EASA PART 66 Module 4_electronic Fundamentals

7/29/2019 EASA PART 66 Module 4_electronic Fundamentals

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Index

1 SEMICONDUCTOR DEVICES...................................................... 1-1

1.1 RECTIFIER DIODES .................................................................1-21.1.1 Circuit Symbols & Identification.............................. 1-21.1.2 Operating Characteristics....................................... 1-31.1.3 Parallel & Serial Arrangements of Diodes .............. 1-41.1.4 Rectification............................................................1-5

1.2 SIGNAL DIODES......................................................................1-10

1.3 ZENER DIODES.......................................................................1-10

1.4 LIGHT EMITTING DIODES..........................................................1-11

1.5 PHOTOCELLS .........................................................................1-111.5.1 Photoconductive Cells............................................1-111.5.2 Photovoltaic Cells................................................... 1-12

1.6 PHOTODIODES .......................................................................1-12

1.7 VARACTOR DIODE ..................................................................1-12

1.8 SILICON CONTROLLED RECTIFIER.............................................1-13

1.9 TRANSISTORS ........................................................................1-13

1.9.1 NPN Transistor....................................................... 1-141.9.2 PNP Transistor.......................................................1-16

1.10 TESTING SEMICONDUCTOR DEVICES ........................................1-181.10.1 Testing Diodes .......................................................1-181.10.2 Testing Transistors.................................................1-19

2 OPERATIONAL AMPLIFIERS...................................................... 2-1

2.1 THE PERFECT AMPLIFIER ........................................................2-1

2.2 OP AMP SPECIFICATION ..........................................................2-1

2.3 POWER REQUIREMENTS..........................................................2-2

2.4 PIN OUTS &CIRCUIT SYMBOL ..................................................2-22.5 OPERATION ...........................................................................2-3

2.5.1 Negative Feedback ................................................ 2-3

2.6 OP-AMP COMPARATOR ...........................................................2-5

2.7 OP AMP SUMMING AMP ...........................................................2-6

3 PRINTED CIRCUIT BOARDS....................................................... 3-1

3.1 BASE MATERIAL .....................................................................3-2

3.2 CONDUCTOR MATERIAL ..........................................................3-2

3.3 BONDING OF CONDUCTOR MATERIAL........................................3-2

3.3.1 Inspections & Tests................................................ 3-33.4 MACHINING OF BOARDS ..........................................................3-4

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3.5 CIRCUIT ARTWORK.................................................................3-43.6 PRINTING OF CIRCUITS ...........................................................3-5

3.6.1 Etching Process .....................................................3-53.6.2 Additive Process..................................................... 3-63.6.3 Inspection...............................................................3-7

3.7 SOLDERING METHODS ............................................................3-73.7.1 Hand Soldering ......................................................3-73.7.2 Mass Soldering ......................................................3-7

3.8 SOLDER SPECIFICATION..........................................................3-9

3.9 FLUXES &THEIR APPLICATION.................................................3-9

3.10 SOLDER RESISTS ...................................................................3-103.11 PLATING OF PRINTED WIRING CIRCUITS ....................................3-10

3.11.1 Through-Hole Plating.............................................3-10

3.12 ORGANIC PROTECTIVE COATINGS ............................................3-11

3.13 FLEXIBLE PRINTED WIRING CIRCUITS ........................................3-11

3.14 HANDLING OF CIRCUIT BOARDS ...............................................3-123.14.1 Electrostatic Discharge Sensitive Devices ............. 3-123.14.2 Removal & Installation of ESDS Printed Circuit Boards 3-153.14.3 Removal & Installation of Metal-Encased ESDS LRU's 3-16

4 SYNCHRONOUS DATA TRANSMISSION................................... 4-14.1 DESYNN SYSTEM....................................................................4-1

4.1.1 The Basic Desynn.................................................. 4-14.1.2 Slab Desynn........................................................... 4-4

4.2 SYNCHRO SYSTEMS ...............................................................4-44.2.1 Synchro Types .......................................................4-54.2.2 Synchro Schematics............................................... 4-74.2.3 XYZ Synchro system.............................................. 4-94.2.4 Synchro Supplies ................................................... 4-94.2.5 Torque Synchro System......................................... 4-104.2.6 Electrical Zero ........................................................4-134.2.7 Differential Torque Synchro System....................... 4-144.2.8 Control Synchro System.........................................4-164.2.9 Differential Control Synchros.................................. 4-20

5 SERVO SYSTEMS........................................................................5-1

5.1 CATEGORIESOFSERVOSYSTEMS ...............................5-15.1.1 open loop ............................................................... 5-15.1.2 closed loop............................................................. 5-2

5.2 REMOTE POSITION CONTROL SERVOMECHANISMS .....................5-35.2.1 Positional Feedback...............................................5-3

5.3 TYPES OF INPUTS ..................................................................5-55.3.1 Step Input............................................................... 5-5







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5.3.2 Ramp Input.............................................................5-55.3.3 Accelerating Input................................................... 5-5

5.4 SYSTEM RESPONSE...............................................................5-6

5.5 DAMPING ..............................................................................5-75.5.1 Frictional Forces which Produce Damping............. 5-75.5.2 Velocity Feedback Damping................................... 5-9

5.6 VELOCITY CONTROL SERVOMECHANISMS .................................5-115.6.1 Residual Error........................................................5-115.6.2 Velocity Lag............................................................5-11

5.7 A.C. SERVOMECHANISM COMPONENTS .....................................5-12

5.7.1 E & I Bar Transducer..............................................5-125.7.2 A.C. Tachogenerators............................................5-13

5.8 PRACTICAL SERVO SYSTEMS ...................................................5-155.8.1 Direct Servo Current System.................................. 5-155.8.2 Alternating Current Servo System.......................... 5-16

6 OTHER TRANSDUCERS ............................................................. 6-1

6.1 LINEAR VARIABLE DIFFERENTIAL TRANSFORMER .......................6-1

6.2 ROTARY VARIABLE TRANSFORMER ...........................................6-2

6.3 INDUCTIVE TYPE TRANSDUCERS ..............................................6-26.3.1 Induced EMF Type................................................. 6-26.3.2 A.C. Current Control............................................... 6-3







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1 SEMICONDUCTOR DEVICES

The early discoveries in the field of electricity made by Volta, AmpHata! Yer iaretitanmlanmam.ere, Gains, Faraday, Hertz and others raised fundamentalproblems concerning the nature of matter.

The first breakthrough came in 1897, when Sir J .J . Thompson discovered theelectron, a discovery soon verified by other investigators. In 1913 Bohr evolved thebasic theory of atomic structure, and that theory has been developed to our presentconcept of the nature of matter.

The electrical characteristics of an atom are determined by how tightly the nucleusholds on to its outer electrons. If the outer electrons are easily removed from theatom, the material will conduct easily and is known as a conductor. If the outerelectrons are difficult to dislodge from their orbits, the material is known as aninsulator.

The material used in diodes and transistors is known as 'semi-conductor' material.One of the attributes of this material is that the number of free electrons in any givenarea can be fixed during the manufacturing process.

Interest in semi-conductors began in 1873, when it was discovered that the

resistance of rods and wires of selenium decreased as they were heated. This wassurprising because the resistance of metals normally increased with an increase intemperature. Furthermore, some lowering of resistance was noted when the rodswere exposed to light. Later investigations found similar effects in other materials,but the change in resistance was so small that no practical applications could befound.

