Avionics Systems

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BASIC

COMPLEMENTARY COURSE

FOR AIRFRAME AND POWER PLANT

ENGINEERS

ELECTRICAL

SYSTEMS

EGYPTAIR TRAINING CENTER

BASIC TECHNICAL TRAINING

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BASIC

COMPLEMENTARY COURSE

FOR AIRFRAME AND POWER PLANT

ENGINEERS

AVIONICS

SYSTEMS

EGYPTAIR TRAINING CENTER

BASIC TECHNICAL TRAINING

(3)

ELECTRONIC FUNDAMENTALS DIODES PAGE 1 of 22

BASIC COMPLEMENTARY COURSE FOR AF & PP ENGINEERS

TECHNICAL TRAINING DEPARTMENT

1 DIODES

Semi-conductor diodes embrace a very wide field of devices using varied modes of operation. Before discussing these, it is necessary to briefly describe semi-conductors themselves.

1.1 SEMI-CONDUCTORS

Germanium and silicon are the most common semi-conductor elements. Figure 1 shows an element in pure crystalline form. The circles represent atoms and the dots valence electrons, electrons able to combine with those of another atom.

Silicon Structure Figure 1

1.1.1 INTRINSIC SEMI-CONDUCTOR

Note that one of the atoms has lost an electron, leaving a 'hole' but the free

electron is still present inside the crystal lattice, so the crystal as a whole remains. A crystal of pure semi-conductor material with no other atoms, such as in Figure 1, is called an intrinsic semi-conductor.

4 4 4 4 4 4 4 4 4 4 4 4 HOLE ELECTRON

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Figure 2 shows current flow in an intrinsic semi-conductor. The electrons (negative charge) are attracted to the positive terminal of the battery, while the holes (positive charge) are attracted to the negative.

Intrinsic Semiconductor Figure 2

1.1.2 EXTRINSIC SEMI-CONDUCTOR

Intrinsic semi-conductors are poor conductors. By adding an impurity to the crystal, conductivity can be improved. Figure 3a shows an impurity having five electrons added. The 'extra electron' is not needed for crystal bonding and so is free to move about the lattice as a conduction electron.

Since it is not a part of the lattice, it does not leave a 'hole' when it moves; but a 'positive ion'. The more impurity atoms added, the more conductive the material. The semi-conductor is now 'extrinsic' and of the 'N type'. Electrons are the

majority carriers, they are negative, and hence 'N' type.

Figure 3b shows a lattice with an element having only three valence electrons added. This time there is a shortage of electrons and this produces 'holes' in the material and negative ions. With fewer negative electrons, the majority carriers are positive 'holes'. Now the material is described as 'P' type.

The impurity added to give more electrons to make N type material is known as a ‘donor impurity’. The impurity added to give more holes to make P type material is known as an ‘acceptor impurity’. The process of adding either type of impurity is known as doping.

SEMICONDUCTOR MATERIAL

HOLES ELECTRONS

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Extrinsic Semiconductor Figure 3 4 4 4 3 4 4 4 3 3 4 4 4 4 4 4 5 4 4 4 5 5 4 4 4

(a)

(b)

EXTRA ELECTRON DONOR IMPURITY ATOM ACCEPTOR IMPURITY ATOM HOLE

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1.2 THE HALL EFFECT

When experimenting in 1879 with current flowing in a strip of metal, E M Hall discovered that some of the charge carriers were deflected to one of the faces of the conductor when a strong magnetic field was applied. This gave rise to an emf (the Hall voltage) between opposite faces of the conductor. The emf is only a few microvolts in the case of a metal conductor, but is much larger when the current flows in a semiconductor.

An experiment, making use of what is known as the “Hall Effect”, can be conducted to demonstrate that the majority carriers in a bar of semiconductor material are electrons in “N” type and holes in “P” type. Figure 4 shows the Hall Effect

The Hall Effect Figure 4 20V +10V +10V SEMICONDUCTOR MAT ERIAL CURRENT FLOW +2 0V 0V 0V P.D. 20V +11V +9V +11V +9V +9V +11V

POSITIVE CHARGE CARRIERS (HOLES)

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Consider the arrangement illustarted in figure 4a, this shows a bar of

semiconductor material, with a D.C. voltage of 20V applied. Conventional current will flow as indicated by the arrow. A further two connections “A” & “B” are taken from opposite faces of the bar at the mid-point along the axis. Thus under static conditions, the voltgae at connect A and B will be +10V relative to the negative terminal, and there is no voltage difference between them, i.e. no potential difference.

No consider what happens when we place this bar in a transverse magnetic field as in figure 4b. the charge carriers moving in the semiconductor are deflected by the magnetic field in the direction given by “Fleming’s Left-Hand rule”. Thus, whether the charge carriers are holes or electrons, they are deflected upwards in figure 4b, towards connection A. This will result in a redistribution of charge carriers between A & B, with the consentration towards A. If the charge carriers are positive (holes), connection A becomes positive with respect to connection B as shown in figure 4c. Conversely, if the charge carriers are negative (electrons), connection A becomes negative with respect to B as shown in figure 4c.

The voltage difference between connection A & B is called the “Hall Voltage” and has many pratical applications such as “Contactless switches (proximity

detectors). It can also be used in a dc starter/generator system as a means of measuring generator output current and providing an input signal to a Generator Control Unit (GCU) which controls generator field current (voltage regulation)m and protection. Figure 5 shows Hall Effect Sensors in a DC starter/generator system as fitted to the ATR 42/72 aircraft.

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Hall Effect Sensors Figure 5 GENERATOR CONTROL UNIT HALL EFFECT SENSOR STARTER GENERATOR HALL EFFECT SENSOR CURRENT MEASURING TO DISTRIBUTION

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1.3 THE JUNCTION DIODE

So far “N” type and P-Type materials have been considered separately.

