Current in the collector, and out the base, gets blocked
    > Current runs in the emitter, and out the collector, at 1000
    > Current in the collector, and out the emitter, gets blocked

    I have no idea what this is suppose to do. Does it look familiar to any of you?
  5. Nov 30, 2009 #4


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    You are using the diagram below to get your names for the wires? Fair enough.

    http://quality-deal.com/osCommerce/images/MPSA06%20NPN%2080V%20500mA%20General%20Purpose%20Transistor.jpg [Broken]

    That one seems like a FET. C is the gate and the drain and source could be either of the other two. Possibly P-Channel.
    Last edited by a moderator: May 4, 2017
  6. Nov 30, 2009 #5


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    Staff: Mentor

    It would probably be better if you listed the tests by the pin numbers, and not make assumptions yet about which are EBC. There are 6 ways to choose 2 things from a set of 3, where the order matters. So use your diode check feature to fill out the following table for each of the transistors:

    Code (Text):

    +  -
    1  2
    2  1
    1  3
    3  1
    2  3
    3  2
    Then look to see if you have two diodes identified, with either common cathode (CC, for PNP transistor), or common anode (CA, for NPN transistor). What are the numbers you are seeing for the diode check? Is that forward voltage in millivolts?
  7. Dec 3, 2009 #6
    Wal mart has a great deal on a DMM that has a transistor test function I think I paid something like $8 for mine.
  8. Dec 3, 2009 #7
    I wikied FET, but the article didn't help much. In layman's terms, what does a FET do?

    I'm assuming the leftmost pin in vk6kro's picture is pin 1? Here are the results translated:
    Code (Text):

    +  -
    1  2    300
    2  1    300
    1  3    1000
    3  1    blocked
    2  3    1000
    3  2    blocked
    In terms of other components, there seems to be a diode between pin 1 and 3 and another between pin 2 and 3, and a resistor or something like it between pin 1 and 2.
    But I still don't know what this is.
    I don't know, the meter doesn't say, this mode just has a little diode symbol on it's switch position with nothing else.
    What's a DMM?
  9. Dec 3, 2009 #8
    DMM is an abbreviation for Digital multimeter. One with a transistor test allows you to plug the transistor into the front of the meter and will display the hfe of the transistor (current gain). This is recommended as simply reading the resistance thru the pins may not give much information on the transistor or may not work at all if the voltage on the multimeter may not be high enough to to put the junctions into conduction mode. Here is a good site about the basics of transistors, pin identification, etc. It should help you out a bit.
  10. Dec 3, 2009 #9


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    Science Advisor

    A FET is a field effect transistor. The resistance between the drain and the source (wire 1 and 2) changes depending on the voltage on wire 3, the gate.

    Usually, most circuits use a N-Channel type which requires a positive power supply. This one looks like a P-Channel one which requires a negative power supply. These are rarely used, but some special circuits require them.

    You could test it like this:

    Testing P ch FET.PNG

    If the variable power source (on the left in the diagram) is set to zero volts, the current shown on the meter would be a maximum. As you made the gate more positive, the current in the meter would decrease. If there is enough voltage on wire 3 the current will drop to zero.

    1 and 2 can be reversed and the FET will work better one way than the other. You can try this. Look for the connection that requires least voltage on wire 3 to cut the meter current to zero.
  11. Dec 3, 2009 #10
    Transistors amplify current, for example they can be used to amplify the small output current from a logic IC so that it can operate a lamp, relay or other high current device. In many circuits a resistor is used to convert the changing current to a changing voltage, so the transistor is being used to amplify voltage.

    A transistor may be used as a switch (either fully on with maximum current, or fully off with no current) and as an amplifier (always partly on).
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