Find the collector, base and emitter leads of a transistor . . . and if it is PNP or NPN . . . in three simple steps.

Types: 2N... 2SA... 2SB... 2SC... 2SD... 2SJ... 2SK...

Types: 3SK... AC... BC... BD... BF... BG... BL... BS... BU... D... G... H...        I... to ......Z

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How many times have you picked up a transistor and wondered which is the COLLECTOR, BASE and EMITTER lead? This chapter explains how you can identify the leads of PNP and NPN transistors. 
There are 3 steps and you only need an "old-style" multimeter 
to help with the test. It's really very simple but we will be expanding the discussion to help the beginner. 
Answer the set of questions at the end and the "computer" will give you a score.

There are no "fixed" pinouts for the leads of some transistors. But in general there is a "common" pinout for each style of case. Most technical data sheets include a pinout diagram but if the transistor is unmarked or 
unknown, what do you do?
Simply follow our 3-step approach:

Firstly, the transistors we are talking about are "ordinary" transistors.
The technical name is Bipolar Junction Transistor  (BJT) (other types of transistors are: Field Effect Transistors, Uni-Junction Transistors, 
and others).
"Ordinary" transistors have three leads:

and come in many different styles and cases. 
Here are a few of the packages, including surface mount:

Note: A small "plastic" case (such as TO-92) generally means the transistor is a low current (called "small signal") device.
A flat pack, such as TO-126 and TO-220, indicates the transistor is a medium current device and 
a "high-hat" or metal can device, such as TO-3 and TO-66, (TO-66 is a small version of TO-3) indicates the transistor is a high current device.
Each style has a case-number called a "JEDEC number" as shown above (TO-3 etc) but this is not the important topic. We are interested in finding  the COLLECTOR, BASE and EMITTER leads. 

All PNP transistors have the same circuit symbol:

PNP symbol

This symbol does not indicate if the transistor is small-signal, high power or the type of package. It just indicates the transistor is PNP. Note the arrow on the emitter is pointing to the base. This is how to remember the symbol. I have shown it this way as the emitter of a PNP normally goes to the positive rail and this is how it will appear in a circuit. 

All NPN transistors have the following symbol:

NPN symbol

The arrow on the emitter points away from the base. 
NPN transistors are the most popular type. In the early days of manufacture, it was easier to make NPN transistors. The voltage and current capability could be made higher. This made them cheaper and most circuits were designed around NPN types. 
In both cases, the arrow points in the direction of current flow (current flows from positive to negative - high-technology instructors like to talk about electron flow - from negative to positive - but this just makes things more complicated). Let's keep things simple.    Luckily, the arrow points in the direction of current flow!!!

A simple transistor tester is a multimeter - the "old style" analogue type 
with the moving needle (pointer). For this test, the multimeter is firstly switched to the HIGH-OHMS RANGE. The high-Ohms range is used so that you will be able to pick up a leaky transistor at the same time. More about this later. 
Before we discuss the multimeter, we need to know how a multimeter 
"sees" a transistor. It "sees" it as two back-to-back diodes.  For the PNP transistor the cathodes are connected to the base and for the NPN the anodes are connected to the base, as shown in the diagram below:

Some digital multimeters may work as a transistor tester (mine does not work) but others will not detect the forward voltage drop of a diode because the voltage delivered by the meter is below 0.7v and the diode is not placed in forward conductivity. Some digital multimeters have a transistor tester built into them but the holes for the leads on the front of the multimeter are so fine that you need to add extension-leads to test the larger transistors!

Every multimeter has one or more Ohms ranges. The lowest range is called the "Ohms Range" as the scale on the meter is read directly. For instance, "500" on the scale is 500 Ohms (500R). The other range is the x1k range. "500" on the scale is read 500k. This is the setting we use for the tests. 
Inside the multimeter is a battery (1.5v or 3v) and this provides the energy to move the needle. One very important point to note is the red probe of a multimeter is connected to negative of the battery (inside the multimeter) and the black probe is connected to the positive of the battery (via a set of resistors and the meter-movement itself).
When the black probe is connected to the anode of a diode and the red probe to the cathode, as shown in the animation below, the needle moves about 90% across the dial. (It does not move fully across because the multimeter is actually detecting the voltage-drop of 0.7v of the diode and not its actual resistance - but this is a technical point we will discuss later). 
When the red probe is connected to the anode and the black probe to the cathode, the needle does not move at all. 
In the first case the diode is forward biased and current flows. In the second case the diode is reverse biased and no current flows. The pointer (needle) clearly indicates these two states. These are the two conditions we need to remember. 

Click = mouseover
Note: the multimeter is on "x1k"  scale

Now that we know how a multimeter reacts to a diode in forward and reverse bias, we can test a transistor and determine the base lead. 

