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I need to know how to distinguish between an NPN transistor and a PNP transistor. If I pluck any transistor from a radio, how can I know its nature using a digital multimeter?

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  • \$\begingroup\$ electricalbasicprojects.com/… but be careful; the pin order of collector, base and emitter are not always as shown. \$\endgroup\$ – Peter Smith Apr 17 '16 at 13:21
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    \$\begingroup\$ Your first move should always be to read the part number from the transistor (most will be marked) and to use the internet to try to find a datasheet for the transistor. That will tell you much more than you could possibly test with a multimeter. Note that Japanese-origin transistor typically omit a '2S' in front of the part number, thus a C1815Y is a 2SC1815 with a gain of 'Y'. You should search for 2SC1815 + datasheet. \$\endgroup\$ – Spehro Pefhany Apr 17 '16 at 13:31
  • \$\begingroup\$ If you can't find a datasheet, at least note the area from which you 'plucked' it. If it came from the RF section of an FM radio it will likely have different characteristics than if you removed it from the audio amplifier portion. \$\endgroup\$ – Spehro Pefhany Apr 17 '16 at 15:57
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This is pretty easy. I used to do this routinely in high school when salvaging parts from unknown discarded boards.

As Spehro said in a comment, sometimes you can find a part number. In that case, you can find the datasheet and get the parameters outright. However, all too often there is no manufacturer's part number, or its just a short code, or its a in-house number. Especially with the smaller packages, you're going to have to experiment.

First, make sure the ohmmeter is not set to some extra low voltage mode intended to not forward bias diodes. Some meters have such a feature. In this case, you definitely want to forward bias the junctions.

A bipolar transistor has only three leads, so only 6 possible two-wire measurements when taking polarity into account. From the view of probing with a two-wire ohmmeter, a bipolar transistor looks like two diodes back to back. There is one B-E and one B-C. In a NPN, it takes positive voltage on the base relative to E or C to make the diodes conduct, and the other way around with a PNP. The "N" and "P" in the names tells you the voltages required to make the diodes conduct.

Figuring out whether you have a NPN or PNP and which lead is the base is therefore easy. The next problem is to figure out which are the C and E leads. On most packages, C is in the middle. On a power package, C is usually connected to the case or tab or whatever.

Another way to test for C versus E is to measure the gain. A transistor will still work with C and E flipped, but the gain will be higher when connected as intended. I usually did this by connecting the meter across C-E. With the base floating, there should be no current so the meter should read infinite resistance. Now use your fingers to bridge C and B. You should see a lower resistance than if you used your fingers to bridge C and E. That apparent lower resistance is due to the transistor amplifying the base current.

Now run the same test with C-E flipped. Most of the time, one orientation has a obvious higher gain. If not, then you can run this test with a real resistor instead of your fingers.

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    \$\begingroup\$ +1 Another way to discern C from E is to measure in diode mode. The BC junction will usually have a bit (maybe 1%) less voltage drop than the BE junction. A few transistors are made to be symmetrical (in gain too) but they are pretty rare. \$\endgroup\$ – Spehro Pefhany Apr 17 '16 at 15:54
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Honestly it is better to type up the first line of numbers/letters you see on the front face of the transistor, add the word "datasheet" and search it up on google (and try to go for the PDF results). You will able to determine if it is PNP, NPN, some other transistor like a mosfet, or a miscellaneous chip. You can also know all their characteristics.

If you don't have Internet and want to test if it is NPN or PNP, set your multimeter mode to diode mode (should have the image of an arrow with a line at the end). Using the two probes, probe the three legs all 6 possible ways (because two probes cannot probe the same pin) and put results as a table on paper with one probe on the same pin on the same row. If your result comes out like this (not in order):

  • Not a number above zero, Not a number above zero
  • Not a number above zero, Not a number above zero
  • A number above zero, A number above zero

Then you got a NPN transistor. Else if your result comes out like this (not in order again):

  • Not a number above zero, A number above zero
  • Not a number above zero, A number above zero
  • Not a number above zero, Not a number above zero

Then you got a PNP transistor.

If your results do not match these, then you probably don't have either a PNP or NPN transistor as there exist many other types of transistors, and some basic computer chips can be made to look like a 3 pin transistor (voltage regulators, Hall effect switches, AM radio IC, temperature sensor).

If you don't want to use the Internet to determine the properties of a transistor, you could buy a Peak Atlas DCA55 semiconductor analyser, but I doubt that's necessary at this stage. (But it's good for determining if the transistor is fake if you compare it with the datasheet online)

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Note that because of the marvel of modern, inexpensive microcontrollers, and clever open-source software, there are small, inexpensive pieces of "test gear" which will identify and measure a wide variety of transistors, NPN, PNP, BJT, FET, etc. and thyristors SCR, Triac, etc, resistors, capacitors, inductors, etc.

There are perhaps two dozen slightly different implementations of this design available on Ebay at any given moment with prices around US$15-25 The more sophisticated ones have graphic display screens that draw the symbol of the device and identify the pins (Emitter, Base, Collector, or Source. Gate, Drain, etc.)

Here is JUST ONE example of these gadgets. There are many variations avaiable with nearly identical features and similar prices...

http://www.ebay.com/itm/2015-12864-LCD-Transistor-Tester-meter-Diode-Triode-Capacitance-LCR-ESR-Meter-/271839788857

enter image description here

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first thing find any number on the transistor this number is the key of transistors with the help of datasheet or the internet, ie you can find the type of transistor and the characteristics of it

secend if you don't find any number on the transistor you can make this circuit to find the kind of the transistor, but befor that you shoult know the Transistor Package Styles chek this link

http://www.interfacebus.com/semiconductor-transistor-packages.html

than you can build your circuit and find the type of your transistor wither is NPN or PNP

1st: NPN transistor

enter image description here

2nd: PNP transistor

enter link description here

and this one

enter image description here

i hope this can help you

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https://www.youtube.com/watch?v=2vk7JS-0ir0 use this vedio u can get full vision about testing the npn and pnp transistors

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    \$\begingroup\$ Answers with (mostly) only a link are often considered low quality. While youtube links are usually long-lived as long as they are unique content, it'd be a more helpful answer to the world at large and in the long run if you added some explanation. For those who cannot watch youtube, have no audio, etc. \$\endgroup\$ – Asmyldof Apr 17 '16 at 17:04
  • \$\begingroup\$ add some resume of the method in the video will be good idea \$\endgroup\$ – m salim Feb 5 '17 at 10:16

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