Module 7 − Introduction to Solid−State Devices and Power Supplies
Pages i ,
4−1 to 4−10,
.................. NUMBER of SEMI- CONDUCTOR IDENTIFICATION First
.................. JUNCTIONS...................................................... NUMBER
You may also find other markings on transistors that
do not relate to the JAN marking system. These markings are manufacturers' identifications and may not conform to
a standardized system. If in doubt, always replace a transistor with one having identical markings. To ensure that
an identical replacement or a correct substitute is used, consult an equipment or transistor manual for
specifications on the transistor.
Transistors are very rugged and are expected to be relatively trouble free. Encapsulation and conformal
coating techniques now in use promise extremely long life expectancies. In theory, a transistor should last
indefinitely. However, if transistors are subjected to current overloads, the junctions will be damaged or even
destroyed. In addition, the application of excessively high operating voltages can damage or destroy the junctions
through arc-over or excessive reverse currents. One of the greatest dangers to the transistor is heat, which will
cause excessive current flow and eventual destruction of the transistor.
To determine if a transistor is
good or bad, you can check it with an ohmmeter or a transistor tester. In many cases, you can substitute a
transistor known to be good for one that is questionable and thus determine the condition of a suspected
transistor. This method of testing is highly accurate and sometimes the quickest, but it should be used only after
you make certain that there are no circuit defects that might damage the replacement transistor. If more than one
defective transistor is present in the equipment where the trouble has been localized, this testing method becomes
cumbersome, as several transistors may have to be replaced before the trouble is corrected. To determine which
stages failed and which transistors are not defective, all the removed transistors must be tested. This test can
be made by using a standard Navy ohmmeter, transistor tester, or by observing whether the equipment operates
correctly as each of the removed transistors is reinserted into the equipment. a word of caution-indiscriminate
substitution of transistors in critical circuits should be avoided.
When transistors are soldered into equipment, substitution is not practicable; it is generally desirable to test
these transistors in their circuits.
Q34. List three items of information normally included in the
general description section of a specification sheet for a transistor.
Q35. What does the number "2"
(before the letter "N") indicate in the JAN marking scheme?
Q36. What is the greatest danger to a
Q37. What method for checking transistors is cumbersome when more than one transistor is bad
in a circuit?
Transistors, although generally more rugged mechanically than electron tubes, are susceptible to damage by
electrical overloads, heat, humidity, and radiation. Damage of this nature often occurs during transistor
servicing by applying the incorrect polarity voltage to the collector circuit or excessive voltage to the input
circuit. Careless soldering techniques that overheat the transistor have also been known to cause considerable
damage. One of the most frequent causes of damage to a transistor is the electrostatic
discharge from the human body when the device is handled. You may avoid such damage before starting
repairs by discharging the static electricity from your body to the chassis containing the transistor. You can do
this by simply touching the chassis. Thus, the electricity will be transferred from your body to the chassis
before you handle the transistor.
To prevent transistor damage and avoid electrical shock, you should
observe the following precautions when you are working with transistorized equipment:
1. Test equipment
and soldering irons should be checked to make certain there is no leakage current from the power source. If
leakage current is detected, isolation transformers should be used.
2. Always connect a ground between
test equipment and circuit before attempting to inject or monitor a signal.
3. Ensure test voltages do
not exceed maximum allowable voltage for circuit components and transistors. Also, never connect test equipment
outputs directly to a transistor circuit.
4. Ohmmeter ranges that require a current of more than one
milliampere in the test circuit should not be used for testing transistors.
5. Battery eliminators
should not be used to furnish power for transistor equipment because they have poor voltage regulation and,
possibly, high-ripple voltage.
6. The heat applied to a transistor, when soldered connections are
required, should be kept to a minimum by using a low-wattage soldering iron and heat shunts, such as long-nose
pliers, on the transistor leads.
7. When it becomes necessary to replace transistors, never pry
transistors to loosen them from printed circuit boards.
8. All circuits should be checked for defects
before replacing a transistor.
