Electrostatic Discharge Sensitive (ESDS) 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  

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.

Triboelectric Series

                Material

Air	Increasingly Positive
Human Hands
Asbestos
Rabbit Fur
Glass
Mica
Human Hands
Nylon
Wool
Fur
Lead
Silk
Aluminium
Paper
Cotton-----------------------------------------------	Increasingly Negative
Steel
Wood
Amber
Sealing Wax
Hard Rubber
Nickel Copper
Brass Silver
Gold Platinum
Sulphur
Acetate Rayon
Polyester
Celluloid
Orion
Saran
Polyurethane
Polyethylene
Polypropylene
PVC (vinyl)
Kelf (ctfe)
Silicon
Teflon

Electronic Instrument Systems EIS

Modern technology has enabled some significant changes in the layout of flight instrumentation on most aircraft currently in service.  The biggest change has been the introduction of Electronic Instrument systems.  These systems have meant that many complex Electro-mechanical instruments have now been replaced by TV type colour displays.  These systems also allow the exchange of images between display units in the case of display failures.

There are many different Electronic Instrument Systems, including:

i.    Electronic Flight Instrument System (EFIS).

ii.   Engine Indicating & Crew Alerting System (EICAS).

iii.  Electronic Centralised Aircraft Monitoring (ECAM).   

The Electronic Instrument System (EIS) also allows the flight crew to configure the instrument layout by allowing manual transfer of the Primary Flight Display (PFD) with the Navigation Display (ND) and the secondary Electronic Centralised Aircraft Monitoring (ECAM) display with the ND.

As well as a manual transfer, the system will automatically transfer displays when either the PFD or the primary ECAM display fails.  The PFD is automatically transferred onto the corresponding ND, with the ECAM secondary display used for the primary ECAM display.

The system will also automatically transfer the primary ECAM information onto the ND if a double failure of the ECAM display system occurs.

Slab Desynn

If the voltage at the 3 tappings of the transmitter of a basic Desynn are measured as the wiper arms are rotated 360°, it will be seen that they produce a sawtooth waveform as opposed to a sinewave. 
This results in the pointer of the indicator not following the transmitter exactly.  In most instances the difference is insignificant, however their may be certain circumstances where it cannot be overlooked.

The solution is to use a modified Desynn transmitter called a 'slab Desynn'.  In a slab Desynn, the resistor is wound on a slab former and has the power supply connected to it, whilst the wiper arms now provide the output to the receiver, there being 3 wiper arms each displaced from the next by 120°.  The output from this device is a sinewave.  It can be connected to the same type of indicator and operates in the same way as the basic Desynn.

Printed Circuit Board PCB

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

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


In this technique, a metallic foil is first bonded to a base board made from an insulating material, and a pattern is then printed and etched on the foil to form a series of current conducting paths, the pattern replacing the old method or wiring. Connecting points and mounting pads, for the soldering of components appropriate to the circuit, are also formed on the board, so that, as a single assembly, the board satisfies 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 circuits may be incorporated in multi-layer assemblies.

Synchronous Data Transmission

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'

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 

Basic Desynn

 

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.

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

Methods Of Biasing

Base Bias

 

The simplest form of biasing is to use a single resistor.  A single source provides the biasing requirements of both PN junctions.  The base of the transistor is returned to  via and the value of  determines the level of

This form of biasing is not as accurate or stable as the aforementioned potential divider bias. 


Feedback base bias

A better more stable form of biasing arrangement is shown below, is to connect the bias resistor  between the collector and base terminals of the transistor.

Now any increase in  increases the potential difference across , lowers the quiescent collector potential set by the biasing arrangement, decreasing  as before.  However, this time the fall is fed back to the base via , decreasing the forward bias on the base , decreasing  and ultimately decreasing , thus correcting the original rise owing to thermal runway.

This NEGATIVE FEEDBACK also occurs when an ac signal is applied to the base and causes a severe reduction in amplification.  As the example above shows, the value of the biasing resistor is dependent upon the gain of the transistor and this circuit does NOT stabilise the quiescent conditions against changes in transistor parameters (production spread).