Specifications

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CONSTRUCTION

ARUP KUMAR SEN

TABLE I

Orientation Test socket Test socket Test socket Base-Id Base-Id Collector-Id for

No. terminal 3 terminal 2 terminal 1 for npn for pnp pnp and npn

1 C B E 02 05 04

2 C E B 01 06 04

3 E C B 01 06 02

4 E B C 02 05 01

5 B E C 04 03 01

6 B C E 04 03 02

B=Base C=Collector E=Emitter Note: All bits of higher nibble are set to zero.

TABLE II

Q2 (MSB) Q1 Q0 (LSB)

001

010

100

TABLE III. SET 1

Q2 Q1 Q0

001

010

100

TABLE IV. SET 2

Q2 Q1 Q0

110

101

011

ransistor lead identification is cru-

cial in designing and servicing. A cir-

cuit designer or a serviceman must be

fully conversant with the types of tran-

sistors used in a circuit. Erroneous lead

identification may lead to malfunctions,

and, in extreme cases, even destruction

of the circuit being designed or serviced.

Though transistor manufacturers en-

capsulate their products in different pack-

age outlines for identification, it is im-

possible to memorise the outlines of in-

numerable transistors manufactured by

the industry. Although a number of

manuals are published, which provide pin

details, they may not always be acces-

sible. Besides, it is not always easy to

find out the details of a desired transis-

tor by going through the voluminous

manuals. But, a handy gadget, called tran-

sistor lead identifier, makes the job easy.

All one has to do is place the transistor

in the gadget’s socket to instantly get the

desired information on its display, irre-

spective of the type and package-outline

of the device under test.

A manually controlled version of the

present project had been published in June

’84 issue of

EFY

. The present model is to-

tally microprocessor controlled, and hence

all manually controlled steps are replaced

by software commands. A special circuit,

shown in Fig. 1, which acts as an interface

to an

8085-based microprocessor kit, has

been developed for the purpose.

Principle

Base and type identification. When a

semiconductor junction is forward-biased,

conventional current flows from the source

into the p-layer and comes out of the junc-

tion through the n-layer. By applying

proper logic voltages, the base-emitter

(

) or base-collector (

) junction of a bi-

polar transistor may be forward-biased.

As a result, if the device is of npn type,

current enters only through the base. But,

in case of a pnp device, current flows

through the collector as well as the emit-

ter leads.

During testing, when leads of the

‘transistor under test’ are connected to

terminals 1, 2, and 3 of the test socket

(see Fig.1), each of the leads (collector,

base, and emitter) comes in series with

one of the current directions indicating

LED

s (

4, and

6) as shown in Fig. 1.

Whenever the current flows toward a par-

ticular junction through a particular lead,

the

LED

connected (in proper direction) to

that lead glows up. So, in case of an npn-

device, only the

LED

connected to the base

lead glows. However, in case of a pnp-

device, the other two

LED

s are lit. Now, if

a glowing

LED

corresponds to binary 1, an

LED

that is off would correspond to binary

0. Thus, depending upon the orientation

of the transistor leads in the test socket,

we would get one of the six hexadecimal

numbers (taking

LED

connected to termi-

nal 1 as

LSB

), if we consider all higher

bits of the byte to be zero. The hexadeci-

mal numbers thus generated for an npn

and pnp transistor for all possible orien-

tations (six) are shown under columns 5

and 6 of Table I. Column 5 reflects the

BCD

weight of

(base) position while col-

umn 6 represents 7’s complement of the

column 5 number.

We may call this 8-bit hexadecimal

number base identification number or, in

short, base-Id. Comparing the base-Id,

generated with Table I, a microprocessor

can easily indicate the type (npn or pnp)

and the base of the device under test,

with respect to the test socket terminals

marked as 1, 2, and 3. The logic num-

bers, comprising logic 1 (+5V) and logic 0

(0V), applied to generate the base-Id, are

three bit numbers—

100, 010, and 001. These

numbers are applied sequentially to the

leads through the testing socket.

Collector identification. When the

base-emitter junction of a transistor is for-

ward-biased and its base-collector junction

is reverse-biased, conventional current

flows in the collector-emitter/emitter-col-

lector path (referred to as

path in sub-

sequent text), the magnitude of which de-

pends upon the magnitude of the base cur-

rent and the beta (current amplification

factor in common-emitter configuration) of

the transistor. Now, if the transistor is bi-

ased as above, but with the collector and

emitter leads interchanged, a current of

much reduced strength would still flow in

the

path. So, by comparing these two

currents, the collector lead can be easily

identified. In practice, we can apply proper

binary numbers (as in case of the base iden-

tification step mentioned earlier) to the ‘de-

vice under test’ to bias the junctions se-

quentially, in both of the aforesaid condi-

MICROPROCESSOR-CONTROLLED

TRANSISTOR LEAD IDENTIFIER

RUPANJANA

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