However, most semiconductor devices contain regions where P-type material is joined to N-type material at one or more places. These places are called P-N junctions and the behaviour of the devices depends upon the electrical behaviour of the region around the junctions.

By doping a semi-conductor so that there is N type material at one end and P type at the other, a Junction Diode is made. Refer Figure 6. In this arrangement, the electrons in the N type are repelled by the like polarity of the negative ions in the P type.

Similarly the positive holes in the P type are repelled by the positive ions in the N Type. This leaves an area at the junction without any majority carriers and it is called the depletion layer.

Junction Diode Figure 6

DEPLETION

LAYER

POSITIVE IONS NEGATIVE IONS

N-TYPE P-TYPE

HOLES ELECTRONS

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By connecting a battery across a junction diode, positive to N type, negative to P type, (reverse biased), majority carriers cannot flow, hence there is no current flow in the circuit.

If the battery is connected positive to P type, negative to N type, (forward biased) majority carriers are allowed to flow and there is current flow in the circuit. This is the characteristic of the diode. It will allow current flow in one direction only, when forward biased, but not in the other direction when reverse biased. Figure 7 shows a junction diode reversed and forward biased.

Junction Diode Reversed/Forward Biased Figure 7

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1.4 DIODE SYMBOL

Figure 8 demonstrates, using the circuit symbol for a diode, how the device is placed in a circuit to allow or block current flow. Note that (conventional) current flows in the direction of the arrow in the symbol.

Diode Symbol Figure 8

1.5 DIODE CHARACTERISTICS

With all diodes there are four parameters to be considered, these are: 1. Maximum permissible forward current (mA).

2. Maximum voltage drop (V) at nominal operating current (mA). 3. Typical reverse current (µA).

4. Maximum permissible reverse voltage (V).

ANODE CATHODE

+

_

NO CURRENT CURRENT FLOW

REVERSED BIASED

FORWARD BIASED

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Figure 9 shows the static characteristics of a silicon diode and figure 10 show s the characteristics for a germanium diode.

Note: That the reverse current axes on both graphs are different.

Silicon Diode Characteristics Figure 9 mA VOLTS 0.25V 0.5V 0.75V 1V -50V -100 -150 -200V µA 50 200 150 100 -0.08 -0.02 -0.04 -0.06 REVERSED BIAS FORWARD BIAS

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Germanium Diode Characteristics Figure 10

1.6 DIODES IN SERIES AND PARALLEL

Diodes may be connected in series or parallel. For carrying high voltage, a series configuration would be used. If a high current carrying capability were required, the diodes would be connected in parallel.

mA VOLTS 0.25V 0.5V 0.75V 1V -50V -100 -150 -200V µA 50 200 150 100 200 50 100 150 REVERSED BIAS FORWARD BIAS

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1.7 RECTIFIER DIODES

Rectifier diodes are designed to convert A.C. to D.C. and to be able to achieve this effectively and efficiently, they must have:

1. Low resistance to current flow in the

forward direction.

2. High resistance to current flow in the

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Because of the need for a very low reverse current and a high breakdown

voltage, almost all semiconductors rectifier diodes are silicon junction types; they usually have a junction area that is large relative to their size to assist in the dissipation of heat. An elementary rectifier circuit is where the diode is inserted in series between the input and output, this is shown in figure 11.

Basic Rectifier Circuit Figure 11

The diode effectively passes current only in the forward bias condition. As can be seen from figure 10, when A.C. input is applied, pulses of unidirectional D.C. voltages are developed across the output load resistance.

Note; The polarity of the output D.C. can be reversed by reversing the diode connections.

+ 0

-A.C. INPUT D.C. OUTPUT

+ 0

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1.8 EXAMPLES OF RECTIFIER DIODES

Silicon rectifier diodes are available that are capable of supplying currents from about 200mA to about 2000A at voltages up to 3000 or 4000 volts. A sample cross-section of such diodes is illustrated in Figure 12. Compared with other rectifying devices, silicon junction rectifiers are small and lightweight. They are also impervious to shock and are capable of working at temperatures up to about 200°C.

Silicon Rectifier Diodes Figure 12

250mA @ 200V

1A @ 1000V

1A @ 1500V 10A @ 400V

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1.9 RECTIFIER DIODES

1.9.1 SELENIUM RECTIFIERS

The aluminium base serves as a surface for the dissipation of heat. The rectifying junction covers one side of the base apart from a narrow strip at the edges and an area around the fixing hole, which is sprayed with insulating varnish. Figure 13 shows the construction of a selenium rectifier element.

Selenium Rectifier Figure 13

The counter electrode is a thin layer of a low melting point alloy, sprayed over the selenium coating and insulating varnish. The counter electrode is the cathode, while the base is the anode.

These rectifiers may be stacked in series, suitable for high voltages, or in parallel, suitable for high current. When stacking, pressure applied during assembly tends to reduce the reverse resistance. This is overcome by application of varnish at the mounting studs.

Reverse resistance is a limiting factor in rectifiers, as is temperature. The maximum operating temperature of these rectifiers is in the order of 70°C.

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1.9.2 SILICON RECTIFIERS

The silicon rectifier is a far smaller unit than the selenium rectifier. This type of rectifier is used in the brushless ac generator. The silicon slice is extremely small. On one face it has a fused aluminium alloy contact to which the anode and lead are soldered. The other face is soldered to a base, usually copper. This is the cathode and acts as a heat sink. The aluminium - silicon junction forms the barrier layer. The whole is enclosed in a hermetically sealed case to protect it from environmental conditions. These rectifiers operate at temperatures up to 150°C. Figure 14 shows a Silicon Rectifier.

Silicon Rectifier Figure 14

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Figure 15 shows the circuit for a “Full-Wave bridge” rectifier.

Full-Wave Bridge rectifier Figure 15

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1.10 THE LIGHT-EMITTING DIODE (LED)

LEDs are made from a semiconductor material, which emits light when current flows through the junction. The most common colour emitted is red but green and yellow are available at a lower intensity. Figure 24 shows the circuit symbol for an LED and its operation.

