I am trying to figure out what is going on here.

I have a simple circuit to send current to a brushless fan running at 12V on the high side, using a NPN transistor 2N2222A:


This is the picture of the datasheet for the pins:


If I set up the circuit for a quick test, it seems that the datasheet (or the transistor) has the pins swapped.

When I apply a voltage between the pin 1 to 3, as soon as the voltage rises over about 8V, the current starts to flow, even when the base has no voltage (pulled down or floating), and the transistors gets pretty hot. It looks like a "diode".

If I rotate the transistor 180°, everything works as I would expect, current flows, and the fan power is proportional to the base voltage.

This is the pic of the transistor, as I understand that it should work, but it doesn't (it is always 'on' with 12V between the collector and the emitter with the base floating).


What do you think? Am I missing something here?

  • \$\begingroup\$ If it is hot throw it away (or test it VERY well before relying on it). You probably burnt it. \$\endgroup\$
    – Gravity
    Jan 16, 2019 at 9:03

3 Answers 3


There really isn't a single '2N2222' transistor, since each manufacturer just seems to use XYZ_2N2222 as a part number for a generic 'NPN silicon amplifier transistor'.

Hence the 2N2222 pin-out is sadly not standardized. Most 2N2222s have the orientation which your diagram shows, where if the flat side is facing you, the pins are E-B-C respectively. However, some manufacturers have the pins going C-B-E for the TO-92 package. For example, the ON Semi P2N2222A variant: datasheet

enter image description here

  • 9
    \$\begingroup\$ With it wired the wrong way round the emitter is more positive than the collector and the base is reverse biased. Absolute maximum rating for emitter-base voltage is 6V hence the E-B junction broke down and started conducting. Would I reuse it? I don't think so - throw it in the bin. \$\endgroup\$
    – Andy aka
    May 6, 2014 at 16:35
  • 1
    \$\begingroup\$ oh my god! this is awful :/ i didn't check other manufacturers \$\endgroup\$ May 6, 2014 at 19:36
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    \$\begingroup\$ It can be even worse, since the On Semi PN2222 has the conventional pinout, despite being the same manufacturer and mostly same specifications: onsemi.com/pub_link/Collateral/PN2222-D.PDF (note the difference in part number, PN2222 vs P2N2222) \$\endgroup\$
    – Zuofu
    May 6, 2014 at 23:45

You can determine the pinout and NPN/PNP of an unknown BJT with the diode function of a digital multimeter in seconds.

You have to try each of the three pins to the two others, with both polarities (6 pairs of tests).

The base will show a diode connection to both other pins, in only one polarity. No other pin will show that. If the red test lead is on the base, then it is NPN, if the black is on the base, then it is PNP. Now you have identified the base pin and whether it is PNP or NPN.

To distinguish collector from emitter, you look at the voltage shown in the diode function with the one successful test that found the base. Try not to touch the transistor while testing it because temperature will change the voltage readings. The slightly lower of the two voltages will correspond to the collector-base junction, and the other will be the emitter-base junction. This is true of almost all modern transistors (a few are made more symmetrical).

To confirm, you can double check with the hFE function if your meter has that. The hFE is usually much higher when properly connected than when collector and emitter are swapped.

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    \$\begingroup\$ Thank you for this terrific post. I wish that SE had a method of bookmarking answers, not just questions. \$\endgroup\$
    – dotancohen
    Oct 4, 2020 at 13:30

If it is to replace a shorted transistor, we will never know if both are the same ebc configuration between your new 2N2222 and the old 2N2222. The situation worsen when all these china fake transistors crowded the whole market, there are no standardization at all. You can have up to 3 types of configurations.


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