9. The power must be removed from the equipment before replacing a
10. Using conventional test probes on equipment with closely spaced parts often causes
accidental shorts between adjacent terminals. These shorts rarely cause damage to an electron tube but may ruin a
transistor. To prevent these shorts, the probes can be covered with insulation, except for a very short length of
Transistor lead identification plays an important
part in transistor maintenance; because, before a transistor can be tested or replaced, its leads or terminals
must be identified. Since there is no standard method of identifying transistor leads, it is quite possible to
mistake one lead for another. Therefore, when you are replacing a transistor, you should pay close attention to
how the transistor is mounted, particularly to those transistors that are soldered in, so that you do not make a
mistake when you are installing the new transistor. When you are testing or replacing a transistor, if you have
any doubts about which lead is which, consult the equipment manual or a transistor manual that shows the
specifications for the transistor being used.
There are, however, some typical lead identification
schemes that will be very helpful in transistor troubleshooting. These schemes are shown in figure 2-17. In the
case of the oval-shaped transistor shown in view A, the collector lead is identified by a wide space between it
and the base lead. The lead farthest from the collector, in line, is the emitter lead. When the leads are evenly
spaced and in line, as shown in
view B, a colored dot, usually red, indicates the collector. If the transistor is round, as in view C,
a red line indicates the collector, and the emitter lead is the shortest lead. In view D the leads are in a
triangular arrangement that is offset from the center of the transistor. The lead opposite the blank quadrant in
this scheme is the base lead. When viewed from the bottom, the collector is the first lead clockwise from the
base. The leads in view E are arranged in the same manner as those is view D except that a tap is used to identify
the leads. When viewed from the bottom in a clockwise direction, the first lead following the tab is the emitter,
followed by the base and collector.
Figure 2-17. - Transistor lead identification.
In a conventional power transistor as shown in views F and G, the collector lead is usually connected to
the mounting base. For further identification, the base lead in view F is covered with green sleeving. While the
leads in view G are identified by viewing the transistor from the bottom in a clockwise direction (with mounting
holes occupying 3 o'clock and 9 o'clock positions), the emitter lead will be either at the 5 o'clock or 11 o'clock
position. The other lead is the base lead.
There are several
different ways of testing transistors. They can be tested while in the circuit, by the substitution method
mentioned, or with a transistor tester or ohmmeter.
Transistor testers are nothing more than the solid-state equivalent of electron-tube testers (although they do
not operate on the same principle). With most transistor testers, it is possible to test the transistor in or out
of the circuit.
There are four basic tests required for transistors in practical troubleshooting: gain,
leakage, breakdown, and switching time. For maintenance and repair, however, a check of two or three parameters is
usually sufficient to determine whether a transistor needs to be replaced.
Since it is impractical to
cover all the different types of transistor testers and since each tester comes with its own operator's manual, we
will move on to something you will use more frequently for testing transistors-the ohmmeter.
Testing Transistors with an Ohmmeter
Two tests that can be done with an ohmmeter are
gain, and junction resistance. Tests of a transistor's junction resistance will reveal leakage, shorts, and opens.
Transistor Gain TEST. - a basic transistor gain test can be made using an ohmmeter and a
simple test circuit. The test circuit can be made with just a couple of resistors and a switch, as shown in figure
2-18. The principle behind the test lies in the fact that little or no current will flow in a transistor between
emitter and collector until the emitter-base junction is forward biased. The only precaution you should observe is
with the ohmmeter. Any internal battery may be used in the meter provided that it does not exceed the maximum
collector-emitter breakdown voltage.
Figure 2-18. - Testing a transistor's gain with an ohmmeter.
With the switch in figure 2-18 in the open position as shown, no voltage is applied to the PNP
transistor's base, and the emitter-base junction is not forward biased. Therefore, the ohmmeter should read a high
resistance, as indicated on the meter. When the switch is closed, the emitter-base circuit is forward biased by
the voltage across R1 and R2. Current now flows in the emitter-collector circuit, which causes a lower resistance
reading on the ohmmeter. a 10-to-1 resistance ratio in this test between meter readings indicates a normal gain
for an audio-frequency transistor.
To test an NPN transistor using this circuit, simply reverse the ohmmeter leads and carry out the
procedure described earlier.
Transistor JUNCTION Resistance TEST. - An ohmmeter can be used to test a transistor for
leakage (an undesirable flow of current) by measuring the base-emitter, base-collector, and collector- emitter
forward and reverse resistances.
