A transistor is made up of two PN junctions joined together. A transistor with a PN junction followed by an NP junction is called a PNP transistor, while an NP junction followed by a PN junction results in an NPN transistor.
That is why it is a three-terminal device. The three pins are called Emitter, Base, and Collector.
In an NPN transistor, the Base region or terminal is the p-type region. Whereas in a PNP transistor, the Base terminal is the n-type region. The remaining two terminals are the Emitter and Collector. Given below are the symbols of both transistors.
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Without any tool
The easiest method to find the terminals of a transistor is by visually inspecting the casing. Every transistor has a marking on its surface- the name of the transistor.
The next step is to look at its datasheet for the pinout or search on Google, ‘2N3906 pinout’.
If the transistor name is not visible or not printed on the casing, using a multimeter to identify its terminals is the next best method. But there’s even a better way- using a multi-function tester!
Using Multi-function tester
This is by far the best method to test a transistor and identify its terminals. Why? This device not only tells you about the terminals but also gives you other important parameters such as current gain and base-emitter voltage drop.
To use it, just insert the transistor terminals in the three slots with different numbers and press the button. The device will soon display the parameters as shown below.
How to identify transistor terminals with a multimeter?
Step 1: Prepare your multimeter
Insert the red probe into the mAVΩ port and the black probe into the COM port on your multimeter. Then using the rotary dial, set your multimeter in the diode test mode (denoted by a diode symbol).
Note: Some multimeters have diode mode included with one of the resistance ranges(mostly with a 2K ohm range). Whereas some multimeters have diode and continuity modes together.
It is important to identify the transistor type first: NPN or PNP.
Step 2: Identify whether the transistor is of NPN or PNP type.
- Touch the first leg of the transistor with the red probe. Then, touch the second and third leg with the black probe one by one.
- If you get a voltage between 0.4V and 0.9V in both cases, it is an NPN diode. If you get this voltage in only one position, it is a PNP diode.
- If you get an open loop or ‘OL’ in both cases continue with the following test.
- Move the red probe to the middle leg, and repeat the above steps. If you get a voltage between 0.4V and 0.9V in both cases, it is an NPN diode. If you get this voltage in only one case, it is a PNP diode.
- If you get an open loop or ‘OL’ in this case also, repeat the test by moving the red probe to the third or last leg of the transistor.
Step 3: Identify the base terminal
For NPN transistor:
Touch the first leg of the transistor with the red probe. Then, touch the second and third leg with the black probe one by one and note down the readings.
If the multimeter displays 0.4V to 0.9V in both conditions, the first pin is the base terminal of the transistor.
If the multimeter displays OL in any condition, repeat this with the red probe touching the second( and moving the black probe to terminals 1 and 3) and then the third leg(black probe-1,2).
Whenever the multimeter displays 0.4V to 0.9V in both conditions, the terminal/leg touching the red probe is the base terminal. Since the base is connected to the P region, the two PN junctions will drop voltage only when a +ve supply is given to the base.
For PNP transistor:
Repeat the above procedure(for PNP), but swap the red and black probe conditions, i.e., keep the black probe on a leg and touch the other two with the red one.
Whenever the multimeter displays 0.4V to 0.9V, the terminal touching the black probe is the base terminal. Since the base is connected to the P region, the two PN junctions will drop voltage only when a +ve supply is given to the base.
Step 4: Identify the emitter terminal
Now that we know which terminal is the Base, it’s easy to identify the emitter out of the remaining two.
NPN transistor:
Touch the base terminal with the red probe and touch the two terminals with the black probe one by one. The multimeter will show a voltage drop in both cases. But it will be higher in one case. That terminal is the Emitter, and the other one with a lower voltage drop is the Collector.
PNP transistor:
Touch the base terminal with the black probe and touch the two terminals with the red probe one by one. The multimeter will show a voltage drop in both cases. But it will be higher in one case. That terminal is the Emitter, and the other one with a lower voltage drop is the Collector.
