How to test a transistor - the easy way

how to test a transistor 2n3906

This is the quickest and easiest way to test a transistor. There is no messing around with pin out diagrams or identifying the base, collector and emitter. No messing around with a test meter and trying to hold one lead on one connection while touching another.

If you want to know how to test a transistor with a multimeter I’ve also shown that method later in the article.

The easiest way is to use this device. DEOK Multifunction Transistor Tester Mega328 NPN/PNP Transistor Checker Capacitor ESR SCR/MOSFET/Resistor/Diodes Meter LCD Display (Small).

This is the best device I have ever bought for my hobby of constructing electronic projects. It’s also one of the cheapest at less than twenty pounds.

I bought it as a kit but you can also buy them ready built. You can read about the kit version here. It’s not particularly difficult and can be assembled in a few minutes with careful soldering.

What you end up with whether you build it yourself or buy it readymade is this.

                             

                        

                                

     

There are some versions of this around with three screw terminals to connect up to. I prefer the zero insertion force socket version just because it’s easier to use.

The zero insertion force socket has numbered terminals around it as shown in the picture below.

how to test a transistor zero insertion force socket 1
how to test a transistor zero insertion force socket 2

It doesn’t matter which terminals you connect to. Just make sure you connect each transistor leg to a 1, 2 and 3 connection. The tester will do the rest and identify the terminals for you as well as test and tell you what the transistor is. It will indicate whether the device is PNP or NPN, the emitter threshold voltage and the current amplification factor.

Just drop the transistor in and throw the leaver. Then just press the test button. It’s as simple as that.

how to test a transistor pnp

Here you can see the tester with a 2N3906 PNP transistor. With this device it's easy just to drop it into the top right of the socket as there are three terminals 1,2 ans 3 next to each other. As you can see the device is working and has been identified as a PNP transistor with a pin out of 1 E 2 B 3 C. The "B=284" on the second line of the display is the current amplification factor or gain as it is more commonly known. I think the "B" is used as it's also the the Greek letter B or beta. The other number "677mV" is the emitter threshold voltage.

how to test a transistor npn

Here you can see the tester with a 2N3904 NPN transistor. It has identifies the pin out as 1 E 2 B 3 C. Just to prove that it doesn't care which legs connect where I've turned the device around and retested it.

how to test a transistor npn reversed

As you can see the pin out now reads 1 C 2 B 3 E.

Here it is testing a 2N3819, a common N-channel JFET.

how to test a transistor 2N3819

The display show that it is a N type JFET with a pin out of 1 source 2 gate a and 3 drain. the other numbers show the ​gate capacitance and the gate threshold voltage.

How to test a transistor with a multimeter

As you can see it doesn’t get much easier than that however if you searching for how to test a transistor and don’t have that piece of kit you can do it with a multimeter with a diode test. Most multimeters have this function.

You will need to know a few things before you begin.

1 make sure you know what the device is. NPN is the more common the other type is a PNP. The easiest way is to look at the number on the device and look it up on the internet.

2 you will also need to know the pin out of the device. That’s which legs are the base collector and the emitter. The easiest way is again to look it up on the internet.

3. Once you have the pin out make a drawing of it. It will make identifying the leads much easier when you’re testing.

how to test a transistor pin out diagram

You need to know the pin out if you're testing a transistor with a multimeter.

Set your multimeter to the diode range. It will look something like below.

how to test a transistor diode range

Testing a NPN transistor

For our testing purposes we are testing the transistor as if it were 2 diodes as shown in the picture below. You may have heard of this analogy before.

how to test a transistor npn diodes

Make sure you have the leads connected correctly on your meter. I have seen people with the red lead connected to the black terminal.

1. Connect the red positive lead to the base of the transistor.

2. Touch the black negative lead on the emitter and you should get an open circuit reading.

3. Touch the black negative lead on the collector and you should get an open circuit reading.

An open circuit will read like this the same is if it’s not connected to anything open circuit that’s like this picture.

how to test a transistor open circuit

4. Now connect the black negative lead to the base of the transistor.

5. Touch the red positive lead on the emitter and this time you should get a reading.

6. Touch the red positive lead on the collector and you should also get a reading.

By reading I mean something like .740 as shown in the picture below.

how to test a transistor with a multimeter circuit

The final check is to connect your meter probes across the collector and emitter. This should also read as an open circuit either way the leads are connected.

Testing a PNP transistor 

how to test a transistor pnp diodes

Once again make sure you have the leads connected correctly on your meter.

1. Connect the black negative lead to the base of the transistor.

2. Touch the red positive lead on the emitter and you should get an open circuit reading.

3. Touch the red positive lead on the collector and you should get an open circuit reading.

An open circuit will read like it's the same is if it’s not connected to anything open circuit that’s like this picture.

how to test a transistor with a mutimeter open circuit

4. Now connect the red positive lead to the base of the transistor.

5. Touch the black negative lead on the emitter and this time you should get a reading.

6. Touch the black negative on the collector and you should also get a reading.

By reading I mean something like .740 as shown in the picture below.

how to test a transistor with a multimeter

The final check is to connect your meter probes across the collector and emitter. This should also read as an open circuit either way the leads are connected.

The biggest problem I have in using this method is that of trying to hold the multimeter probes steady while checking the readings. I have found it much easier to use the miniature crocodile clips and clip them one. It also helps to use the insulated ones to avoid shorting the legs out while testing.

how to test a transistor crocodile clips

I’m sure you’ll agree it’s not as easy as using the transistor tester.