For simplicity, consider the transistor under test in each view of
figure 2-19 (view A, view B and view C) as two diodes connected back to back. Therefore, each diode will have a
low forward resistance and a high reverse resistance. By measuring these resistances with an ohmmeter as shown in
the figure, you can determine if the transistor is leaking current through its junctions. When making these
measurements, avoid using the R1 scale on the meter or a meter with a high internal battery voltage. Either of
these conditions can damage a low-power transistor.
Figure 2-19A. - Testing a transistor's leakage with an ohmmeter. Collector-to-Emitter TEST
Figure 2-19B. - Testing a transistor's leakage with an ohmmeter. Base-to-Collector TEST
Figure 2-19C. - Testing a transistor's leakage with an ohmmeter. Base-to-Emitter TEST
Now consider the possible transistor problems that could exist if the indicated readings in figure 2-19
are not obtained. a list of these problems is provided in table 2-2.
Table 2-2. - Possible Transistor Problems from Ohmmeter Readings
By now, you should recognize that the transistor used in figure 2-19 (view A, view B and view C) is a
PNP transistor. If you wish to test an NPN transistor for leakage, the procedure is identical to that used for
testing the PNP except the readings obtained are reversed.
When testing transistors (PNP or NPN), you
should remember that the actual resistance values depend on the ohmmeter scale and the battery voltage. Typical
forward and reverse resistances are insignificant. The best indicator for showing whether a transistor is good or
bad is the ratio of forward-to- reverse resistance. If the transistor you are testing shows a ratio of at least 30
to 1, it is probably good. Many transistors show ratios of 100 to 1 or greater.
Q38. What safety
precaution must be taken before replacing a transistor?
Q39. How is the collector lead identified on an
Q40. What are two transistor tests that can be done with an ohmmeter?
Q41. When you are testing the gain of an audio-frequency transistor with an ohmmeter, what is indicated by a
10-to-1 resistance ratio?
Q42. When you are using an ohmmeter to test a transistor for leakage, what is indicated by a low,
but not shorted, reverse resistance reading?
Up to now the various semiconductors, resistors, capacitors, etc., in our discussions have been
considered as separately packaged components, called DIsCRETE COMPONENTS. In this section we will introduce some
of the more complex devices that contain complete circuits packaged as a single component. These devices are
referred to as INTEGRATED Circuits and the broad term used to describe the use of these devices to miniaturize
electronic equipment is called MICROELECTRONICS.
With the advent of the transistor and the demand by the
military for smaller equipment, design engineers set out to miniaturize electronic equipment. In the beginning,
their efforts were frustrated because most of the other components in a circuit such as resistors, capacitors, and
coils were larger than the transistor. Soon these other circuit components were miniaturized, thereby pushing
ahead the development of smaller electronic equipment. Along with miniature resistors, capacitors, and other
circuit elements, the production of components that were actually smaller than the space required for the
interconnecting wiring and cabling became possible. The next step in the research process was to eliminate these
bulky wiring components. This was accomplished with the PRINTED Circuit BOARD (PCB).
A printed circuit
board is a flat insulating surface upon which printed wiring and miniaturized components are connected in a
predetermined design, and attached to a common base. Figure 2-20 (view a and view B) shows a typical printed
circuit board. Notice that various components are connected to the board and the printed wiring is on the reverse
side. With this technique, all interconnecting wiring in a piece of equipment, except for the highest power leads
and cabling, is reduced to lines of conducting material (copper, silver, gold, etc.) deposited directly on the
surface of an insulating "circuit board." Since printed circuit boards are readily adapted as plug-in units, the
elimination of terminal boards, fittings and tie points, not to mention wires, results in a substantial reduction
in the overall size of electronic equipment.
Figure 2-20A. - a typical printed circuit board (PCB). FRONT SIDE
Figure 2-20B. - a typical printed circuit board (PCB). REVERSE SIDE
After the printed circuit boards were perfected, efforts to miniaturize electronic equipment were then
shifted to assembly techniques, which led to MODULAR CircuitRY. In this technique, printed circuit boards are
stacked and connected together to form a module. This increases the packaging density of circuit components and
results in a considerable reduction in the size of electronic equipment. Since the module can be designed to
perform any electronic function, it is also a very versatile unit.