Light Emitting Diode (LED) Figure 24

The voltage drop across a LED is around 2 volts. Above this voltage, the current passing through it increases rapidly. For this reason a series resistor is used to limit the current to around 10 ma to prevent burnout of the junction.

1.10.1 USE OF LEDS

LEDs can be used to replace filament lamps, with the advantage of less current consumption, less heat and no filament to burn out. They are often found on aircraft fault panels.

+5V EARTH

OFF

DIODE IS REVERSED BIASED

+5V EARTH

DIODE IS FORWARD BIASED

EMITS LIGHT

ON

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1.11 THE PHOTO CONDUCTIVE DIODE

This device is a normal PN junction with a transparent case or window. All semi-conductor diodes are subject to some movement of hole/electron pairs when the junction is at room temperature and this gives rise to a small leakage current, even with the diode reversed biased but the current is measured in

microamperes.

When light falls on the junction, its energy produces a much larger number of hole/electron pairs and the leakage current is greatly increased. These devices have a rapid response to light and are used in the encoding altimeter to encode the grey code into binary code. Figure 25 shows the circuit symbol and

construction of a Photo Conductive Diode.

Photo Conductive Diode Figure 25

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1.12 VARISTORS

The varistor is a semi-conductor device used for clipping 'noise spikes' off ac voltage. Noise spikes are of very short duration and large amplitude. They may pass through a power supply and appear on a dc regulated output voltage. Low pass filters are often ineffective against noise spikes so the spikes are attenuated before rectification of ac to dc.

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1.13 TESTING DIODES

Before testing a diode, the cathode must be identified and then an ohmmeter is applied as in Figure 27. In one direction the ohmmeter reading should be low but a very high resistance should be detected in the other direction.

Testing Diodes Figure 27

FLUKE 23SERIES MUL TIMET ER

0 10 20 30 0 0 0 . 2 3O HM S OFF _V V 300 m V Ω A A COM VΩ 10A 300 m A FUSED ! 10 00V 75 0V P RE S S RANGE A UT ORANGE SYMBOL P N STRUCTURE LOW RESISTANCE CATHODE

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ELECTRONIC FUNDAMENTALS TRANSISTORS PAGE 1 of 8

1 TRANSISTORS

The transistor can be a high or low resistance device, hence the name, which is derived from TRANSfer resISTOR.

It is used in many switching and amplifier circuits where its resistive properties are controlled by small currents.

1.1 TRANSISTOR CONSTRUCTION

The properties of semi-conductor materials, P and N type, were discussed in Module 4.1.1. A transistor is made up of these materials in the configurations shown in Figure 1. The circuit symbols for these transistors are also shown.

PNP & NPN Transistors Figure 1

N

P

BASE COLLECTOR EMITTER B C E CIRCUIT SYMBOL NOT POINTING IN THE NPN TRANSISTOR

P

N

BASE COLLECTOR EMITTER B C E CIRCUIT SYMBOL THE PNP TRANSISTOR

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As can be seen from figure 1, there are two possible types of physical arrangement:

1. TheN-P-N transistor, which consists of a thin region of P-type material, sandwiched between two N-type regions.

2. TheP-N-P transistor, which consists of a thin region of N-type material, sandwiched between two P-type regions.

The centre region of the device is called the “Base”; one outer region is called the “Emitter”, and the other the “Collector”. Although the emitter and collector regions are the same type of extrinsic semiconductor (N-type in N-N and P-type in P-N-P), they are constructed and doped differently and are not

interchangeable on a practical device.

The circuit symbol for both P-N-P and N-P-N are shows in figure 1. The only difference between them is the direction of the arrowhead on the emitter lead. For either type, the arrowhead indicates the direction of “Conventional”

current flow when the base/emitter junction is forward biased (i.e. base +ve with respect to emitter for an N-P-N device, and base –ve relative to emitter for a P-N-P device).

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1.2 TRANSISTOR OPERATION

Figure 2 shows a NPN transistor and the corresponding diode circuit. It can be seen from the diode circuit that to operate, the base/emitter must be forward biased, whereas the base/collector is reversed biased.

NPN Transistor & Diode Circuit Figure 2 N - TYPE N - TYPE P - TYPE DIODE MODEL

B

C

E

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Figure 3 shows a simple transistor circuit using electron flow to explain the operation. NPN Transistor Operation Figure 3

C

B

E

I

_E

HIGH

(100%)

I

_B

LOW

(1%)

I

_C

HIGH

(99%)

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1.3 SWITCHING TRANSISTORS

When a transistor is to be used as a switching device, it operates either as an open circuit (i.e. in the cut-off region) or as a short circuit (i.e. in the saturation region). Figure 3 shows the solenoid switch and an alternative transistor switch. Switching Transistors Figure 3 SOLENOID ANALOGY LAMP E C B

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For a common base circuit, such as in figure 3, the output voltage taken from the collector is either equal to the supply voltage (saturated region), or zero volts. (cut-off).

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1.4 TRANSISTOR CONFIGURATIONS

Before a transistor can be used, it must be connected into an input circuit (by two wires) and an output circuit (two wires). However, because the transistor has only three terminals, one of the terminals must be in both the input and output circuits; this is then called “The Common terminal”. Three circuit configurations are possible and are illustrated in figure 9.

Transistor Configurations Figure 9

Note that the word ‘common’ refers to the transistor component connected to both the INPUT and OUTPUT. In the common emitter configuration for example, the emitter is connected to both the input and output.

