TRANSISTOR

Contents: Definition Identifying transistor leads

Identified by identifying marks Identified based on the configuration of the body Identified by ohmmeter test

Identifying emitter and collector by test Determining base by test

Testing transistor with an ohmmeter Identifying by test PNP or NPN 3 tests to evaluate if a transistor is good or defective Tests of good transistor Tests of defective transistor

Transistors -a small low-powered solid-

state electronic device consisting of a semiconductor and at least three electrodes, used as an amplifier and rectifier and frequently incorporated into integrated circuit chips

There’s a transistor that the leads are positioned closely together in a triangular form. A dot on the body indicates collector. The base lead is always in the center, therefore the remaining lead is the emitter. This method is standard for this kind of transistors.

a. There is a transistor with a flat portion on a round body, the nearest to the flat part of the body lead is emitter

b. In another case there is a tab on the body which indicates the emitter

c. A transistor with two leads, the body of this transistor serves as collector. Some transistors with this type of body have identification labels. However, some are without them. In this case hold the transistor so that the two leads and the hole nearest to them in the body are in the upper position. The left terminal is the base and the other lead is the emitter

d. The body of a power transistor is made of plastic and metal. The metal part is provided with a hole for the purpose of mounting the transistor. The metal tab is connected to the collector lead. This lead arrangement is standard

e. Another transistor, is the base is identified by its outward bent angular shape

Ohmmeter tests are helpful in identifying emitter and collector. The DC resistances of base-emitter and base-collector junctions are not equal. The reverse DC resistance of the base-emitter junction tends to be lower than that of the base-collector. For these tests, set the ohmmeter to the highest range x10K. Conduct the tests similarly to the diode test of the transistor. The test should be in reverse mode.

Some transistors have equal DC resistances and it is difficult to distinguish between the two leads. In such a case your experience with transistors will soon educate you in determining the leads.

EB C E B

C

RR

Higher resistance

Lower resistance

The base of a transistor can be determined by forward or reverse tests. Small signal transistors particularly with half round plastic bodies have two different locations of the base. It is located either at the center or right side lead if you are facing the flat side of the body. Let us assume the center lead is the base. If the pointer of the ohmmeter both deflect, it means that our assumption is correct because both tests show low resistance. The same would be true if both indicated high resistance. If the readings are opposite, one shows forward (low) and the other reverse (high) resistances the assumption is wrong. You may pick the right hand and follow the same testing procedures.

RR

NPN (Negative Positive Negative)

PNP (Positive Negative Positive)

To simplify the understanding, a bipolar transistor can be viewed as two diodes facing each other in NPN and with their backs to each other in PNP transistors.

Both terms “front” and “forward” mean low resistance. On the other hand “back” and “reverse” mean high resistance.

forward

forward

forward

forward

NPN PNP

R

R

R

base-collector

base-emitter

NPN

emitter-collector

The first and second tests are diode tests of front and back resistances. The third is leakages current test and should indicate high resistance in a good transistor. forward reverse leakage current

CB E E B

C E BC

CB E E B

C

base-emitter

CB E E B

C

RR

base-collector

CB E E B

C

RR

emitter-collector

EB C C B

E

RR

base-emitter shorted

CB E E B

C

RR

base-collector open

EB C C B

E

RR

emitter-collector

leak shorted

CB E E B

C

Silicon transistor Germanium transistor

emitter-collector

no

leakage

little

leakage

good

Just as you are about to check transistors it is important that you observe precautions to avoid possible damage to a transistor or arriving at false test results.

Ranges x1-x10 are intended for checking large transistors. The current available in these ranges is great enough to possibly damage small signal transistors. Ranges x1K-x10K are suitable for small transistors. A single test may not be very conclusive, double check it. Range x10K is very sensitive and a contact of transistor leads or test prods with your fingers will give you false results. After some practice transistor testing will be easy.

Checking transistors with an ohmmeter is not always conclusive in cases when the readings indicate a good transistor. However, the ohmmeter check provides fairly reliable test results. If you have doubts about the quality of transistor its replacement will confirm whether the component is good or bad. The ohmmeter test is that of conduction by means of the internal battery of a tester. In actual circuit conditions a transistor may operate under higher voltages than the one provided by the internal ohmmeter battery. This explains why the ohmmeter test is not always conclusive. Sometimes a partly defective transistor can display its defects only under higher supply voltages.

Prepared by: Maika A. De Torres

IV-Narra