By 1906 a number of crystalline semi-conductors were being used as radio signaldetectors, but the introduction of thermionic valves put an end to them. The valveswere more reliable and had the advantage of being able to amplify the signal as wellas detect it.

During the development of radar systems in WW, it was discovered that valve typemixers would not operate at the high frequencies being used. Research turned tosemi-conductor type mixers, and silicon proved the most successful. After the war,the peculiar properties of Germanium and Silicon were rigorously investigated, anda germanium diode detector was made and used extensively in radio and television.

During development of the Germanium detectors an important discovery was made.It was found that when two very close contacts are made with a piece of germanium,the current flow through one of the contacts affects the amount of current flowthrough the other.

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Bell Telephone Laboratories latched onto this phenomena and eventually in 1948they announced the manufacture of the first solid-state amplifying device, thetransistor. This triggered renewed interest in semi-conductor diodes, resulting in thedevelopment of a huge variety of semi-conductor devices that we now take forgranted.

1.1 RECTIFIER DIODES

A rectifier diode is the electrical equivalent of a one way valve, it is a semiconductordevice which allows current to flow in one direction but not in the other.

When conducting, the diode is said to be 'forward biased'. Under these conditionsthe diode offers little resistance to current flow.

When opposing current flow, the diode is said to be 'reverse biased'. Under reversebiased conditions the diode has a high resistance.

1.1.1 CIRCUIT SYMBOLS & IDENTIFICATION

The various symbols used for diodes are shown below.

Whether the triangles are filled or unfilled depends only on the drawing officepreference. Where it is considered necessary, it is possible to show that one of theelectrodes is connected to the case of the device by adding a dot to the symbol, butthis is not often used. In every symbol, the arrow indicates the direction ofconventional current flow.

The base of the triangle is the end where conventional current enters the diode, thisend is called the anode. The end through which current leaves the diode is thecathode. In some cases the arrow symbol is marked on the diode, where it is not,the cathode is identified by a band or distinctive shape as shown above.

Two identification codes are used for diodes. In the American system the codealways starts with 1N and is followed by a serial number, i.e. 1N4001. In thecontinental system, the first letter gives the semiconductor material; A forgermanium; B for silicon, and the second letter identifies the use; A - signal diode; Y

- rectifier diode and Z for zener diode. To complicate the situation somemanufacturers have their own codes.







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1.1.2 OPERATING CHARACTERISTICS

Most semiconductor diodes are made from silicon or germanium, these twomaterials have different operating characteristics, although the principle of operationand circuit symbols are both the same.

1.1.2.1 Biasing

A diode is said to be 'biased' when a voltage is applied between the terminals suchthat the diode operates as required.

An external voltage applied so that the anode is positive and the cathode negative is

called 'forward bias'. There are many ways of achieving this, for example:

Connect the anode to +3V and the cathode to 0V.

Connect the anode to +1V and the cathode to -1V.

Connect the anode to -50V and the cathode to -52V.

So far as the diode is concerned, it is the voltage of the anode with respect to thecathode which determines the bias.

If the voltage is applied so that the anode is negative with respect to the cathode,

the diode is reverse biased, again, there are many ways of achieving this.The forward voltage required to make the diode conduct depends on the material itis made from. Germanium diodes require a voltage of approximately 0.1 to 0.2 voltsand silicon diodes 0.6 to 0.7 volts.

1.1.2.2 Forward Voltage Drop

Ideally a diode should have zero resistance when conducting and should cause novoltage drop, unfortunately this does not happen. Germanium diodes create avoltage drop of approximately 0.6V and silicon diodes a drop of approximately 1.1V.

This needs to be taken into account when doing circuit calculations.

1.1.2.3 Reverse Leakage Current

When a diode is reverse biased, it should ideally have infinite resistance and nocurrent should flow. Unfortunately when a diode is reverse biased, a small currentcalled 'reverse leakage current' flows, generally this is too small to be ofsignificance, however, it should be noted that the value of this current increases withan increase in diode temperature. The reverse current of silicon diodes is muchsmaller than that of germanium diodes, (approx. one thousandth), therefore silicondiodes can be used more successfully at high temperatures (150 - 200C) thangermanium diodes (80 - 100C).







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1.1.2.4 Reverse Breakdown Voltage

If the reverse bias voltage is increased, eventually the diode breaks down andcurrent flows in the wrong direction through the diode. This causes permanentdamage and the diode has to be replaced.

The breakdown voltage can have any value from a few volts, up to 1000V for silicondiodes and 100V for germanium, depending on the construction and forms ofmaterial used.

The maximum reverse voltage is an important diode characteristic. Under normalconditions this value should not be exceeded.

1.1.2.5 Graphical Representation

Shown below is a graphical representation of the operating characteristics of atypical silicon and germanium diode.

1.1.3 PARALLEL & SERIAL ARRANGEMENTS OF DIODES

It is possible to operate silicon rectifier diodes in parallel or in series to providerespectively, higher current or higher voltage capabilities.

1.1.3.1 Parallel Arrangements

In parallel arrangements used for higher currents, some method must be used toensure that the current divides equally through the individual diodes. This is difficultto do.







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1.1.3.2 Series Arrangements

Series arrangements can be used if the applied voltage is greater than themaximum rated value of a single diode. Some method must be used to ensure theapplied voltage divides equally among the individual diodes. Resistors or capacitorsin parallel can be used in an effort to achieve this.

1.1.4 RECTIFICATION

Rectifier diodes are designed to convert ac to dc. To do this effectively andefficiently they must have:

Low resistance to current flow in the forward direction.

High resistance to current flow in the reverse direction.

Almost all semiconductor rectifier diodes are silicon, junction types. The symbolused in circuit diagrams can be any of those shown earlier in the notes.

1.1.4.1 Basic Rectifier Circuit

A basic rectifier circuit is shown below. The diode is inserted in series between thea.c. supply and the load.

The diode only passes current when forward biased. Thus when an a.c. signal isapplied, pulses of uni-directional (d.c.) voltage are developed across the output loadresistance.

Note from the diagrams that the d.c. polarity can be reversed by reversing the diodeconnections.

If the average value of wave rectified a.c. is calculated it will be found to be 32%of the peak value of the output voltage.







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1.1.4.2 Centre Tap Full Wave Rectifier

In full wave rectification, both halves of every cycle of input voltage produce currentpulses through the load resistor.

In the circuit shown above, two diodes D1 and D2 and a transformer with a centre-tapped secondary are used.

During the positive half cycle of the input waveform, A is positive with respect to Oand D1 conducts, the current flowing top to bottom through the load resistor. Duringthis time diode D2 is reversed biased and does not conduct.

During the negative half cycle of the input waveform, B is positive with respect to Oand D2 conducts, the current again flowing top to bottom through the load resistor.During this time diode D1 is reverse biased and does not conduct.

In effect, the circuit consists of two half wave rectifiers working into the same load onalternate half cycles of the input. The current through R is in the same directionduring both half cycles and a fluctuating d.c. is created across R.

The average value of this full wave rectified a.c. is 64% of the peak value of thevoltage across the load resistor R.

The output frequency is double that of the input frequency.







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1.1.4.3 Full Wave Bridge Rectifier

The circuit of a Full Wave Bridge rectifier is shown below. The rectifier has 4 diodesas opposed to 2 and does not have a centre tapped transformer.