INPUT OUTPUT COMMON EMITTER B C E INPUT _B OUTPUT C E COMMON BASE INPUT OUTPUT B C E COMMON COLLECTOR

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Table 1 shows the comparisons of the three transistor configurations

Common Emitter Common

Base

Common Collector

Current Gain 20 -200 (0.95 – 0.995) 20 - 200

Voltage Gain 100 – 600 500 – 800 <1

Power Gain Highest Medium Lowest

Input Impedance 500 - 2000Ω 50 - 200Ω 20kΩ - 100kΩ

Output Impedance 10 – 50 KΩ 100 kΩ - 1MΩ 20 – 500Ω

I/P – O/P Phase 180° In Phase In Phase

Typical Use Normal Amp

Impedance matching (low to high)

Impedance matching (high to low) Table 1

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ELECTRONIC FUNDAMENTALS INTEGRATED CIRCUITS PAGE 1 of 19

1 INTEGRATED CIRCUITS

1.1 GENERAL

Integrated circuits, or IC’s, have changed the entire electronics industry. Before IC’s were developed, all electronic circuits consisted of individual (discrete), components that were wired together, often requiring a large amount of physical space. Printed circuit Board (PCB) technology made it possible to reduce the amount of space required. Electronic circuits can be quite complex, requiring a large number of components, since discrete components have a fixed size, there is a practical limitation on the amount of size reduction that can be achieved.

The development of integrated circuit technology has made it possible to fabricate large numbers of electronic components onto a single silicon chip. As a result, the physical size of a circuit can be significantly reduced, making it possible to design circuits and devices that would otherwise be impractical. IC’s are complete circuits containing many transistors, diodes, resistors and capacitors as may be necessary for the circuit operation. They are

encapsulated in packages that are often no larger than a single discrete transistor. The technology and materials used in the manufacture of IC’s are basically the same as those used in the manufacture of transistors and other solid-state devices. In addition, IC’s are manufactured for a wide variety of applications and, as a result, are used throughout the electronics industry.

1.1.1 ADVANTAGES

The small size of the IC is its most apparent advantage. A typical IC can be constructed on a piece of semiconductor material that is less than 4mm2. Even when the IC is suitably packaged, it still occupies only a small amount of space. The small size of the IC also produces other benefits such as they consume less power than the equivalent conventional circuit. They generate less heat and therefore generally do not require elaborate cooling or

ventilation systems.

IC’s are also more reliable than conventional circuits. This greater reliability result because every component within the IC is a solid-state device and is permanently connected together with a thin layer of metal. They are not soldered together like the components in a conventional circuit and a circuit failure due to faulty connections is less likely to occur.

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1.1.2 DISADVANTAGES

It might appear that the IC has only advantages to offer and no real disadvantages. Unfortunately, this is not the case, since IC’s are an extremely small device it cannot handle large currents or voltages. High currents generate heat within the device and small components can be easily damaged if the heat becomes excessive.

High voltages can break down the insulation between the components in the IC because the components are very close together. This can result in shorts between the adjacent components, which would make the IC completely useless. Therefore, most IC’s are low power devices, which have a low operating current (milliamps) and low voltages (5 – 20V). Also, most IC’s have a power dissipation range of less than 1 watt.

At the present only four types of component are commonly constructed within an IC. This makes only a narrow selection of components available, these are:

1. Diode. 2. Transistor. 3. Resistor. 4. Capacitor.

Diodes and transistors are the easiest components to construct and are used extensively to perform as many functions as possible within each IC.

Resistors and capacitors may also be formed, but it is much more difficult and expensive to construct these components. The amount of space occupied by a resistor increases with its value and in order to conserve space, it is

necessary to use resistors with values as low as possible.

Capacitors occupy even more space than resistors and the amount of space required increases with the value of the capacitor.

Ic’s cannot be repaired because their internal components cannot be

seperated. When one internal component becomes defective, the whole IC becomes defective and musty be replaced. This means that good

components are often thrown away with the defective ones. This disadvantage is not as bad as it sounds, as the task of fault finding is

simplified because it is only necessary to trace the problem to a specific circuit instead of an individual component. This greatly simplifies the task of

maintaining highly complex systems and reduces the demands on maintenance personnel.

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1.2 IC CONSTRUCTION

There are basically four methods of construction used for IC’s. These are: 1. Monolithic.

2. Thin-Film. 3. Thick Film. 4. Hybrid.

1.2.1 MONOLITHIC IC’S

The monolithic IC is constructed in basically the same manner as a “Bipolar Transistor”, although the overall process requires a few additional steps because of the greater complexity of the IC. Its fabrication begins with a circular semiconductor wafer (usually silicon). This wafer is usually very thin (0.015mm – 0.3mm) and either 2.5cm or 5cm in diameter. The

semiconductor serves as a base on which the tiny integrated circuits are formed and is commonly referred to as a “Substrate”. Figure 1 shows the IC construction. IC Construction Figure 1 2.5 - 5 CM DIAMETE R 0.015 - 0.30mm SILICON WAFER

IC’S ARE FORMED ON THE WAFER

NUMBER OF IC’S FORMED DEPENDS ON THE SIZE

OF THE WAFER

ONCE THE IC’S HAVE BEEN FORMED, THE WAFER IS SLICED INTO

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When all of the IC’s have been simultaneously formed, the wafer is sliced into many sections, which are commonly referred to as “Chips” or “Dice”. Each chip represents one complete integrated circuit and contains all the

components and wiring associated with that circuit. Once the IC’s have been separated into individual chips, each IC must be mounted in a suitable

package and tested.

1.2.2 BIPOLAR IC CONSTRUCTION

As mentioned earlier, the components that are commonly used in IC’s are diodes, Transistors, resistors and capacitors. Diffusing impurities into selected regions of a semiconductor wafer (substrate) can form these components. This process produces PN junctions at specific locations and the basic manner in which these four components are formed and the manner in which they are interconnected are shown at Figure 2.

Basic Construction of Bipolar IC Figure 2

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The circuit shown in figure 2 is a simple circuit consisting of a capacitor, a PN junction diode, an NPN transistor and a resistor. Operating voltages and currents can be applied to the circuit through terminals 1,2 and 3 as shown. This circuit could be easily constructed using four discrete components, however, it can also be produced as a monolithic IC.

