DIODES

Diodes are the simplest of the solid-state devices. They consist of a piece of P-type material fused to a piece of N-type material. The most common forms of diodes are constructed from Silicon. Germanium is less stable at high temperatures than Silicon. Its use is generally reserved for those special applications where a low forward bias is essential. Germanium is less stable at high temperatures than Silicon.

Symbol for a Diode

As outlined in the downloadable booklet in the section on the "P-N junction", a diode will only conduct in one direction, with electrons flowing from the N-type end to the P-type end.(ie from the "Cathode" to the "Anode")

If a voltage is applied which reverse biases the junction, the depletion layer will widen until a point is reached where the voltage exceeds the 'breakdown voltage' of the diode and large currents flow; destroying the device. One type of diode, the 'zener', actually makes use of breakdown voltage in an interesting way. (See below for information on zener diodes.)

There are many different forms of diodes, from simple 'point-contact' signal diodes to multi-coloured light emitting diodes. A few of the more common varieties are discussed below.

Signal Diodes
Signal diodes are physically small devices usually used where small currents, high voltages and high frequencies are involved. As discussed in the section on 'vari-caps' in the downloadable booklet, the size of the junction has an effect on the signal capabilities of the diode. A small junction offers less resistance to high frequencies than does a wide thick junction. The name comes from the fact that these diodes are suitable for use in radio detectors to isolate the radio 'signal'.

Signal diodes are very small and often glass encapsulated, with a red or black band on one end. (The glass is sometimes painted over to reduce unwanted photo-voltaic effects.)

Power Diodes
Where larger currents are involved, a larger junction is needed to dissipate the heat generated. A small junction would be in danger of literally melting with currents in excess of a few hundred milli-amps.

Since the power diode has a large junction, it is not suited to high frequency applications. (High frequency, high current diodes are available, but the cost is substantial.)

One advantage of the larger junction is its ability to withstand higher voltages without sustaining damage. While a signal diode may only be able to take 30 to 50 volts reverse potential, it is quite common to find power diodes rated up to several thousand volts maximum reverse bias. (Termed "Peak Inverse Voltage", or PIV.)

Power diodes are able to pass large loads varying from the 1N400X series rated at 1 amp up to industrial diodes capable of carrying hundreds of amps!

These diodes come in a variety of encapsulations, the most common being a black cylinder of plastic about 3mm long with a white band indicating the cathode (negative) end. Large-current devices are often encased in metal to provide efficient heat transfer.

It is possible to purchase a 'pack' of four power diodes arranged in a full-wave bridge configuration. (See the section on power supplies in the downloadable booklet.) The physical construction of bridge depends on the current demand. All have four leads - two for AC input and one each for positive and negative output.The 'bridge' converts an alternating input current into a fixed polarity DC output. (AC = alternating current. DC = Direct Current)

Light Emitting Diodes
Light emitting diodes (LEDs) are among the most widely used of all types of diodes. Colours available range from red, orange, yellow, green and blue. Sizes are 3mm, or 5mm. It is also possible to purchase 'rectangular' LEDs and special-purpose LEDs, for example LEDs that have been moulded to represent a small dot. Most LEDs have a couloured 'lens', but it is possible to buy "water-clear" LEDs that have no colouring in the plastic lens. LEDs are also available in different "intensities" ranging up to several "candle-power".

The most common (and cheapest) LED is the 5mm red LED.

LEDs are also available in "packages" arranged to produce letters and numerals. The price and availability of these packages depends to a large extent upon current industrial requirements. The once common "FND500" could be obtained for less than a dollar until quite recently. At the moment it costs considerably more, if it can be found at all! (The problem of changing commercial demand for components needs to be kept in mind when choosing student projects.)

Numerals are produced by arranging LEDs in a seven-segment arrangement as indicated below. Integrated circuits (ICs) are available for 'driving' displays directly. (The 4026 IC for example, will take pulses, count them and display the count on a seven-segment display, all for a few dollars!)

A red LED and a "Seven-segment Display".

A LED may be thought of as a 1.5 volt globe for design purposes. If voltages of more than this are involved, a 'dropping' resistor is needed, as indicated below. The maximum current flowing through a LED should be limited to around 20 milli-amps. The table below shows how to calculate the resistance required to use LEDs with different voltages. (NOTE: 20 milli-amps = 0.02 amps. In the formula below, "I" = 0.02)

As with all diodes, orientation of LEDs is critical. If you connect the legs the wrong way around it will not conduct. The following diagram should provide a useful guide. This property is very useful when using a diode to provide protection against voltage 'reversal' - also called: idiot-proofing. If a diode is built into the power section the rest of the circuit will be protected in the event of somebody connecting the power supply the wrong way around.

Zener Diodes
If a reverse bias is applied to a diode it will resist conduction until a point is reached where current is forced to flow. This voltage is called the Peak Inverse Voltage, or; breakdown voltage.

Under normal circumstances, the diode would be destroyed.

It was discovered that precise production techniques could produce a diode with a predetermined 'breakdown' voltage which was less likely to be damaged by 'reverse' current flow. The effect is called the "zener" effect after Clarence Melvin Zener. This type of diode is called a "Zener Diode".

The result is device which maintains a constant voltage across its ends regardless of the input voltage.

Zeners are available in a variety of ratings, the most economical being a one watt version. 'Voltages' usually follow the E12 series, i.e. 1.5, 1.8, 2.2, 2.7, 3.3, etc.

It must be remembered that Zeners are used in REVERSE mode, i.e. the anode connects to the negative supply.

Diodes - QUIZ