During the positive half cycle diodes D1 and D2 conduct, the current flowing top tobottom through the load RL. During the negative half cycle D3 and D4 conduct, the

current again flowing top to bottom through the load. The output from this rectifier isthe same as that obtained from the centred tapped transformer type. The averagevalue again being 64% of the peak voltage across the load resistor.

It should be noted that in this rectifier, the peak voltage across R L is equal to thewhole of the secondary transformer output voltage, whereas in the previous rectifier,the peak voltage across RL is only half the transformer secondary voltage.

1.1.4.4 Smoothing

The rectifier circuits previously discussed produce pulsating d.c. outputs. A

smoothing circuit changes these outputs into a steady d.c. voltage level.

1.1.4.4.1 Half Wave Rectifier

The diagram below shows a simple half wave rectifier with a reservoir capacitor, C,connected in parallel with the load RL. The capacitor charges towards the peakvalue of the input voltage whenever the input voltage is greater than VC and thediode is conducting. When the input voltage is less than VC the diode cuts-off andthe capacitor discharges through the load.







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This results in a mean d.c. output level less than the peak of the input, with a ripplesuperimposed at the input frequency.

1.1.4.4.2 Full Wave Rectifier

The diagram above shows a centre tapped full wave rectifier with a reservoircapacitor. The charge is now topped up twice during each cycle of the inputwaveform which results in:

A lower amplitude ripple, at twice the frequency of that from the half waverectifier.

A higher mean d.c. output than that from a similarly loaded half wave rectifier.







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1.1.4.5 Ripple Factor

A measure of the amount of ripple present at the output of a d.c. supply is given bythe ripple factor, which is usually expressed as a percentage and defined as:

Ripple factor = Hata! 100%

1.1.4.6 Peak Inverse Voltage

The peak voltage across a rectifier diode in the reverse direction is known as the'peak inverse voltage'. In a half wave rectifier with a reservoir capacitor, the peakinverse voltage is twice the amplitude of the peak voltage across the load. i.e. mean

d.c. level to maximum negative peak. The diode must be able to withstand thisvoltage without breaking down.

1.1.4.7 Voltage Regulation

Voltage regulation is a measure of the ability of a power supply to provide anincreased load without a fall in output voltage.

Regulation = Hata! 100%

1.1.4.8 Filter Circuits

Smaller ripple factors and improved voltage regulation is obtained by using R-C andL-C filter circuits across the output of the rectifier.

1.1.4.9 Ripple Frequency

The ripple frequency on the d.c. output from a half wave rectifier is equal to thesupply frequency. For a full wave rectifier, the ripple frequency is double the supplyfrequency.







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1.2 SIGNAL DIODES

Signal diodes are used to detect radio signals (a process similar to rectification inwhich radio frequency a.c. is converted to d.c.), because of their very lowcapacitance. A capacitor passes a.c. The higher the frequency of the a.c. and thegreater the capacitance, the less opposition it offers. At radio frequencies, a normaldiode would be of little use as a detector because of its large junction area. Thelarge junction area resulting in a large capacitance value and little opposition tocurrent flow.

A point diode type signal diode has a very small junction area resulting in a lowvalue of capacitance and a large opposition to current flow.

Germanium is used for signal diodes since it has a lower 'turn-on' voltage thansilicon, and so lower signal voltages start it conducting in the forward direction.

1.3 ZENER DIODES

In an ordinary diode, if the reverse bias is increased, the diode breaks down and thediode suffers permanent damage. A zener diode is designed to be used in thebreakdown region. The zener diode looks like a rectifier diode, the cathode oftenbeing marked by a band. Its symbol is shown above.

From the characteristic graph, it can be seen that the reverse current is negligible asthe reverse bias is increased until the breakdown voltage is reached, then itsuddenly increases. The breakdown voltage is called the zener or referencevoltage.







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The important thing is that the voltage across the diode remains almost constantover a wide range of reverse currents. It is this property of a zener diode that makesit useful in stabilised power supplies.

To limit the reverse current at breakdown and prevent overheating, the power ratingof the diode must not be exceeded. This is achieved by using a resistor in serieswith the diode.

1.4 LIGHT EMITTING DIODES

A light emitting diode is a specially constructed and doped diode type device which

emits light when operated in the forward bias condition. The colour of light emitteddepends on the semi-conductor material used.

Gallium arsenide phosphide - red light

Gallium phosphide - green light

Symbols used are similar to the photodiode.

Unless an LED is of the constant current type, which incorporates an integratedcircuit regulator, it must have an external resistor connected in series to limit theforward current which typically may only be 10mA. The voltage drop across aconducting LED is about 1 to7 volts.

In seven segment LED displays, each segment is a separate LED and depending onwhich segments are energised, the display lights up the number 0 to 9. Suchdisplays are usually designed to operate from a 5V supply - each segment needs aseparate current limiting resistor and all the cathodes or anodes are joined together

to form a common connection.

1.5 PHOTOCELLS

Photocells change light into electrical signals. There are two basic types,Photoconductive cells and Photovoltaic cells.

1.5.1 PHOTOCONDUCTIVE CELLS

The resistance of certain semiconductors decreases as the intensityof light falling on them increases. They are therefore light sensitive

resistors and sometimes referred to as light dependent resistors.







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1.5.2 PHOTOVOLTAIC CELLS

When illuminated, a photovoltaic cell produces a voltage. If anexternal circuit is connected to the cell, current flows through it. Thesource of energy is the light.

The voltage available depends on the material used, the intensity of the light and theamount of current drawn from the cell. For a silicon cell in full sunlight the voltageon open circuit is 0.45V. With a maximum current of 35mA for each square cm of

cell. Only about 10% of the light is turned into electrical energy.

1.6 PHOTODIODES

Photodiodes are operated under reverse bias conditions. Theleakage current increasing in proportion to the amount of light fallingon the device. Photodiodes are used as fast counters and lightmeters.

1.7 VARACTOR DIODE

A varactor diode is a special type of diode constructed to act as a voltage controlledcapacitor. It is also known as a varicap diode. The diode is operated under reversebias conditions, with an increase in bias decreasing the value of capacitance. Thecircuit symbols are as shown below.

There are 3 main uses for varactor diodes:

As remotely controlled capacitors in RF tuned circuits.

As variable capacitors in amplifiers.

As variable capacitors in frequency modulator circuits.







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1.8 SILICON CONTROLLED RECTIFIER

Silicon controlled rectifiers (SCR's) are now more commonly known as thyristors.They are semiconductor devices which rectify a.c. and control the power supplied toa load in a way that wastes very little energy. They are commonly used inhousehold lighting dimmer switches. The general symbol is shown below, togetherwith the symbol for 'P' and 'N' types.

SCR's normally block the flow of current in both directions, but can be triggered soas to allow current to flow in the forward direction as in a normal diode, whilst stillblocking current flow in the reverse direction. In the triggered condition thecharacteristics are similar to rectifier diodes.

An SCR will continue to conduct until the load current is reduced to zero, or until it is

reverse biased, when it automatically returns to the blocking state.

The SCR is triggered by applying a pulse to a third terminal called the gate. Theduration of the pulses can be extremely short.

1.9 TRANSISTORS

Transistors are the most important device in electronics today. Not only are theymade as discrete components, but integrated circuits may contain severalthousands on a tiny slice of silicon. They are 3 terminal devices used as amplifiersand as switches, and are classed as active devices.