1.2.3 MOS IC’S

Not all IC’s are constructed using bipolar components, IC’s are often designed to utilize either bipolar transistors or “Field-Effect transistors” (FETS). The Field effect transistor is one in which the emitter-collector current is controlled by voltage rather than by a current. Figure 3 shows the construction and operation of a MOSFET.

MOSFET Figure 3

The FET may be constructed of a channel of either N-type or P-type silicon with a controlling gate sitting on top. One end of the channel is called the source, and the other end is called the drain. An N-channel FET has a P-type gate, so that when a positive voltage ios applied to the gate, the FET is

forward biased. There will be current flow between the source and the drain. When a negative voltage is applied to the gate, the FET will be reversed biased, and the flow between the source and the drain will be pinched off.

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The source and drain regions are diffused into the substrate. A thin layer of silicon oxide is formed over the substarte and the appropriate windows are cut into it so that metal electrodes ) terminals) can be formed at the proper

locations. Note that the gate terminal is separated from the substrate by an extremely thin oxide layer, which is only 1 X 10-10 metres thick, but it

completely isolates the gate from the substrate.

1.2.4 THIN-FILM IC

Unlike the monolithic IC’s, which are formed within a semiconductor material (substrate), the thin-film circuit is formed on the surface of an insulating substrate. In the thin-film circuit, components such as resistors and

capacitors are formed from extremely thin layers of metals and oxides, which are deposited onto a glass or ceramic substrate. Interconnecting wires are also deposited on the substrate as thin strips of metal. Components such as diodes and transistors are formed as separate semiconductor devices and then permanently attached to the substrate at the appropriate locations. The substrate on which the thin-film circuit is formed is usually less than 2.5cm2_._{Depositing tantalum or nichrome as thin films or strips on the surface}

of the substrate forms the resistors. These films are usually less than

0.00254cmthick. The thickness, length and width of each strip that is formed on the substrate determine the value of each resistor. The interconnecting conductors are extremely thin metal strips, which have been deposited on the substrate. Low resistance metals, such as gold. platinum, or aluminium, are generally used as conductors. The substrate is made from an insulating material that will provide a rigid support for the components. Glass or ceramic materials are often used as substrates. Figure 4 shows a portion of a thin-film circuit. Thin-Film IC Figure 4 INSULATING SUBSTRATE THIN-FILM CONDUCTORS THIN-FILM RESISTORS

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1.2.5 THICK-FILM IC’S

Thick-film IC’s components are formed on an insulating substrate by using a “Silk-screen” process. In this process, a very fine wire screen is placed over the substrate and a metalized-ink is forced through the screen using a squeegee. Only certain portions of the wire screen are open (the remaining portions are filled with a special emulsion), thus allowing the ink to penetrate and coat the specific portions of the substrate. A pattern of interconnecting conductors is formed on the substrate, which is then heated to over 6000°C to harden the painted surface and become low resistance conductors.

Resistors and capacitors are also silk-screened on top of the substrate by forcing the appropriate materials (in paste form) through the appropriate screen and then heating the substrate to a high temperature. This process is repeated using various pastes until the circuit is formed. Components such as diodes and transistors are formed as separate semiconductor devices and then permanently attached to the substrate at the appropriate locations.

1.2.6 HYBRID IC’S

Hybrid IC’s are formed by utilizing various combinations of monolithic, thin-film and thick film techniques and may in certain circumstances contain discrete semiconductor components in chip form. Therefore many types of hybrid circuit arrangements can be produced. A typical hybrid circuit might consist of a thin-film circuit on which various monolithic IC’s have been attached or it could utilize monolithic IC’s thick-film components and discrete diodes and transistors that are all mounted on a single insulating substrate.

A portion of a hybrid IC is shown at figure 5. An insulated substrate is used to support the circuit components as shown. The monolithic IC is mounted on the substrate along with thich-film resistors and a small discrete capacitor. All the components are interconnected with conductors that are formed on the substrate using film techniques. The monolithic IC is connected to the conductors with fine wires that are bonded in place. Thick-film resistors will usually have notches cut into them to trim their values. The capacitor used in these circuits can be formed either by using film techniques or miniature devices can be installed between conductors as shown.

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Hybrid IC Construction Figure 5

1.2.7 IC PACKAGES

Like transistors and other types of solid state components, IC’s are mounted in packages, which protect them from moisture, dust and other types of contaminations. Many different types of IC packages are available and each type has its own advantges and disadvantages. The most popular IC

package is the “Dual In-Line (DIL) package. The packages also make it easier to install the IC’s in various types of equipment, since each package contains leads which can be either plugged into matching sockets or plugged into DIL mounting frames.

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Figure 6 shows typical DIL packages.

DIL Packages Figure 6

The IC package shown in figure 6 contains three monolithic IC’s, also a network of conductors have been formed on the same base that supports the chip. Various conductor pads on the chips are connected to these conductors with fine gold wires that have been bonded in place. The conductors in turn are connected to two rows of connecting pins along the edge of the package. A lid or cover (not shown) is placed over the opening in the package and soldered into place to provide an air tight (hermetically sealed) unit.

Integrated circuits may also be mounted in “Metal cans” that are similar to the types used to house transistors. The metal can have 8 or more connecting leads and can used to house either monolithic or hybrid type IC’s. The advantage of these packages is that they may be installed in a variety of ways. Metal cans can be used over a wide temperature range (-55° -

+125°C) and are therefore suitable for military and space applications. Figure 7 shows the DIL and metal can type of packages.