However, the drawback to this approach
was that the modules required a considerable number of connections that took up too much space and increased
costs. In addition, tests showed the reliability was adversely affected by the increase in the number of
A new technique was required to improve reliability and further increase packaging density.
The solution was INTEGRATED Circuits.
An integrated circuit is a device that integrates (combines) both active components (transistors, diodes,
etc.) and passive components (resistors, capacitors, etc.) of a complete electronic circuit in a single chip (a
tiny slice or wafer of semiconductor crystal or insulator).
Integrated circuits (ICs) have almost
eliminated the use of individual electronic components (resistors, capacitors, transistors, etc.) as the building
blocks of electronic circuits. Instead, tiny CHIPS have been developed whose functions are not that of a single
part, but of dozens of transistors, resistors, capacitors, and other electronic elements, all interconnected to
perform the task of a complex circuit. Often these comprise a number of complete conventional circuit stages, such
as a multistage amplifier (in one extremely small component). These chips are frequently mounted on a printed
circuit board, as shown in figure 2-21, which plugs into an electronic unit.
Figure 2-21. - ICs on a printed circuit board.
Integrated circuits have several advantages over conventional wired circuits of discrete components.
These advantages include (1) a drastic reduction in size and weight, (2) a large increase in reliability, (3)
lower cost, and (4) possible improvement in circuit performance. However, integrated circuits are
composed of parts so closely associated with one another that repair becomes almost impossible. In
case of trouble, the entire circuit is replaced as a single component.
Basically, there are two general
classifications of integrated circuits: HYBRID and MONOLITHIC. In the monolithic integrated circuit, all elements
(resistors, transistors, etc.) associated with the circuit are fabricated inseparably within a continuous piece of
material (called the SUBSTRATE), usually silicon. The monolithic integrated circuit is made very much like a
single transistor. While one part of the crystal is being doped to form a transistor, other parts of the crystal
are being acted upon to form the associated resistors and capacitors. Thus, all the elements of the complete
circuit are created in the crystal by the same processes and in the same time required to make a single
transistor. This produces a considerable cost savings over the same circuit made with discrete components by
lowering assembly costs.
Hybrid integrated circuits are constructed somewhat differently from the
monolithic devices. The PASSIVE components (resistors, capacitors) are deposited onto a substrate (foundation)
made of glass, ceramic, or other insulating material. Then the ACTIVE components (diodes, transistors) are
attached to the substrate and connected to the passive circuit components on the substrate using very fine (.001
inch) wire. The term hybrid refers to the fact that different processes are used to form the passive and active
components of the device.
Hybrid circuits are of two general types: (1) thin film and (2) thick film.
"Thin" and "thick" film refer to the relative thickness of the deposited material used to form the resistors and
other passive components. Thick film devices are capable of dissipating more power, but are somewhat more bulky.
Integrated circuits are being used in an ever increasing variety of applications. Small size and weight and
high reliability make them ideally suited for use in airborne equipment, missile systems, computers, spacecraft,
and portable equipment. They are often easily recognized because of the unusual packages that contain the
integrated circuit. a typical packaging sequence is shown in figure 2-22. These tiny packages protect and help
dissipate heat generated in the device. One of these packages may contain one or several stages, often having
several hundred components. Some of the most common package styles are shown in figure 2-23.
NEETS Table of Contents
- Introduction to Matter, Energy, and Direct
- Introduction to Alternating Current and Transformers
- Introduction to Circuit Protection, Control,
- Introduction to Electrical Conductors, Wiring
Techniques, and Schematic Reading
- Introduction to Generators and Motors
- Introduction to Electronic Emission, Tubes,
and Power Supplies
- Introduction to Solid-State Devices and
- Introduction to Amplifiers
- Introduction to Wave-Generation and Wave-Shaping
- Introduction to Wave Propagation, Transmission
Lines, and Antennas
- Microwave Principles
- Modulation Principles
- Introduction to Number Systems and Logic Circuits
- Introduction to Microelectronics
- Principles of Synchros, Servos, and Gyros
- Introduction to Test Equipment
- Radio-Frequency Communications Principles
- Radar Principles
- The Technician's Handbook, Master Glossary
- Test Methods and Practices
- Introduction to Digital Computers
- Magnetic Recording
- Introduction to Fiber Optics