Hundreds of different transistors are available. The same identification code is usedas for diodes, but in the American system transistors always start with 2N followedby a number. In the continental system the first letter gives the semiconductormaterial and the second letter gives the use:

C indicates an audio frequency device.

F a radio frequency device.

S a switching transistor.

An example being BC108, a silicon audio frequency amplifier device.







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The two basic types of transistors are: The bipolar or junction transistor.

The unipolar or field effect transistor.

In this element of the course we concentrate on bipolar transistors, of which thereare two basic types. The NPN and the PNP, both of which are active deviceshaving three terminals labelled; Base, Collector and Emitter.

1.9.1 NPN TRANSISTOR

NPN transistors are made from 3 pieces of semi-conductor material joined togetherin a manner similar to two diodes, as shown in the diagram below. Also shown isthe circuit diagram with each terminal identified.

If the base is made positive with respect to the collector, the diode, orjunction as itis called, is forward biased and current flows (conventional current flows from baseto collector).

If the base is made positive with respect to the emitter, again the junction (diode) isforward biased and conventional current flows from base to emitter.

If the collector is made positive with respect to the emitter, or the emitter is madepositive with respect to the collector no current will flow, because in either directionone of the junctions (diodes) is reverse biased and will prevent current flow.

The last three paragraphs should be noted, as their contents is invaluable when itcomes to determining the terminals and testing transistors. This will be discussedlater.







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1.9.1.1 NPN Transistor as a switch

If the NPN transistor isconnected as shown, it can beused as a switch.

In this diagram the transistor isbeing used to turn on a lamp, itcould however be used tooperate any type of d.c. device

such as a relay, solenoid,another transistor or an LED.

When the base is made positive with respect to the emitter, the junction is forwardbiased and current flows in through the base and out of the emitter. The flow ofcurrent from base to emitter creates a reaction in the transistor that causes thereverse biased collector/base junction to break down and conduct. Current can thenflow from the battery positive terminal, through the lamp, through the reverse biasedcollector base junction, through the forward biased base emitter junction and back tothe battery, illuminating the lamp.

When the base is made sufficiently positive with respect to the emitter (approx. 0.6Vfor silicon, 0.2V for germanium) so that current flows from collector to emitterthrough the transistor, the transistor is said to be switched or turned 'ON'.

If the base / emitter potential is reduced below the switch 'ON' potential, or removedtotally, the collector / base junction will return to its reverse bias condition and willprevent current flowing around the circuit through the lamp. Under these conditionsthe transistor is said to be switched or turned 'OFF'.

If should be noted that it may be necessary to limit the current through the transistorwhen it is switched on, this can be achieved by a series resistor as in the LED

circuit.

1.9.1.2 NPN Transistor as an amplifier

When the base is made positive with respect to the emitter so that the transistor isswitched 'ON', the amount of base emitter current required is very small. If the base/ emitter current is increased slightly, by increasing the base emitter voltage, thetransistor will turn 'ON' more, its effective resistance will decrease and the collector /emitter current will increase.







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If the base / emitter current is decreasedslightly by reducing the base / emittervoltage, the transistor will turn 'OFF' more,it's effective resistance will increase and thecollector / emitter current will decrease.

The transistor can therefore be likened to a variable resistor. As the base / emitterbias increases, the resistance of the transistor effectively decreases and morecurrent flows from collector to emitter. The change in current and resistance causes

the output voltage to decrease.As the base / emitter bias decreases, the effective resistance of the transistorincreases and less current flows from collector to emitter. The change in currentand resistance now causes the output voltage to increase.

When set up correctly, millivolt changes across the base / emitter junction producechanges at the output of 10's or even 100's of volts, depending on the collectorvoltage.

If a small sinusoidal a.c. signal is applied to the base / emitter junction, the bias willvary sinusoidally as will the resistance of the transistor and the output voltage,

however the output voltage will vary sinusoidally 10's of volts for millivolt changes inthe input signal. (Using the example voltage in the diagram).

It should be noted, that although the changes in output voltage are much greaterthan the changes in input voltage, the bipolar transistor is a current device.Small changes in base / emitter current result in large changes in collector / emittercurrent. It is these changes in collect / emitter current that produce the large outputvoltage swings.

1.9.2 PNP TRANSISTOR

PNP transistors are made in a similar manner to NPN transistors, except thedirection of the junctions is reversed.

If the base is made negative with respect to the collector, the diode, or junction isforward biased and current flows.







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If the base is made negative with respect to the emitter, the junction is forwardbiased and current flows.

Current cannot flow between collector and emitter, because irrespective of the biasapplied, one junction will be reverse biased.

Again these three statements are worth remembering when it comes to determiningthe terminals and testing transistors.

1.9.2.1 PNP Transistor as a switch

When connected as shown, the PNP

transistor can also be used as aswitch, however, for the transistor tobe tuned 'ON', the base must bemade negative with respect to theemitter. For a silicon transistor thebase needs to be about 0.6Vnegative with respect to the emitter,for a germanium transistor 0.2Vnegative.

Once turned 'ON', conventional current flows from the emitter to the collector, which

is in the opposite direction to that in the NPN transistor.

1.9.2.2 PNP Transistor as an amplifier

The PNP transistor can also be used an amplifier. It operates in a similar manner tothe NPN transistor except the transistor must be turned 'ON' by making the basenegative with respect to the emitter, as seen above. If the base / emitter potential isincreased by making the base more negative with respect to the emitter, thetransistor turns 'ON' more, its effective resistance decreases and more emitter /collector current flows. If the bias potential is decreased, by making the base lessnegative with respect to the emitter, the transistor turns 'OFF' slightly, the effective

resistance increases and less emitter / collector current flows.

A small sinusoidal signal applied to the base will vary the effective resistance of thetransistor and produce much larger changes in the output voltage as with the NPNtransistor. Again it must be realised that the transistor is a current device. Thesmall changes in base emitter bias potential created by the input signal results insmall changes in base emitter current, resulting in large changes in collector /emitter current.







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1.10 TESTING SEMICONDUCTOR DEVICES

1.10.1 TESTING DIODES

Diodes only conduct in one direction, it is therefore relatively easy to determine theterminals and serviceability using a multimeter, however, 2 points need noting:

When AVO's are used on a resistance range, the black terminal is positive withrespect to the red terminal.

The potential difference between the red and black terminals of a digital metermay be insufficient to forward bias a silicon diode (remember: requires 0.6V).

This would indicate that the diode was non conducting in both direction leading tothe false assumption that the diode is unserviceable.

1.10.1.1 Determining the Terminals

When forward biased, a diode has a resistance of approx. 1k. When reversebiased the resistance is in the order of megohms. To determine the terminals of adiode, it is simply a matter of connecting the meter across the diode to see if it willconduct, if it will not, the terminals should be reversed to confirm conduction andserviceability. When conducting, the black terminal of an AVO, or the red terminal ofa digital meter, is connected to the anode (flat end of symbol).

1.10.1.2 Confirming Serviceability

The serviceability of a diode is determined by ensuring it has a resistance in the

order of 1K in one direction and a resistance in the order of megohms in theopposite direction. Remember the points made about the two types of meter.







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1.10.2 TESTING TRANSISTORS

As we have seen, transistors basically comprise 2 back-to-back diodes, thereforethe process of confirming the serviceability and determining the terminals is similarto that used for diodes.