MONOLITHIC IC’s

CONNECTING PINS INTERCONNECTING

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DIL and Metal Can Packages Figure 7

1.3 TYPES OF INTEGRATED CIRCUIT

Integrated circuits are placed into two general groups, these are:

1. Digital IC’s. 2. Linear IC’s.

1.4 DIGITAL IC’S

Digital circuits use discrete values (0 or 1) to perform 3 general functions. These are:

1. AND Function.

2. OR Function.

TYPICAL MINIATURE DUAL IN-LINE (DIL)

PACKAGES

TYPICAL METAL CAN IC PACKAGES

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3. NOT Function.

Thgese three function are performed by logic circuits that are called the AND, OR and NOT logic gates. These gates or circuit configurations can be

combined to make decision based on digital input information. In a digital logic gate it is only possible to have an output of either a 0 or 1.

1.4.1 AND GATE

Figure 8 shows the AND gate truth table and logic circuit and a corresponding circuit to carry out this function.

AND Gate Figure 8

The AND gate has an output of 1 only when all of its inputs are equal to 1. This is similar to a multiplier function since the only possibilities in a digital circuit are 0 X 1 = 0 and 1 X 1 = 1. The schematic circuit in figure 8 shows two switches connected in series. Unless both switches are closed, there is no current flow to the output.

A B A.B SYMBOL A B A.B TRUTH TABLE SCHEMATIC DIAGRAM

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1.4.2 OR GATE

Figure 9 shows the OR gate truth table and logic circuit and a corresponding circuit to carry out this function.

OR Gate Figure 9

1.4.3 NOT GATE

The NOT gate provides an output that is always the opposite the input. This is called inversion or 180° phase shift. Thus, the NOT gate is commonly referred to as an inverter. In the bipolar transistor, the common emitter amplifier configuration was the only one capable of inverting the input so is used to carry out the NOT function.

A

B

A+B

SYMBOL

A B A+B

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Figure 10 shows the NOT gate truth table and logic circuit and a correspondingcircuit to carry out this function.

NOT Gate Figure 10

1.4.4 COMBINATION LOGIC CIRCUITS

The three basic logic circuits can be combined into a single decision making circuit with more than 1 distinct outputs. Consider a circuit that compares two inputs and calculates three outputs as shown below.

Output X1

Input A < Input B

Output X2

Input A > Input B

Output X3

Input A = Input B

A

1

0

1

INPUT OUTPUT +VE SYMBOL TRUTH TABLE SCHEMATIC DIAGRAM

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A combined logic circuit that would carry out the function is shown at Figure

11.

Combination Logic Circuit Figure 11

A

B

X1 (A<B) X2 (A>B) X3 (A=B) A A BB X1X1 X2X2 X3X3 TRUTH TABLE

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1.5 LINEAR (OR ANALOGUE) IC

Figure 12 shows the type of analogue signal handled by the Linear Integrated Circuit.

Analogue Signal Figure 1

1.6 THE OPERATIONAL AMPLIFIER (OP AMP)

The integrated circuit operational amplifier is one of the most useful and versatile electronic devices available today. The name ‘operational amplifier’ is not new; it refers to a type of amplifier originally used in analogue

computing to perform mathematical operations – e.g. multiplication or division by a constant. The modern integrated circuit device can be adapted (by feedback) to perform most general-purpose amplifier duties, as well as its use in mathematical operations.

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The Op Amp can consist of many stages of amplification to ensure high gain, and will be arranged to have two input terminals, two power supply terminals and an output terminal. In addition it will normally have terminals for setting the output to zero when the input is zero.

The Op Amp consists of a transistor circuit of considerable complexity, which has been found so useful that the whole circuit is manufactured on a single piece of silicon, fitted with input and output leads, and covered in plastic. It is the first “Integrated Circuit”, and can be treated just as if it were a new

component. Figure 2 shows a type 741 Op Amp and circuit.

Op Amp and Circuit Figure 2 1 2 3 5 4 7 8 6 INVERTING INPUT NON-INVERTING INPUT VOLTAGE OUTPUT POWER SUPPLY (+) POWER SUPPLY (–) 1 4 3 2 5 6 7 8 V– V+ NON-INVERTING INPUT INVERTING INPUT VOLTAGE OUTPUT GROUND

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In the Op Amp, two pins are marked supply + and supply - and are connected to the amplifiers power supply. The device also has two inputs, the “Inverting input” (VΙ) identified by a negative symbol. A “Non inverting input” (VN)

identified by a positive sign and a single output (VO).

Note: The negative/Positive signs on the inputs does not mean that negative/positive signals are applied, but identify the inverting and non-inverting terminals.

The VΙ, VN and VO are the values of the voltages applied to the inputs and

obtained form the output. These voltages are joined by the equation:

V

O

= A

O

(V

N

– V

Ι

)

Here we have a slight problem. Voltages are measured between one point in a circuit and another. Usually one point is the negative or zero line. When calculating VN & VΙ it does not matter were the reference is as long as it is the

same for both voltages. When we obtain the output VO we need to know the

reference point used by the Op Amp. This is not the zero line but a voltage halfway between the positive supply and the zero line.

The other unknown quantity in the equation is AO, the “Open Loop Gain”. This

gain is constant for each particular Op Amp and is the ratio between two voltages. Open Loop gain in Op Amps is normally 105.

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The following example will make use of the equation. Figure 3 shows an Op Amp with an open loop voltage gain of 400, connected between a 12V supply.

Op Amp Figure 3

V

Ι

= 5.88V V

N

= 5.87 A

O

= 400

Using the equation:

V

O

= A

O

(V

N

- V

Ι

)

V

O

= 400(5.87 – 5.88)

= 400(-0.01)

= -4V

The voltage is relative to a point halfway between +12v and zero, that is 6V. The output voltage is therefore 4V below 6V, i.e. 2V. What would the output be if the input values were reversed?

