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