1.10.2.1 Determining the Base

The base of the transistor can be found by considering the transistor as two back-to-back diodes, and using a multimeter set on ohms.

1.10.2.1.1 NPN Transistors

Connect the positive terminal of the meter to one of the three transistor terminals.

Measure the resistance between this terminal and the other two. If both indicate alow resistance then the positive terminal is connected to the base. If the resistanceto the other two terminals is not low, the positive terminal is not connected to thebase. Connect the positive terminal of the meter to another terminal and repeat theprocess until the base is determined.

1.10.2.1.2 PNP Transistors

The procedure used to identify the base of a PNP transistor is the same as thatused to determined the base of the NPN transistor, except that the negative terminalof the meter is connected to each transistor terminal in turn, and it is this negative

terminal that indicated the base.

1.10.2.2 Confirming the Serviceability

Both types of transistor are serviceability tested by confirming that each forwardbiased junction (Diode) has a low resistance, and each reverse biased junction ahigh resistance. The high resistance between collector and emitter should also beconfirmed. Remember the points made about AVO's and Digital meters, otherwiseincorrect conclusions may be drawn from the observations.







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NPN Base to Emitter - forward biased - low resistanceBase to Collector - forward biased - low resistance

Emitter to Collector - reverse biased - high resistance

PNP Emitter to Base - forward biased - low resistance

Collector to Base - forward biased - low resistance

Emitter to Collector - reverse biased - high resistance






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2 OPERATIONAL AMPLIFIERS

Operational amplifiers are integrated circuit devices designed to be a closeapproximation to the perfect amplifier.

2.1 THE PERFECT AMPLIFIER

Although a theoretical device, the specification of a perfect amplifier would be asfollows:

Gain infinitely high. This has to be controlled in some way otherwise thesmallest input would result in maximum output.

Input impedance. Infinitely high so as not to load the source.

Output impedance. Zero, so that the amplifier can be connected to any loadwithout the output voltage being affected.

Bandwidth. Infinite, so that signals from d.c. to infinite frequency are allamplified by the same amount.

Supply voltage. The amplifier should be unaffected by variations in the power

supply voltage.

2.2 OP AMP SPECIFICATION

The following specification is for a SN72741 operational amplifier. This is a verypopular operational amplifier generally simply referred to as a 741 op-amp.

Gain - 200 000 voltage gain (106db approx.)

Input impedance - 2M.

Output impedance - 75

Bandwidth - d.c. to 1MHz.

Supply voltage - The op-amp will operate with a supply of plus and minus 5 to15 volts, and take a quiescent current of about 2mA. The output voltage will

change less than 150V per volt change in supply voltage.

It can be seen that the 741 Op Amp approximates the specification of a perfectamplifier.







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2.3 POWER REQUIREMENTS

Operation is most convenient from a dual balanced d.c. power supply giving equalpositive and negative voltages (+ Vs) in the range +5V to +15V. The centre point ofthe power supply, i.e. 0V is common to input and output and is taken as their voltagereference.

The input signs on the circuit symbol for an Op Amp should not be confusedwith those for the supply polarities.

An op-amp can be operated from a single power supply. The voltage differenceavailable from, for example, a 0V to 18V supply is the same as that from a +9V to

0V to -9V one, however, if a single power supply is used, extra components arerequired.

2.4 PIN OUTS & CIRCUIT SYMBOL

The circuit symbol and pin outs of a typical operation amplifier are shown below.

Most of the terminals are self-explanatory or will be explained in the course of thesenotes. Terminals 1 and 5, the offset null terminals however require furtherexplanation.







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If the same input signal is applied to the input terminals 2and 3 the output (terminal 6) should be zero, in practice it isnot. For d.c. amplification this not acceptable. The outputis zeroed by connecting a resistor between terminals 1 & 5as shown, and adjusting it until the output falls to zero. Fora.c. amplification a coupling capacitor in series with theoutput removes any unwanted d.c. offset.

2.5 OPERATION

An operational amplifier has one output and two inputs as seen on the circuit andpin-out diagrams. The two inputs are referred to as, the non-inverting input, markedwith a +, and the inverting input, marked with a -.

If the voltage applied to the non-inverting input (+) is positive relative to the otherinput, the output voltage is positive. If the voltage applied to the non-inverting inputis negative relative to the other input, the output voltage is negative. That is, thenon-inverting input and the output are in-phase.

If the voltage applied to the inverting input (-) is positive relative to the other input,the output voltage is negative. If the voltage applied to the inverting input (-) isnegative relative to the other input, the output voltage is positive. That is, the

inverting input, and the output are anti-phase.

Basically an op-amp is a differential amplifier. It amplifies the difference betweenthe two input voltages.

There are 3 cases:

If V+ > V- the output is positive

If V+ < V- the output is negative

If V+ = V- the output is zero

In general to output is given by V0 = A0 ((V+) - (V-)) where A0 is the gain.

2.5.1 NEGATIVE FEEDBACK

As already mentioned, and as can be seenfrom the transfer characteristic to the left.

There is only a very small range of inputvalues giving an output that is directlyproportional (A to B). It takes very littleinput to drive the amplifier into saturationdue to its extremely high gain.







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Assuming a gain of 10

5

, the maximum input voltage swing (for linear amplification) is9V/105 = 90V. This is of little practical use.

To reduce this gain and allow larger input signals, requires the use of negativefeedback. Part of the output is fed back to the input in such a way that it produces avoltage at the output that opposes the one from which it was taken. This basicallymeans taking part of the output and feeding it back to the inverting input. (Feedbackapplied to the non-inverting input would be positive and would increase the output).

The application of negative feedback also gives greater stability, less distortion andincreased bandwidth, it also becomes possible to exactly predict the gain of theamplifier. The relatively small loss in gain is far outweighed by the advantages

obtained.

A simple feedback network is shown in the diagram of an inverting amplifier below.

The signal to be amplified is applied to the inverting input via the resistor, the outputis therefore antiphase with respect to the input. The non-inverting input isconnected to ground. Negative feedback is provided by resistor Rf, called the'feedback resistor', it feeds back a certain amount of output voltage to the invertinginput.

Using this arrangement the gain can be calculated from;

-Rf/R1 if Rf = 1M and R1 = 10kthe gain A = Hata! = -100 and,

an input of 0.01V will cause an output change of 1.0V.

It should be noted that the gain depends entirely on the values of resistors Rf andR1, and is totally independent of the parameters of the operational amplifier.







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2.6 OP-AMP COMPARATOR

If both inputs of an Op Amp are used together, then V0, the output voltage, is givenby:

V0 = A0 (V2 - V1)

Where V1 is the inverting input and V2 the non-inverting input. (Note: no feedback isused).

The difference in voltage at the input terminals is amplified and appears at the

output, however the gain is so large, that about 90V difference in the two inputs willcause the output to fall or rise to the +ve or -ve supply voltage.

When V1 > V2 the output is almost -Vs, when V1< V2 the output is almost +Vs. Theop-amp basically behaves like a two state switch, switching 'high' or 'low' dependingon the difference in the inputs.

By connecting a reference voltage to the inverting input and a signal to the non-inverting input, the output will swing to +Vs when the signal is greater than thereference voltage and to -Vs when the signal is smaller than the reference signal.







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ELECTRONICengineering FUNDAMENTALSuk

Hata!

2.7 OP AMP SUMMING AMP

When connected as multi-input inverting amplifier (see previous topic on feedback),an op amp can be used to add a number of voltages, either a.c. or d.c.

