How to Test a Transistor Using Analog and Digital Multimeter (whether it is PNP or NPN)

This post is about troubleshooting the unknown transistor.It helps you to identify a transistor whether it is PNP or NPN. It is very easy to identify. Here, I will discuss the procedure how to do that.

A transistor is actually like the figures below,

Fig(a) 

Fig(b)

Fig(c)

Fig(a) & (b) are for PNP and NPN transistor respectively. The Transistors are actually the two diodes connected together like the above figures. It's almost like a diode test. For PNP transistor, if you connect the multimeter positive to the base and negative to emitter and collector separately, you will see a reading of resistance. And for NPN if you connect a the multimeter negative to base and positive to emitter and collector separately, you will see a reading of resistance. Now for testing the transistor follow the step below,

1.  Take your multimeter (Digital or Analog) and set it to resistance measuring mode or continuity test mode at a medium range.
2. Take your Transistor and mark the pin 1, 2 & 3 as you like. This is only for your notation to describe or identify. See figure (c).
3. Now Just connect as below and observe the readings.

          a). Multimeter pos (+) to 1 & Neg (-) to 2;

          b). Multimeter pos (+) to 1 & Neg (-) to 3;

          c). Multimeter pos (+) to 2 & Neg (-) to 1;

          d). Multimeter pos (+) to 2 & Neg (-) to 3;

          e). Multimeter pos (+) to 3 & Neg (-) to 1;

          f). Multimeter pos (+) to 3 & Neg (-) to 2.

Decision:

If you get any positive data for above a & b or, c & d or, e & f observations from step 3, make sure that your transistor is a PNP transistor.Because, with a PNP transistor the base will have two diodes facing towards it.On the other hand, if you get any positive data for above a & f or, b & d or, c & e from step 3, make sure that your transistor is a NPN transistor. Because, with an NPN transistor the base will have two diodes facing away from it.

The above procedure is based on using a ohmmeter. If you have a multimeter with diode test mode. Then another criteria can be done. Set your multimeter to diode test mode. connect as below,

(i) Pin 1 to Positive & Pin 2 to Negative;
(ii) Pin 2 to Positive & Pin 1 to Negative;
(iii) Pin 1 to Positive & Pin 3 to Negative;
(iv) Pin 3 to Positive & Pin 1 to Negative;
(v) Pin 2 to Positive & Pin 3 to Negative;
(vi) Pin 3 to Positive & Pin 2 to Negative.

Observe the readigs. You can see the readings as volt. Since there is two diode on a transistor, the voltage you can see approximately 0.7 V ( Silicon) or 0.3 V (Germanium). If you get values for observations (i) & (iii) or, (ii) & (v) or, (iv) & (vi), make sure that your transistor is a PNP transistor.

If you get values for observations (i) & (vi) or, (ii) & (iv) or, (iii) & (v), make sure that your transistor is an NPN transistor.