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FUNDAMENTALS
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 electrostatic discharges. Such components are referred to as electrostatic discharge sensitive (ESDS) devices. Decals installed on ESDS LRU's, indicate that special handling is required. Some decals are shown below, the lower four are typical Boeing ESDS decals.
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 handle having 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 charge as materials are rubbed together or pulled apart. The different charge levels create potential differences between the different materials, and when materials of different electrical potential are brought into close proximity with each other, a discharge occurs as the potentials equalise.
The different levels of charge with respect to cotton (the reference material) are shown 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 discharge when the two materials are brought together.
JAR 66 CATEGORY B1 MODULE 4
ELECTRONIC FUNDAMENTALS
JAR 66 CATEGORY B1 MODULE 4
ELECTRONIC FUNDAMENTALS
JAR 66 CATEGORY B1
JAR 66 CATEGORY B1
The typical voltages that can occur are shown in the table below, note the importance of humidity.
Walking across carpet 35,000 1,500
Walking over vinyl floor 12,000 250
Worker at bench 6,000 100
The last table shows a list of static sensitive devices and the voltages that can cause damage. The damage may vary from a slight degradation of performance, giving rise to intermittent and spurious indications, to complete destruction, giving rise to total system failure. The amount of damage varies with the amount of energy that strikes the component.
The less obvious damage can cause considerable and expensive maintenance headaches which may lead to lack of confidence in the equipment.
Static Sensitive Device
Bipolar Transistors 4,000 - 15000 volts Silicon-Controlled Rectificers
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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 Boards
1. Remove system electrical power.
Warning: use only wrist straps with a grounding lead resistance of greater than 1 megohm. Inadvertent contact between a low resistance wrist strap and a high voltage, is a shock hazard to personnel.
2. Connect wrist strap assembly to a convenient ground on component containing PC 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 with an ESD label. Use an ESDS label or 100% cotton twine to close the conductive bag.
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 of conductive 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 greater than 1 megohm. Inadvertent contact between a low resistance wrist strap and a high voltage, is a shock hazard to personnel.
2. Connect wrist strap assembly to a convenient ground on unit where the printed circuit 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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JAR 66 CATEGORY B1 MODULE 4
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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 bolted on.
Equipment and Material 9. Dust caps.
Note: Conductive or anti-static dust caps should be used when available. If conductive or anti-static dust caps are not available, non-conductive dust caps may be used but with caution, since they do not provide complete ESDS
protection 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 being removed.
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 touch electrical pins in connectors.
16. Place unit in position and secure.
JAR 66 CATEGORY B1 MODULE 4
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JAR 66 CATEGORY B1 MODULE 4
ELECTRONIC FUNDAMENTALS
JAR 66 CATEGORY B1
Synchronous data transmission systems are designed to indicate the position of a component or control surface that cannot be directly observed. The systems fall into one of two categories; d.c. systems called 'Desynn Systems' and a.c. systems which are generally grouped under the heading of 'Synchro Systems'.
Both a.c. and d.c. systems comprise two main components, a transmitting element and a receiving element. The two being interconnected by wiring that provides the signal path. The word 'synchronous' means 'happening at the same time', which infers that when the transmitter is moved, the receiving element, normally an indicator, 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 linear
versions are also found. The conversion from linear to rotary motion being achieved by 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 basic Desynn system. In the vast majority of instances the errors in the basic Desynn could 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 that the 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 these that system power is applied.