In the above circuit, 3 input voltages, Vin 1, Vin 2 and Vin 3 are applied throughresistors R1, R2 and R3 respectively.

Hence: = Hata! + Hata! + Hata!

V0ut = - Hata!

Thus the input voltages are added and amplified if Rf is greater than each of the

input resistors.

If R1 = R2 = R3 = Rin, the input voltages are amplified equally

and V0ut = Hata! (Vin 1 + Vin 2 + Vin 3)

If R1 = R2 = R3 = Rin = Rf

then V0ut = (Vin 1 + Vin 2 + Vin 3)

The output voltage is the sum of the input voltages but is of opposite polarity.

This device can be used as a digital to analog converter by making R2 twice the sizeof R1, and R3 twice the size of R2. If a 3 bit digital word is then be applied to theresistors, with the least significant bit applied to R1 and the most significant bit

applied to R3, the output will be the analogue equivalent of the binary word.







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3 PRINTED CIRCUIT BOARDS

The assembly of the various circuits which form part of the units employed in aircraftelectronic systems, necessitates the interconnection of many components by meansof electrical conductors. Before the introduction of printed wiring, these conductorswere formed by wires which connected to the components either by soldering, or byscrew and crimped terminal methods.

In the development of circuit technology, micro-miniaturisation, rationalisation ofcomponent layout and mounting, weight saving, and the simplification of installationand maintenance become essential factors; and as a result, the technique of printingthe required circuits was adopted.







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In this technique, a metallic foil is first bonded to a base board made from aninsulating material, and a pattern is then printed and etched on the foil to form aseries of current conducting paths, the pattern replacing the old method or wiring.Connecting points and mounting pads, for the soldering of components appropriateto the circuit, are also formed on the board, so that, as a single assembly, the boardsatisfies the structural and electrical requirements of the unit which it forms a part.

If the circuit is a simple one, the wiring may be formed on one side of a board, but,where a more complex circuit is required, wiring is continued on to the reverse side,which also serves as the mounting for components. In addition, complex circuitsmay be incorporated in multi-layer assemblies.

3.1 BASE MATERIAL

The base material, or laminate as it is sometimes called, is the insulating material towhich the conducting material is bonded. The base material also serves as amounting for the components which comprise the circuit. The base material iscommonly made up either of layers of phenolic resin impregnated paper, or of epoxyresin impregnated fibre glass cloth which has been bonded to form a rigid sheet,which can be readily sawn, cut, punched or drilled. The thickness of the basematerial depends on the strength and stiffness requirements of the finished board,which, in turn are dictated by the weight of the components to be carried, and by the

size of the printed conductor area.

3.2 CONDUCTOR MATERIAL

The most commonly used conducting material is copper foil, the minimum purityvalue of which is 99.5%.

3.3 BONDING OF CONDUCTOR MATERIAL

For the manufacture of a typical circuit board, the base material and copper foil arecut into sheets, and are then inspected and assembled inside a clean room inalternate layers with stainless steel separator plates (known as cauls) interposedbetween the layers, as shown below. The steel plates, which are accurate inthickness to within 0.001 inch, are very hard, and have a delicately grained surfacewhich is imparted to the finished boards.







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The layered sheets (the assembly) are then passed out of the clean room to bebonded in a hot press. During the pressing operation, the heat melts the resin in thebase material , so that it flows and fully wets out the material and the copper foil.

The pressure applied is adjusted so as to exclude all air and vapour from anyresidual volatiles. As polymerisation of the resin mix proceeds, each layer of thebase material reaches the fully cured state with the copper foil firmly bonded to it.

After cooling has taken place, the individual copper-clad boards are trimmed to therequired size, inspected, and packed in sealed polythene bags.

3.3.1 INSPECTIONS & TESTS

After manufacture, all boards are inspected, and tests are carried out on selectedsamples, in accordance with the relevant specifications. Tests will include:

Inspection of appearance

Checks on thickness

Measurement of bow and twist

Measuring the peel strength of the foil

Checking the heat resistance by solder

Measurement of pull-off strength

Electrical tests







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3.4 MACHINING OF BOARDS

All boards require machining, e.g. guillotining, sawing, punching, and drilling duringthe various stages of production.

Guillotining is one of the quickest and most economical methods of cutting sheetsof copper-clad laminates into strips and panels, and it is frequently employed inconjunction with subsequent punching operations. Correctly performed guillotiningresults in a clean, burr-free edge, with no wastage of stock.

Cutting with a circular saw is superior to guillotining as it gives a cleaner edge,especially so as the thickness of the laminate increases. Wood-cutting machinery is

satisfactory for laminates.

The type of resin, base material and the degree of cure, are the main factorsaffecting the drilling characteristics of a laminate. All laminates are abrasiveparticularly those with glass fibre base material, and drilling techniques should beadapted to suit.

Where large quantities of laminates are required, and cost of tools is acceptable,punched parts can be produced by conventional pierce and blank methods, suchmethods are most commonly adopted for copper-clad phenolic / paper baselaminates.

3.5 CIRCUIT ARTWORK

The quality of a printed wiring board is, in the fist instance, dependent on theproduction of master artwork which must show precisely the circuit conductorpattern required, where components are to be located, circuit module designationsand other essential references. Artwork production requires the use ofdimensionally stable base materials, and the application of skilled draftingtechniques, because, unlike conventional electrical drawings, which are used as aguide to the build-up of an assembly of wiring and connections, a printed wiringboard is an actual reproduction of the original artwork produced for it.

Human error in drafting can be reduced, and, in certain cases, eliminated, by theuse of numerically-controlled drafting machines. These are accurate X and Y co-ordinate plotting machines which are capable of automatically plotting a point, orline, on a surface whether it be on a film or glass base.







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3.6 PRINTING OF CIRCUITS

The printing of circuits is carried out using either an etching process or an additiveprocess. Both of these processes, are briefly described in the following paragraphs.

3.6.1 ETCHING PROCESS

In this process the copper foil is first cleaned, either chemically or mechanically, andis then coated with a photo-sensitive solution known as a 'resist', which has theproperty of becoming soluble when exposed to strong light.

A photographic positive of the circuit artwork is then placed over the sensitisedboard and time-exposed in a special printing machine. After exposure, the resist iswashed away to leave unprotected areas of copper around the circuit pattern. Theboard is dried by a clean, oil and water free air blast. The complete board is theninspected to ensure that no resist has been removed from any part of the conductorpattern itself, and that no resist particles are present in areas which are to be etchedaway. The board is then placed in a bath which contains an etching solution, suchas ferric chloride or ammonium persulphate, which etches away all the unprotectedcopper.

When the etching process has been satisfactorily completed, the board is thoroughlywashed in water in order to remove all traces of etching solution, and is then driedand given a final inspection.

As printed circuit boards with the same circuit pattern are often required in largenumbers, the simple 'print and etch' process is generally superseded by a screenprinting process.







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3.6.2 ADDITIVE PROCESS

In this process, copper is deposited only in the areas where conductors arerequired. To achieve this the base material is pre-coated with a suitable adhesive,the circuit holes are pre-fabricated, and the board is sensitised with a photo-resistsolution. A negative of the circuit pattern is then screen printed onto the board sothat the exposed areas define the conductor network. These exposed areas arechemically activated, and the board is then immersed in an electrolyses copperplating solution. After a period of time consistent with the deposition of the requiredthickness of copper, the board is removed from the bath. The major advantages ofthe additive process are: no chemical etching takes place, thereby eliminating

wastage of copper, the thickness of the deposited copper can be reduced and mademore uniform, the conductor widths and spacing are less restricted, and the holediameter can be reduced, thereby increasing the board area available for routing ofconductors.