Ans:………. V_OUT 5.87V 5.88V +12V GAIN = 400 ZERO LINE

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1.7 THE IDEAL OPERATIONAL AMPLIFIER

Although the characteristics of an ideal operational amplifier are unattainable, modern integrated circuit types can provide an extremely close approximation. The ideal characteristics are:

* A very large open loop gain, near infinite,

* Output unaffected by signal frequency, no signal phase shift with change in frequency,

* A very large (infinite) input impedance so that the amplifier takes negligible current,

* A very small output impedance so that the output of the amplifier is unaffected by loading,

* Output voltage is zero for zero input voltage (offset zero applied). Naturally, no practical operational amplifier will be this perfect, which means of course that there will be small operational errors with such devices.

Therefore, the closer to the ideal properties the amplifier is made, the smaller will be these errors.

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ELECTRONIC FUNDAMENTALS PRINTED CIRCUITS BOARD PAGE 1 of 3

1 PRINTED CIRCUIT BOARDS

Aircraft electronic systems necessitate the interconnection of many

components; in the past this was done by soldered or crimped terminations. With the development of circuit technology and micro miniaturisation, weight saving and simplification of installation and maintenance became needful and these needs were met by the development of the printed circuit board.

1.1 CONSTRUCTION

Printed circuit board is a laminated paper or fibreglass board coated on one side with a thin layer of copper. The areas of copper, called 'lands', required to connect the components are marked out by painting over the copper, and the remaining copper is etched away by a solution of ferric chloride. Holes are then drilled in the board for the component leads. The advantage is that the copper strips can be any shape and few additional wires are required. Industry can produce printed circuit boards in large numbers very cheaply so they have become the standard circuit construction method. Figure 1 shows the front face of a PCB, with Figure 2 showing the rear face.

Printed Circuit Board Figure 1 BASE

BOARD

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Printed Circuit Board Figure 2

IC3 IC4

IC6 C2

IC1 IC2 IC5

REAR CIRCUIT MODULE DESIGNATION (E.G. SIGNAL SELECTOR) CIRCUIT REFERENCE FINGER OR EDGE CONNECTOR INTEGRATED CIRCUIT CHIPS

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1.2 MULTI-LAYER CIRCUITS

In order to save weight and space, and to provide for the interconnection of integrated circuits (which are a feature of a large majority of electronic equipment) the relevant circuits are assembled as a multi-layer moulded package. This consists of three or more single and/or double-sided printed boards and insulating layers of ‘impreg’ material.

1.3 HANDLING PRINTED CIRCUIT BOARDS

Since various types of semi-conductor components are mounted on printed circuit boards, care must always be taken in handling techniques.

General techniques are as follows: -

a) Do not remove or replace units with electrical power applied. b) Do not touch the connectors, leads or edge connectors of circuit

boards unnecessarily.

c) Use conductive packaging, shorting plugs, bands or wire when

provided or prescribed by the relevant aircraft Maintenance Manual. d) Pay strict attention to stores procedures to ensure that protective

packaging is not removed during any goods-inwards inspection.

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ELECTRONIC FUNDAMENTALS SERVOMECHANISM PAGE 1 of 23

1 SERVOMECHANISMS

A servomechanism (servo) is a type of control system whose output is the position of a shaft. They may be controlled remotely when used in

conjunction with synchro devices. Synchros themselves transmit position information but cannot amplify torque to move heavy loads. Used with

servomechanisms, an output to control such a load can be obtained to give a desired result in relation to an input.

1.1 OPEN LOOP SYSTEM

In this system, an input is applied and an output obtained. Figure 1 shows an example; assume an aircraft rudder controlled by an open loop system.

Open Loop System Figure 1

The demand, made by the pilot on the rudder bar, is picked up by the transducer which converts it to an electrical signal; i.e. the demand signal. This signal is amplified and fed to the motor, which responds by moving the load; i.e. the rudder. There is no positional feedback and the pilot does not know if the rudder has adopted the position requested.

INPUT TRANSDUCER MOTOR

LOAD

DEMAND RESPONSE DEMAND SIGNAL AMP

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1.2 CLOSED LOOP SYSTEM

In the closed loop system, the demand is made in the same way. In a basic system, positional feedback would be given to the pilot who would make adjustments accordingly but this is not practical with systems such as aircraft flying controls. Figure 2 shows a closed loop automatic system.

Closed Loop System Figure 2

An output position transducer has been added to the servomotor and this feeds back any difference between input demand and output to an error detector. The error detector outputs an error signal to the amplifier to make any positional corrections necessary at the servo motor and thus the load (or rudder) is positioned as demanded.

If for example the pilot wanted to move the rudder 5°, a demand is made at the rudder bar and this is converted to a voltage at the transducer, say +5 volts. The error detector immediately gives an output signal corresponding to +5 volts input and this is amplified to drive the motor, moving the rudder. The output position transducer converts the output position to an electrical signal, which corresponds to the new position of the rudder. As this happens, this signal, (feedback), is fed back to the error detector until the demanded position is achieved and the input is negated. Now, there is no error signal and no output. The feedback has reached -5 volts.

INPUT TRANSDUCER SERVO MOTOR LOAD OUTPUT POSITION TRANSDUCER ERROR DETECTOR POSITION FEEDBACK ERROR SIGNAL AMP

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1.3 FOLLOW UP

If in our example the rudder were to be displaced from its demanded position, or from the optimum speed at which the demanded position may be achieved, an error signal occurs. In the way described, there is a feedback signal and the system returns to its demanded position or speed. This process is called 'follow up'.

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1.4 FEEDBACK

1.4.1 POSITIONAL FEEDBACK

Positional feedback is obtained from transducers positioned at the output. The feedback element, or transducer, converts the output shaft angle into a signal suitable for operating the error detector. In this case a voltage signal. The simplest form of element is a R-pot, or a helical potentiometer similar to that used as a control element. In practice, helical potentiometers are used since they give 360° coverage, which a R-pot cannot provide. Figure 3 shows positional feedback in a dc system.

