Aircraft Electronics- Diodes

Rectifier Diodes

A rectifier diode is the electrical equivalent of a one way valve, it is a semiconductor device 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 conditions the diode offers little resistance to current flow. 

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

Symbols & Identification

The various symbols used for diodes 

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

The base of the triangle is the end where conventional current enters the diode, this end is called the anode.  The end through which current leaves the diode is the cathode.  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 Two identification codes are used for diodes.  In the American system the code always starts with 1N and is followed by a serial number, i.e. 1N4001.  In the continental system, the first letter gives the semiconductor material; A for germanium; 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 some manufacturers have their own codes.

Operating Characteristics

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

Biasing

A diode is said to be 'biased' when a voltage is applied between the terminals such that 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 the cathode 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 it is made from.  Germanium diodes require a voltage of approximately 0.1 to 0.2 volts and silicon diodes 0.6 to 0.7 volts.

Forward Voltage Drop

Ideally a diode should have zero resistance when conducting and should cause no voltage drop, unfortunately this does not happen.  Germanium diodes create a voltage 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.

Reverse Leakage Current

When a diode is reverse biased, it should ideally have infinite resistance and no current should flow.  Unfortunately when a diode is reverse biased, a small current called 'reverse leakage current' flows, generally this is too small to be of significance, however, it should be noted that the value of this current increases with an increase in diode temperature.  The reverse current of silicon diodes is much smaller than that of germanium diodes, (approx. one thousandth), therefore silicon diodes can be used more successfully at high temperatures (150º - 200ºC) than germanium diodes (80º - 100ºC).

Reverse Breakdown Voltage

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

The breakdown voltage can have any value from a few volts, up to 1000V for silicon diodes and 100V for germanium, depending on the construction and forms of material used.The maximum reverse voltage is an important diode characteristic.  Under normal conditions this value should not be exceeded.

Graphical Representation

graphical representation of the operating characteristics of a typical silicon and germanium diode

Aircraft Aerodynamics Airflow types

Steady Streamline Flow

The flow parameters (eg speed, direction, pressure etc) may vary from point to point in the flow but, at any point, are constant with respect to time.  This flow can be represented by streamlines and is the type of flow which it is hoped will be found over the various components of an aircraft.  Steady streamline flow may be divided into two types:

Classical Linear Flow.  The flow found over a conventional aerofoil at low incidence in which the streamlines all more or less follow the contour of the body and there is no separation of the flow from the surface.

Controlled Separated Flow or Leading Edge Vortex Flow.  This is a half-way stage between steady streamline flow and unsteady flow described later.  Due to boundary layer effects, generally at a sharp leading edge, the flow separates from the surface;  the flow does not then break down into a turbulent chaotic condition but, instead, forms a strong vortex which, because of its stability and predictability, can be controlled and made to give a useful lift force.  Such flows are found in swept and delta planforms particularly at the higher incidences.

Unsteady Flow

 In this type of flow the flow parameters vary with time and the flow cannot be represented by streamlines.

Two-Dimensional Flow

If a wing is of infinite span, or, if it completely spans a wind tunnel from wall to wall, then each section of the wing will have exactly the same flow pattern round it except near the tunnel walls.  This type of flow is called two-dimensional flow since the motion is confined to a plane parallel to the free stream direction.

As the air flows round the aircraft its speed changes.  In subsonic flow a reduction in the velocity of the streamline flow is indicated by an increased spacing of the streamlines whilst increasing velocity is indicated by decreased spacing of the streamlines.  Associated with the velocity changes there will be corresponding pressure changes.

As the air flows towards an aerofoil it will be turned towards the low pressure (partial vacuum) at the upper surface;  this is termed ‘upwash’.  After passing over the aerofoil the airflow returns to its original position and state;  this is termed ‘downwash’ as shown.  The reason for the pressure and velocity changes around an aerofoil is explained in later paragraphs.  The differences in pressure between the upper and lower surfaces of an aerofoil are usually expressed as relative pressures by ‘-‘ and ‘+’.  However, the pressure above is usually a lot lower than ambient pressure and the pressure below is usually slightly lower than ambient pressure (except at high angles of attack), ie. both negative.

Three-Dimensional Flow

The wing on an aircraft has a finite length (ie a wing tip) and, therefore, whenever it is producing lift the pressure differential tries to equalise around the wing tip.  This induces a span-wise drift of the air flowing over the wing, inwards on the upper surface and outwards on the lower surface, producing a three-dimensional flow.

Because the effect of the spilling at the wing tip is progressively less pronounced from tip to root, then the amount of transverse flow reduces towards the fuselage.  As the upper and lower airflows meet at the trailing edge they form vortices, small at the wing root and larger towards the tip.  These form one large vortex in the vicinity of the wing tip, rotating clockwise on the port wing and anti-clockwise on the starboard wing;  viewed from the rear.  Tip spillage means that an aircraft wing can never produce the same amount of lift as an infinite span wing.  If the wing has a constant section and angle of incidence from root to tip then the lift per unit span of the wing may be considered to be virtually constant until about 1.2 chord distance of the wing tip.

The overall size of the vortex at the trailing edge will depend on the amount of the transverse flow.  Therefore, the greater the force (pressure difference) the larger it will be.  The familiar pictures of wing-tip vortices showing them as thin white streaks, only show the low pressure central core and it should be appreciated that the influence on the airflow behind the trailing edge is considerable.