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3.6.3 INSPECTION

After printing, circuit patterns are inspected with particular attention being paid to thefollowing:

Dimensional Accuracy and Condition of the Edges of Conductors

Condition of the Pattern Surfaces

Particles of Copper in Unwanted Areas

Insulation Areas

Lack of Resin Bond in etched Areas

3.7 SOLDERING METHODS

There are two main methods of soldering employed in connection with printedcircuits boards, (a) hand soldering and (b) mass soldering.

3.7.1 HAND SOLDERING

This method is used for soldering joints separately, e.g. in limited batch production,

and when a component or a wire is replaced after a test or a repair has been carriedout. This method involves the use either of electrically heated hand irons, or ofresistance type hand tools when the use of these is permitted.

3.7.2 MASS SOLDERING

In this method, all joints of a finally assembled board are soldered simultaneously,by bringing the board into contact with an oxide-free surface of molten solder, whichis contained in a special type of bath. Mass soldering may be carried out in any oneof five different ways:

Flat or Static Dipping - one edge of the board is first lowered on to the solder andthe other edge is then lowered slowly to allow flux and solvent vapour to escape.

Wave Soldering solder is pumped from the bottom of the solder bath through anarrow slot, so that a symmetrical 'standing wave' of solder is produced across thewidth of the bath. The circuit board after being fluxed, is then either manually orautomatically passed against the crest of the solder wave by a conveyor.







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Weir and Cascade Soldering systems are of the moving solder type, the solderflowing down a trough by gravity, and then being returned to the main bath by apump. In weir-soldering (diagram (a)) a circuit board is lowered on to the solder;while in cascade soldering (diagram (b)) a board is conveyed across the crests ofsolder waves in a direction opposite to the solder flow.

Reflow soldering is an automated process also known as 'heat cushion' soldering.

It is applied particularly to circuit boards on which microcircuits and associateddevices are to be assembled. These efficient but costly components require aspecial soldering technique, so that their full potential as surface-mounted devicescan be realised. The reflow technique is generally recognised as the best method,since the soldered joints are easier to inspect and to remake when a faultycomponent has to be replaced. In addition, soldering times and the risk ofoverheating sensitive components are reduced, and distortion of leads is prevented.

The sequence of reflow soldering is shown in the diagram on the following page.The leads of the circuit or component and the relevant lands on the circuit board,which have been pre-tinned by such methods as wave soldering or dip soldering,

are first brought into contact with each other and accurately aligned. The sequenceis then initiated by lowering the electrode on the lead to be soldered.







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Shortly before the electrode makes contact with the lead, the pre-set heating poweris automatically switched on. The electrode is then pressed on to the lead under aload which gradually increases until the pre-selected value is reached. The soldermelts, and in reflowing, it forms a 'cushion' through which the lead is pressedagainst its corresponding land of the circuit board. As soon as the cushion isformed, the timing device cuts off the heating supply. After a 0.75 second delay, anair blast is delivered to cool the soldered joint, this accelerates the completion of thesoldering process, and also improves the quality of the joint. At the end of thecooling period, the load is relieved, and the electrode is automatically raised readyfor the next operation.

3.8 SOLDER SPECIFICATION

For the mass-soldering of printed wiring boards, solder complying with BS 219Grade K (60/40 tin/lead) is the one most commonly used, since it has a free-flowcharacteristic which permits good joint formation in the short period during whichboards are in contact with the solder.

The solder temperature is chosen for each individual combination of board andtypes of material being processed, but it should normally be within the range 220C

to 260C.

3.9 FLUXES & THEIR APPLICATION

To assist in the wetting of surfaces by molten solder, a flux must be used both toprevent oxidation during joint formation, and to dissolve the thin oxide films whichmay already be present on the surfaces which are to be joined, and on the solderitself.







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3.10 SOLDER RESISTS

There are organic coatings which are designed for use on both rigid and flexibleprinted circuit, to mask off those areas where soldering is not required. Someimportant advantages of the use of solder resists are as follows:

Elimination of bridging and icicling between closely spaced conductors andmountings.

Protection is afforded against corrosion and contamination during storage,handling subsequent life of the circuit.

Flexibility of circuit patterns is maintained since a resist flexes with theconducting material.

The surface resistance values of the circuit patterns are improved.

Minimising of solder contamination from large surfaces of copper and otherplated materials, thereby maintaining a high level of solder purity and anextension of bath life.

Heat distortion is minimised, since a resist acts as a heat barrier.

3.11 PLATING OF PRINTED WIRING CIRCUITS

Plating finishes for printed wiring circuits are used as aids to the performance ofcircuits under specific conditions of use, and are not intended to be decorative. Thechoice of finish is, therefore, governed strictly by the functional and environmentalconditions in which the circuit will be used. In many cases, the different parts of acircuit may be subjected to different conditions of use, and provided there is cleatdemarcation between these parts, they can be plated with the appropriate finishes.A typical example of this differential plating method, is a circuit that is tin/lead platedfor solderability over the component area, and nickel/gold plated for durability onedge-connector finger contacts.

3.11.1 THROUGH-HOLE PLATING

Through-hole plating is a process which is widely employed to provide a conductingsurface in the holes of single-sided and double-sided boards, and also to provide aland or pad for the connection of components.







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3.12 ORGANIC PROTECTIVE COATINGS

After printed wiring boards have been manufactured, organic coatings are applied totheir surfaces, to protect them from oxidation and contamination. The coatingsvary, depending on whether temporary protection is required, e.g. for maintainingclean copper surfaces during normal handling prior to soldering, or, whetherpermanent protection is to be applied after soldering for protecting the circuit andcomponents form subsequent environmental contaminants.

For temporary protection the coating is usually of a resin-based type which does not

require removal before soldering, since it also serves as a flux. Permanentprotective coatings are usually epoxide or polyurethane-based resin, havingexceptionally low oxygen absorption, high humidity resistance, and resistance tocracking and discoloration.

3.13 FLEXIBLE PRINTED WIRING CIRCUITS

Unlike rigid printed wiring boards, flexible circuits serve only as a means ofinterconnecting units, particularly those which require to be moved relative to eachother, and those which may be mounted in different planes. Flexible circuits alsopermit easier assembly and higher density packaging of units. Flexible circuits are

laminated form, consisting of a flexible base insulation material (e.g. polyester,epoxy glass cloth and polyimide) copper foil, and an insulating coverlay of the samematerial as the base.







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3.14 HANDLING OF CIRCUIT BOARDS

3.14.1 ELECTROSTATIC DISCHARGE SENSITIVE DEVICES

Many electronic Line Replaceable Units (LRU's) on aircraft contain printed circuit

boards containing components that are susceptible to damage from electrostaticdischarges. Such components are referred to as electrostatic discharge sensitive(ESDS) devices. Decals installed on ESDS LRU's, indicate that special handling isrequired. Some decals are shown below, the lower four are typical Boeing ESDSdecals.