All you have to do is place the black probe on any lead of a transistor. Then place the red probe on each of the other leads. If the needle moves across the dial, the transistor is NPN. If the needle moves for only one test, try the black probe on another lead. This may take up to 6 tests to get a final answer. See animation below:

Note: the multimeter is on "x1k"  scale

If the pointer doesn't move twice: 

Place the red probe on any lead and repeat the above. When the needle moves for both the other leads, the transistor is PNP. See animation below:

Note: the multimeter is on "x1k"  scale

If the needle does not move for the two other leads, the transistor is faulty. It is "OPEN." 
If the needle moves for ALL tests, the transistor is faulty. It is "SHORTED."
If the needle moves slightly for one of the tests, the transistor is "LEAKY."  

We have found the BASE:
The lead connected to the black probe in the first test (the NPN transistor) is the BASE
The lead connected to the red probe in the second test (the PNP transistor) is the BASE

To find the collector and emitter leads we create the SIMPLEST AMPLIFIER IN THE WORLD. It consists of the "transistor under test" (sometimes called "tut"), a multimeter on 1k range and YOUR FINGER!
We will take the example of the NPN transistor as this is the most common type.
The diagram below shows how an NPN transistor is connected. 
When making the test, you must not touch the third lead with any part of your body as this will upset the reading on the multimeter. 
You already know the transistor is NPN and also the base lead. 
Connect the multimeter to the two leads that are not the base. It does not matter if you get the orientation correct as the circuit will not work until the placement is correct. The needle will not move.  Place a MOIST finger between the base and collector and the pointer will deflect almost 80% across the dial. The harder you press, the further the needle will move across the scale. 
The transistor is amplifying the current you are delivering to the base and causing about 100 times more current to flow in the collector-emitter circuit. 
This current flow effectively reduces the resistance between the two leads and the multimeter indicates the result. You have created the world's simplest amplifying circuit. The diagram below shows the collector and emitter leads connected to the meter.

Note: the multimeter is on "x1k"  scale

If the transistor is a PNP type, you will need to use the arrangement below:

Note: the multimeter is on "x1k"  scale

In circuit testing simply means to test the transistor while it is still in the circuit. This can be done provided you take into account the components surrounding the transistor. In other words the components directly connected to the transistor. If we take the circuit below, for example, we have 4 transistors in different "impedance situations." You can also say different "resistance situations." The term impedance takes into account the resistive effect of the surrounding capacitors, diodes, transistors and coils. Make sure the power is off before making any tests and wait until any capacitors have lost their charge (this circuit has a 120u capacitor and is charged to almost 330v).
If you test the first transistor (BC 557) (It is a surface-mount type in the circuit - 2P or M6 - for more details on testing surface-mount transistors click HERE) with the multimeter switched to the x1k range, the base to collector and base to emitter reading will check-out ok but when you test the collector to emitter and emitter to collector readings, you will get a low value in one direction. This is not a faulty transistor but the base-emitter reading of the second transistor! If you don't know how the circuit is laid out, you will think the transistor is faulty (leaky). The solution is to switch to the Ohms range and measure between the collector and emitter again. If the transistor is faulty it will measure very low in both directions. The pointer will not stop at 90% full scale deflection.
The same situation applies with the second transistor. The third transistor can be tested on high ohms range. The fourth transistor has a 10k between base and collector, which must be taken into account. 
If you are not sure about the results you are getting, remove the transistor completely or at least desolder two leads. 

This is only a simple test for transistors that have "completely broken down." Other faults such as heat stress, over-voltage breakdown, high-frequency failure, or intermittent breakdown can also occur. You may need a can of "freeze," a hot soldering iron or a hair dryer to simulate the effect of overheating etc. 
Try this test and see if you get 100%:

1. Transistors are separated into two types. Name them.

Positive and Negative
P and N

2. Name the three leads of a common transistor:

Collector Bias Omitter
Base Collector Case
Emitter Collector Bias
Collector Base Emitter

3. The positive of the battery in a multimeter is connected to the:

Black probe
Red Probe

4. When testing a transistor with a multimeter, it is set to:

Low Ohms
High Ohms
High Volts

5. When testing a transistor, the first test finds the:

Collector lead
Base lead
Emitter lead

6. The easiest transistor to test is:

Both equal

7. The lead marked with the arrow is:

The Collector
The Base
The Emitter
The case

8. If the voltage on the base of a transistor increases, does it:

Turn on
Turn off
Not enough information
Remain the same

9. In the test above, the collector and emitter leads are found by
putting the transistor in an amplifying mode.

not enough information

10. In the final test (as explained above), the harder you press on the base-emitter leads, the further the needle will swing across the display.


11. In the animation below, name the type of transistor being tested:



12. In the diagram below, what will happen to the pointer when a finger is applied to the leads:


The pointer will move across the scale
The pointer does not move

13. In the diagram below, name the fault with the transistor:


The transistor has shorted between collector and base
The transistor has shorted between collector and emitter
The transistor is not faulty

14. In the diagram below, is the diode ok?


No. It is "shorted"
No. It is "open"

This is a new way to present a topic.  By asking questions with "computer" scoring, you know if you are grasping the topic or sliding over the theory without it "sinking-in." 
I hope you got a perfect score!


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