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How to Identify NPN and PNP Transistor using Multimeter

Published on April 08, 2016 By
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Bipolar Junction Transistors (BJTs) and you don’t know whether they are NPN or PNP  type transistors… (Almost everyone would have face this problem)
In this post, we will discuss about how to find out the NPN and PNP transistor using multimeter…
Before proceeding further, let us refresh about how to identify the transistor terminals.
Identifying BJT Terminals:
We know that the Bipolar junction transistor has three terminals namely 
  1. Emitter (E)
  2. Base (B)
  3. Collector(C)
The transistors are available with various packages in the market. Let us discuss about the TO-92 package.
Keep the transistor such that the flat surface facing towards you as shown in the below figure:
BJT_Terminal_no_identificationNow starting from left, mark like 1,2 and 3. They are respectively 
  1. Emitter (E)
  2. Base (B)
  3. Collector(C)
The schematic symbol of the BJT is given below:
BJT_Schematic_SymbolIdentifying BJT Types:
Both NPN and PNP transistor looks similar in physical appearance. We can not differentiate by seeing them. We need a multimeter to identify the type of BJT.
Remember the following points:
  1. The transistor internally has  two diodes (NPN ≡ N – P – N ≡ NP Junction + PN Junction and PNP ≡ P – N – P ≡ PN Junction + NP Junction).
    ie,Emitter to  base is one PN junction ( diode) and Base to collector another PN junction (diode).
  2. In the diode mode, the multimeter will show the voltage when we keep the positive probe of the multimeter to the anode of the diode and negative probe  to the cathode.
  3. If the multimeter positive probe is connected to the cathode of the diode and the negative probe to the anode, then it will not give any voltage (showing zero).
Steps to identify the NPN type transistor:
  1. Keep the Multimeter in the Diode mode.
  2. Keep the positive probe to the center pin (Base) of the transistor. 
  3. Touch the negative probe to the pin-1 (Emitter). You will see some voltage in the multimeter.
  4. Similarly touch the negative probe to the pin-3 (collector) with respect to the pin-2. You will see some voltage in the multimeter.
  5. It will ensure that it is a NPN transistor. The logic behind this is, in NPN transistor
    Emitter (E) – N type material – Equivalent to cathode of the diode
    Base (B) – P type material – Equivalent to anode of the diode
    Collector(C) – N type material – Equivalent to cathode of the diode
  6. If the multimeter positive probe is connected to anode and negative probe is connected to cathode, then it will show voltage. If the connections are interchanged it will not show any value.
Steps to identify the PNP type transistor:
  1. Keep the Multimeter in the Diode mode.
  2. Keep the positive probe to the pin-1 (Emitter) of the transistor. 
  3. Touch the negative probe to the center pin (Base). You will see some voltage in the multimeter.
  4. Similarly touch the negative probe to the center pin (Base) with respect to the pin-3 (collector). You will see some voltage in the multimeter.
  5. It will ensure that it is a PNP transistor. The logic behind this is, in PNP transistor 
    Emitter (E) – P type material – Equivalent to anode of the diode
    Base (B) – N type material – Equivalent to cathode of the diode
    Collector(C) – P type material – Equivalent to anode of the diode
  6. If the multimeter positive probe is connected to anode and negative probe is connected to cathode, then it will show voltage. If the connections are interchanged it will not show any value.
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