Positional Feedback Figure 3 ERROR DETECTOR SERVO MOTOR LOAD TACHO GEN FEEDBACK ELEMENT POSITIONAL FEEDBACK VELOCITY FEEDBACK CONTROL ELEMENT

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Figure 4 shows a R-Pot & Helical Potentiometer

R-Pot & Helical Potentiometer Figure 4

In ac systems, other components are used to provide positional feedback. Synchros are employed in some servomechanisms. These will be discussed later. E θ i PROPORTIONAL TO θ i Ei R-POT HELICAL POTENTIOMETER E θ i Ei PROPORTIONAL TO θ i

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1.5 ROTARY VARIABLE DIFFERENTIAL TRANSDUCER (RVDT)

The RVDT is an inductance transmitter having a primary stator coil, an iron rotor coil and two secondary stator coils. Figure 5 shows the operation of a RVDT.

RVDT Operation Figure 5

The mechanical input changes the position of the iron core. The position of the core changes the magnetic coupling between the primary and the secondary stator coils. When the input rotates, one of the secondary coils receives more magnetic flux and this induces a higher voltage in that coil.

R S T L3 L1 L2 IRON CORE CONNECTED TO MECHANICAL INPUT PRIMARY COIL R S T R S T

1. ZERO POSITION 2. ROTATED CLOCKWISE

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The other secondary coil receives less magnetic flux, so a lower voltage is induced. The difference between voltages induced in the secondary stator coils is proportional to the rotated angle. This is an AC Ratio Signal.

Figure 5.1: The position of the iron core is zero. The magnetic field induced by primary coil L3 is equally divided between L1 and L2.

Therefore the voltage R-T is zero.

Figure 5.2: The iron core is turned clockwise. Now there is more coupling between L3 and L2, and less coupling between L3 and L1. The voltage between T and S increases and the voltage between R and S decreases.

Figure 5.3: The iron core turned counter-clockwise. Now there is more coupling between L3 and L1, and less coupling between L3 and L2. The voltage between T and S decreases, while the voltage between R and S increases.

The difference between figure 5.2 and 5.3 is that the output-voltage between R and T is of opposite phase. The output measured between R and T is an AC RATIO signal.

The Linear Variable Differential Transducer (LVDT) is also an inductance transmitter with similar components and similar in operation but of course, the movement detected is linear and not rotary.

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1.6 CAPACITANCE TRANSMITTER

An example of a capacitance transmitter can be seen in a simple fuel gauging system as in Figure 6.

Capacitance Transmitter Figure 6

This system depends upon the comparison of two capacitance values. One in Loop A, which is the variable capacitance of a tank unit and the other in Loop B, which is fixed. A current is developed in each loop; IS in loop A; IB in loop

B. The two loops form a bridge with resistor R across it. If the tank is full, then current IS is the greater. With the tank empty, IS falls so that IB is the

greater.

Note: The currents act in opposite directions so that a potential is developed across resistor R of a polarity dependent on the direction of current flow and of a magnitude dependent on the size of the current. This signal is

transmitted to an amplifier, which powers a 2-phase motor to drive an indicator and a balance potentiometer.

TANK UNIT FULL EMPTY AMPLIFIER STAGE REF C 2 - PHASE MOTOR AMPLIFIER UNIT REF PHASE INDICATOR LOOP A LOOP B

I

S

I

B DISCRIMINATION STAGE

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When the balance potentiometer moves as a result of change in fuel level, it adjusts IB, rebalancing the bridge formed by loop A and loop B. Now, no

current flows through resistor R, no signal is developed across R and the new fuel level is displayed at the indicator.

1.7 SYNCHROS

1.7.1 INTRODUCTION

AC transmission systems are generally known as synchros because of their synchronous action in reproducing the angular movement of a shaft. As mentioned previously, they cannot transmit torque to any appreciable degree but can be used in conjunction with servomechanisms.

1.8 TORQUE SYNCHRO

1.8.1 PRINCIPLE OF OPERATION

The principle of a synchro is that of the transformer, where the primary winding is wound onto a rotor and is rotated with respect to a fixed stator winding. The size and phase of the output voltage is dependent on the direction and angular displacement between the primary and secondary windings.

The torque synchro comprises two electrically similar units: the transmitter (TX) and the receiver (TR) which are interconnected by transmission lines. The TX and TR have very similar construction. Each has a rotor carrying a single winding concentrically mounted in a stator of three windings, the axes of which are 120° apart. It should be noted that the TX and TR torque synchros are not identical. The difference is that the TR synchro has an oscillation damper added, so that when its rotor rotates to a given position, it does not oscillate as it comes to rest.

The rotors of both TX and TR synchros are energized from the ac supply and produce an alternating flux which links with their corresponding stators S1, S2

and S3. This process is the normal transformer action, with the rotors

corresponding to the transformer primary winding and the stators to the secondary windings.

Consider the case when the two rotors are not aligned. The three voltages induced in each of the two sets of stator windings are different. Currents therefore flow between the two stators and a torque is produced in each synchro which is directed in such a way that the two rotors must align themselves. Normally, the TX rotor position is controlled by the input shaft, while the TR rotor is free to turn, so it is the one which aligns itself with the TX rotor. In this way, any movement of the TX rotor due to movement of the input shaft is repeated synchronously by movement of the receiver rotor.

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Torque synchros are used for the transmission of angular position information and flight instrument systems is a typical application. Figure 9 shows a

Torque Synchro and circuit symbol.

Torque Synchro Figure 9 ROTOR FIELD CURRENT FLOW STATOR FIELD S1 S2 S3 R1 R2 INPUT SHAFT OUTPUT SHAFT S1 _S1 S3 S2 S3 S2 CIRCUIT SYMBOL

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Figure 10 shows the construction of a torque synchro.

Torque Synchro Construction Figure 10

STATOR

ROTOR

COMPLETE

ASEMBLY

STATOR WINDINGS SHELL LOWER END CAP SHAFT BEARING COILS CORE LEADS TO SLIP RINGS SLIP RINGS STATOR