3.14.1.1 Static Electricity & Electrostatic Discharge

The most common conception of static electricity and its accompanying discharge,is the miniature lighting shock you receive when you touch a metal door handlehaving walked across a nylon carpet. If the door handle is touched with a key first,

the discharge will be seen but not felt.

The discharge occurs because different materials receive different levels of chargeas materials are rubbed together or pulled apart. The different charge levels createpotential differences between the different materials, and when materials of differentelectrical potential are brought into close proximity with each other, a dischargeoccurs as the potentials equalise.

The different levels of charge with respect to cotton (the reference material) areshown on the following page, in what is known as the Triboelectric Series.

The further up or down, the greater the charge and hence the greater the dischargewhen the two materials are brought together.







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Triboelectric Series

Material

Air

Human Hands

Asbestos

Rabbit Fur

Glass

Mica

Human Hands

Nylon

Wool

Fur

Lead

Silk

Aluminium

Paper

Increasingly Positive

Cotton

Steel

Wood

Amber

Sealing Wax

Hard Rubber

Nickel Copper

Brass Silver

Gold Platinum

Sulphur

Acetate Rayon

PolyesterCelluloid

Orion

Saran

Polyurethane

Polyethylene

Polypropylene

PVC (vinyl)

Kelf (ctfe)

SiliconTeflon

Increasingly Negative







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The typical voltages that can occur are shown in the table below, note theimportance of humidity.

Electrostatic Voltages

Means of StaticGeneration

10 to 20 PercentRelative Humidity

65 to 90 PercentRelative Humidity

Walking across carpet 35,000 1,500

Walking over vinyl floor 12,000 250

Worker at bench 6,000 100

Vinyl envelopes for workinstructions

7,000 600

Common poly bag pickedup from bench

20,000 1,200

Work chair padded withpolyurethane form

18,000 1,500

The last table shows a list of static sensitive devices and the voltages that cancause damage. The damage may vary from a slight degradation of performance,giving rise to intermittent and spurious indications, to complete destruction, givingrise to total system failure. The amount of damage varies with the amount of energythat strikes the component.

The less obvious damage can cause considerable and expensive maintenanceheadaches which may lead to lack of confidence in the equipment.

Static Sensitive DeviceSensitivity Range

where damage can

occur

Field Effect Transistor (MOS /FET)

150 - 1000 volts

CMOS 250 - 1000 volts

Bipolar Transistors 4,000 - 15000 volts

Silicon-Controlled Rectificers(SCR)

4,000 - 15000 volts

Thin-Film Resistors 150 - 1000 volts







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3.14.2 REMOVAL & INSTALLATION OF ESDS PRINTED CIRCUIT BOARDS

Equipment and Material

Conductive bags

Wrist straps

100% cotton twine - commercially available

ESDS Labels

Removal of Boards1. Remove system electrical power.

Warning: use only wrist straps with a grounding lead resistance of greaterthan 1 megohm. Inadvertent contact between a low resistance wrist strapand a high voltage, is a shock hazard to personnel.

2. Connect wrist strap assembly to a convenient ground on component containingPC board and to skin of person removing PC board.

3. Gain access to printed circuit board.

4. Remove printed circuit board using extractors provided.

5. Immediately insert static sensitive board into a conductive bag and identify withan ESD label. Use an ESDS label or 100% cotton twine to close the conductivebag.

Caution: Do not use staples or adhesive tape to close conductive bag.Damage to bag will expose contents to electrostatic discharge.

6. Close and secure unit unless replacement card is to be installed immediately.

7. Disconnect wrist strap from ground and operator.

8. Place bagged printed circuit card in a rigid container to maintain integrity ofconductive bag during transportation.

Installation of Boards

1. Check that system electrical power is off.

Warning: use only wrist straps with a grounding lead resistance of greaterthan 1 megohm. Inadvertent contact between a low resistance wrist strapand a high voltage, is a shock hazard to personnel.

2. Connect wrist strap assembly to a convenient ground on unit where the printedcircuit board is to be installed and to skin of person installing PC board.

3. Gain access to receptacle that PC board is to be installed into.

4. Remove static sensitive PC board from conductive bag.







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Caution: Do not touch connector pins or other exposed conductors.Damage to components can result.

5. Install PC card in position using extractors provided. Lock extractors.

6. Close and secure unit.

7. Disconnect wrist strap.

3.14.3 REMOVAL & INSTALLATION OF METAL-ENCASED ESDS LRU'S

General

8. Metal-encases ESDS units can be either rack mounted, panel mounted or boltedon.

Equipment and Material

9. Dust caps.

Note: Conductive or anti-static dust caps should be used when available. Ifconductive or anti-static dust caps are not available, non-conductive dust capsmay be used but with caution, since they do not provide complete ESDSprotection during handling.

Remove metal encased LRU's with ESDS labels

10. Remove system electrical power.

11. Remove ESDS labelled unit from rack, panel, or mounted position.

Caution: Do not touch connector pins or other exposed conductors.Damage to components may result.

12. Install dust caps on all connectors. Do not touch electrical pins in connectors.

Note: Dust caps from unit being installed may be used on the unit beingremoved.

13.Transport unit per standard practices with dust caps installed.

Install metal encased LRU's with ESDS labels

14. Check that system electrical power is off.

Caution: Do not touch connector pins or other exposed conductors.Damage to components may result.

15. Remove all dust caps from connectors of unit being installed. Do not touchelectrical pins in connectors.

16. Place unit in position and secure.






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4 SYNCHRONOUS DATA TRANSMISSION

Synchronous data transmission systems are designed to indicate the position of acomponent or control surface that cannot be directly observed. The systems fall intoone of two categories; d.c. systems called 'Desynn Systems' and a.c. systems whichare generally grouped under the heading of 'Synchro Systems'.

Both a.c. and d.c. systems comprise two main components, a transmitting elementand a receiving element. The two being interconnected by wiring that provides thesignal path. The word 'synchronous' means 'happening at the same time', whichinfers that when the transmitter is moved, the receiving element, normally anindicator, will follow that movement instantly.

4.1 DESYNN SYSTEM

There are a variety of different types of Desynn systems available:

The Basic Desynn is generally operated by a rotary motion, however linearversions are also found. The conversion from linear to rotary motion being achievedby a push rod and gear wheel.

The Micro Desynn was designed to magnify the small movement obtained by such

items as pressure measuring devices. They are operated by linear motion.

The Slab Desynn was designed to overcome signally errors inherent in the basicDesynn system. In the vast majority of instances the errors in the basic Desynncould be considered insignificant.

4.1.1 THE BASIC DESYNN

4.1.1.1 Construction

In the basic Desynn system the transmitter comprises an endless resistance wound

on a circular former, this arrangement being referred to as a 'Toroidal Resistance'.Equally spaced at 120 intervals around the resistor are 3 tappings, it is to these thatthe signal wires are connected. Running on the resistor are two wiper arm type

contacts that are spaced apart by 180 and insulated from one another, it is to thesethat system power is applied.







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The indicator comprises a two pole permanent magnet rotor, pivoted to rotate insidea soft iron stator, the pointer being attached to the spindle. The stator carries threestar connected windings that are connected to the three wires coming from thetappings of the transmitter.

4.1.1.2 Operation

When dc power, is applied to the wiper arms of the transmitter, current will enter thepositive wiper arm and divide to flow in both directions, left and right, around thetorroidal resistor. Both halves of the resistor have the same resistance, thereforethe current in each path will be equ

EASA PART 66 Module 4_electronic Fundamentals

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