How To Test A Transistor Using Multimeter (DMM+AVO) - 4 Ways

BC 557 PNP	Measuring Points	Result
	1-2	OL
	1-3	OL
	2-1	0.733 VDC
	2-3	0.728 VDC
	3-1	OL
	3-2	OL

Finding BASE of Transistor:

As mentioned in the above tutorial, the common number found in the tests above is base. In our case, 2nd terminal is Base and 2 is common out of 1-2 and 2-3.

2nd Method using DMM to find the Base of the Transistor.

If you follow the same pattern and connecting method of multimeter leads and transistor terminals one by one in the figure shown above, in fig “c” and “d” , The Red (+Ve) test lead is connected to the middle one i.e. 2nd terminal of lead and the Black (-Ve) test lead is connected to the 1st one terminal of transistor.

Again, The Red (+Ve) test lead is connected to the middle one i.e. 2nd terminal of lead and the Black (-Ve) test lead is connected to the 3rd one terminal of transistor and multimeter shows some reading i.e. 0.717 VDC & 0.711 VDC respectively in the case of BC 547 NPN.

The common lead is 2nd one connected to Red (+Ve) test lead (i.e. P and yes, the other two leads are N) which is base. The case is reversed in the case of the BC 557 PNP transistor.

NPN or PNP?

It’s simple. If the Black (-Ve) test lead of the multimeter is connected to the base of the transistor (2nd terminal in our case), then it is PNP transistor, and when Red (+Ve) test lead is connected to the base of the terminal, It is NPN transistor.

Related Posts:

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Emitter or Collector?

EB (Emitter – Base) forward bias is greater than CB (Collector – Base) i.e. EB > CB in PNP Transistor e.g. BC557 NPN. Hence, It is a PNP Type Resistor. In NPN Transistor, BE (Base – Emitter) forward bias is greater than BC (Base – Collector) i.e. BE > BC, e.g. BC 547 PNP.

Here is the conclusion.

1. Point 2 is Transistor Base in BC547 Transistor
2. BC 547 is a NPN Transistor where the 2nd (middle terminal is base) is connected to Red (+Ve) test lead of multimeter.
3. At all, Terminal 1 = Emitter, Terminal 2 = Base, and Terminal 3 = Collector (BC 547 NPN Transistor) because, the test result for 1-2 = 0.717 VDC and 2-3 = 0.711 VDC, i.e. 1-2 > 2-3.

BC 547 NPN	Measuring Points	Result
	1-2	0.717 VDC
	1-2	OL
	1-3	OL
	1-3	OL
	2-3	OL
	2-3	0.711 VDC

Check a Transistor using Analog or Digital Multimeter in Ohm (Ω) Range Mode:

Steps:

1. Disconnect the power supply to the circuit and remove the transistor from the circuit.
2. Rotate the selector switch and put the Multimeter knob in Ohm Range (OHM)
3. Connect the Black (common or -Ve) test lead of the multimeter to the 1st terminal of the transistor and Red (+Ve) test lead to the 2nd terminal (Fig 1 (a). (You have to perform 6 tests by connecting the Black (-Ve) test lead to 1 to 2, 1 to 3, 2 to 1, 2 to 3, 3 to 1, 3 to 2 respectively by just replacing the multimeter test leads or reverse the transistor terminals to connect, test, measure and note the reading in the table(shown below). (Numbers in Red colors show the transistor leads connected to the Red (+Ve) test lead of multimeter and the numbers in black colors show the transistor leads connected to the Black (-Ve) test lead of multimeter. (Better explanation in the table & fig below)
4. If the multimeter shows high resistance in both first and second tests by changing the polarity either of transistor or multimeter, as shown in Fig 1 (a) and (b). (Note that, the result will be shown only for 2 tests out of 6 as mentioned above). i.e.  In our case, the 2nd terminal of the transistor is BASE, because it shows high resistance in both tests of 2 to 3 and 3 to 2 where Red (+Ve) test lead of the multimeter is connected to 2nd terminal of the transistor. In other words, the common number in tests is Base which is 2 out of 1, 2 and 3.

Click image to enlarge

PNP or NPN ?

Now, it is a NPN transistor because, it shows reading only when the RED (+Ve) test lead (i.e. P terminal where P = Positive) is connected to the Base of the transistor (See fig below). If you do the reverse, i.e. Black (-Ve) test lead (i.e. N = where N = Negative) of multimeter connected to the transistor terminal in sequence of (1 to 2 and 2 to 3) and shows reading in both tests as above, The 2nd Terminal is still BASE, but the transistor is PNP ( see fig below).

Testing a Transistor using Digital Multimeter in the Transistor or hFE or Beta Mode

hFE also known as beta is dc gain stands for “Hybrid parameter forward current gain, common emitter” used to measure the hFE of a transistor which can be found by the following formula.

hFE = βDC = IC / IB

It also can be used to check a transistor and its pin terminal as shown in fig 1.

To check a transistor in hFE mode, there are 8 pins slot in the multimeter indicated by PNP and NPN as well as E C B (Emitter, Collector and Base). Simply put the three pins of transistor in the multimeter slot one by one in different slots i.e. ECB or CBE (Rotary knob should on hFE mode).

If they display reading (It would be the hFE reading of transistor) , In our example, We used BC548 Transistor which shows the beta value of 368 (CBE position) the current position on the C, B, E slot are the exact terminals of transistor i.e. collector, base and emitter) and transistor is in good position, otherwise, replace with new one.

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