Newsletter!

How to Test a Transistor Using a Multimeter (DMM + AVO)?

How to Test a Transistor Using a Multimeter (DMM + AVO)? . Transistors are integral components in electronic circuits, acting as switches or amplifiers. Testing a transistor is a crucial skill for technicians and hobbyists to diagnose faults and ensure functionality. Using a digital multimeter (DMM) or an analog volt-ohm meter (AVO), you can effectively test a transistor’s health. This guide explains step-by-step how to test a transistor using these tools.

How to Test a Transistor
How to Test a Transistor

What is a Transistor?

A transistor is a semiconductor device used to amplify or switch electronic signals. Its functionality is determined by three terminals: base, collector, and emitter. Transistors are categorized into:

  • Bipolar Junction Transistors (BJTs): NPN and PNP types.
  • Field Effect Transistors (FETs): MOSFETs and JFETs.

In testing, we focus primarily on BJTs due to their widespread use.

Read More : What Is the Difference Between Analog and Digital Multimeter?

How to Remember the direction of PNP & NPN Transistor?

How-to-remember-the-direction-of-PNP-and-NPN-Transistor-Pin-Identification-Check-if-it-is-Good-or-Bad.
How-to-remember-the-direction-of-PNP-and-NPN-Transistor-Pin-Identification-Check-if-it-is-Good-or-Bad.
  • PNP = Pointed In
  • NPN = Not Pointed In.

If you think that is a little bit complex, then try the more simple one as follows.

Remember-the-direction-of-PNP-NPN-Transistor
Remember-the-direction-of-PNP-NPN-Transistor

PNP  NPN

  • P = Points                      N = Never
  • N = IN                             P = Points
  • P = Permanently           N = iN

Read More : What is a Multimeter? How Analog and Digital Multimeters Work

Tools Needed for Testing

To test a transistor, you’ll require the following:

  1. Digital Multimeter (DMM): Provides accurate readings with a diode test mode.
  2. Analog Volt-Ohm Meter (AVO): Measures resistance for older systems.
  3. Probes: Red (positive) and black (negative).
  4. Protective Equipment: Gloves and anti-static wrist straps.

Types of Multimeter Tests

Diode Test Mode

This mode helps measure voltage drop across junctions.

Resistance Test Mode

Measures ohmic resistance between terminals to determine continuity.

How to Test a Transistor Using a Multimeter (DMM + AVO)?

Test a Transistor using Digital Multimeter in Diode or Continuity Mode

To do so, follow the instructions given below.

  1. Remove the transistor from the circuit i.e. disconnect the power supply across the transistor which has to be tested. Discharge all the capacitors (by shorting the capacitor leads) in the circuit (If any).
  2. Set the meter on “Diode Test” Mode by turning the rotary switch of the multimeter.
  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 below). You have to perform 6 tests by connecting the Black (-Ve) test lead and Red (+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 are Red Test Lead and numbers in Black are connected to Black (-Ve) test lead of multimeter.
  4. Test, measure and note the display reading shown in the multimeter in the table below.
Check-Transistor-with-Digital-Multimeter-in-Diode-or-Continuity-Mode
Check-Transistor-with-Digital-Multimeter-in-Diode-or-Continuity-Mode

How to Test a Transistor Using a Multimeter (DMM + AVO)?

Read More : How to Test a Diode Using Digital and Analog Multimeters?

We have the following data from the table given below.

Out of 6 tests, we got data and results only on two tests i.e. points 2 to 1 and 2 to 3. Where we got at points 2 to 1 is 0.733 VDC and 2 to 3 0.728 VDC. Now, we can easily find the type of transistor as well as their collector, base and emitter.

  1. Point 2 is Transistor Base in BC55 Transistor.
  2. BC 557 is a PNP Transistor where the 2nd (middle terminal is base) connected to Red (+Ve) test lead of the multimeter.
  3. At all, Terminal 1 = Emitter, Terminal 2 = Base, and Terminal 3 = Collector (BC 557 PNP Transistor) because, the test result for 2-1 = 0.733 VDC and 2-3 = 0.728 VDC, i.e. 2-1 > 2-3.
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.

BC-547-NPN-and-BC557-PNP-Transistor-Checking
BC-547-NPN-and-BC557-PNP-Transistor-Checking

Read More : How to Measure Voltage using Digital and Analog Multimeter?

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.
Check-Transistor-with-Analog-or-Digital-Multimeter-in-Ohm-Ω-Range-Mode
Check-Transistor-with-Analog-or-Digital-Multimeter-in-Ohm-Ω-Range-Mode

Read More : How to Measure Current Using Multimeter: The Ultimate Guide

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.

Understanding Transistor Terminals

Knowing the roles of the terminals is essential for accurate testing:

  • Base (B): Acts as the control terminal.
  • Emitter (E): Emits carriers (electrons or holes).
  • Collector (C): Collects carriers from the emitter.

Transistor pin configurations differ between models, so refer to the datasheet for guidance.

Read More : How to Test Grounding / Earthing System Using a Multimeter: A Step-by-Step Guide

Testing NPN Transistors

Step-by-Step Instructions:

  1. Identify the Terminals: Determine base, collector, and emitter using the datasheet.
  2. Test Base-Emitter Junction:
    • Connect the red probe to the base and the black probe to the emitter.
    • A reading of 0.6–0.7V indicates the junction is functioning.
  3. Test Base-Collector Junction:
    • Connect the red probe to the base and the black probe to the collector.
    • Expect a similar reading of 0.6–0.7V.
  4. Check Reverse Bias:
    • Reverse the probes. No reading should appear if the transistor is good.

Testing PNP Transistors

How to Perform Accurate Tests:

  1. Test Base-Emitter Junction:
    • Connect the black probe to the base and the red probe to the emitter.
    • Expect a reading of 0.6–0.7V.
  2. Test Base-Collector Junction:
    • Connect the black probe to the base and the red probe to the collector.
    • Similar readings confirm proper functioning.
  3. Reverse Bias Test:
    • Reverse the probes. No reading should appear.

Identifying Common Faults

Open Circuit:

Occurs when no readings are displayed in both forward and reverse bias tests.

Short Circuit:

A reading of 0V across terminals indicates a shorted transistor.

Leakage Issues:

Low resistance in reverse bias suggests leakage.

How to Test a Transistor Using a Multimeter (DMM + AVO)?

Read More : Difference Between Schottky Diode and Shockley Diode

Testing Transistors in Circuits

Testing transistors while they are part of a circuit can be tricky due to interference from surrounding components.

Steps for Live Testing:

  1. Identify the Transistor’s Role: Locate its position in the circuit.
  2. Perform Voltage Checks: Measure voltage drop across terminals.
  3. Compare with Specifications: Match results to the manufacturer’s datasheet.

Frequently Asked Questions (FAQs)

Q1. Can I test a transistor without removing it from the circuit?

Yes, but ensure the circuit is powered off to avoid interference from parallel components.

Q2. What does a 0V reading indicate?

It typically means the transistor is shorted.

Q3. What if both forward and reverse bias tests show continuity?

This suggests the transistor has failed and should be replaced.

Related Topics
How to Test a Transistor Using a Multimeter (DMM + AVO)?
Engineering Reference
Subscribe to our

Newsletter

We will update you with all the latest books and references.