Diode Tutorial

What are Diodes?

Diodes are semiconductor devices which might be described as passing current in one direction only. The latter part of that statement applies equally to vacuum tube diodes. Diodes however are far more versatile devices than that. They are extremely versatile in fact. It might pay you to review the topic of Electron theory and atoms

Diodes can be used as voltage regulators, tuning devices in rf tuned circuits, frequency multiplying devices in rf circuits, mixing devices in rf circuits, switching applications or can be used to make logic decisions in digital circuits. There are also diodes which emit "light", of course these are known as light-emitting-diodes or LED's. As we say diodes are extremely versatile.

Types of Diodes

The first diode in figure 1 is a semiconductor diode which could be a small signal diode of the 1N914 type commonly used in switching applications, a rectifying diode of the 1N4004 (400V 1A) type or even one of the high power, high current stud mounting types. You will notice the straight bar end has the letter "k", this denotes the "cathode" while the "a" denotes anode. Current can only flow from anode to cathode and not in the reverse direction, hence the "arrow" appearance. This is one very important property of diodes.

The second of the diodes is a zener diode which are fairly popular for the voltage regulation of low current power supplies. Whilst it is possible to obtain high current zener diodes, most regulation today is done electronically with the use of dedicated integrated circuits and pass transistors.

The next of the diodes in the schematic is a varactor or tuning diode. Depicted here is actually two varactor diodes mounted back to back with the DC control voltage applied at the common junction of the cathodes. These cathodes have the double bar appearance of capacitors to indicate a varactor diode. When a DC control voltage is applied to the common junction of the cathodes, the capacitance exhibited by the diodes (all diodes and transistors exhibit some degree of capacitance) will vary in accordance with the applied voltage. A typical example of a varactor diode would be the Philips BB204G tuning diodes of which there are two enscapsulated in a TO-92 transistor package. At a reverse voltage Vr (cathode to anode) of 20V each diode has a capacitance of about 16 pF and at Vr of 3V this capacitance has altered to about 36 pF. Being low cost diodes, tuning diodes have virtually replaced air variable capacitors in radio applications today.

The next diode is the simplest form of vacuum tube or valve. It simply has the old cathode and anode. These terms were passed on to modern solid state devices. Vacuum tube diodes are mainly only of interest to restorers and tube enthusiasts.

The last diode depicted is of course a light emitting diode or LED. A led actually doesn't emit as much light as it first appears, a single LED has a plastic lens installed over it and this concentrates the amount of light. Seven LED's can be arranged in a bar fashion called a seven segment LED display and when decoded properly can display the numbers 0 - 9 as well as the letters A to F.

Application of Diodes

Silicon diodes - also germanium diodes. Used as general purpose rectifiers and for small signal applications.
Schottky diodes - a special type of silicon diode that has a lower forward voltage drop and faster reverse recovery than ordinary silicon diodes. They are generally for medium to high current, and used for switching at high speed (switching regulators) and when a low forward voltage drop is desirable.
Fast recovery diodes - another special type of silicon diode, needed for high speed switching at higher voltages, since schottky diodes can’t be made with a high reverse breakdown voltage.
Variable capacitance diodes - small silicon diodes designed to have a high junction capacitance when reverse biased. Used in RF tuned circuits, where the tuned frequency can be changed by varying the (DC) reverse bias.
Tunnel diodes - diodes with a negative resistance curve. Used as RF oscillators in the early days of semiconductors, and now very common (in the form of Gunn diodes) in microwave devices like police radar.
Zener diodes - diodes with a specific reverse breakdown voltage, normally used as shunt voltage regulators.
Light emitting diodes - diodes made from a variety of exotic materials from gallium arsenide to silicon carbide, that emit light from the junction when forward current flows. I include laser diodes with LEDs though the construction is somewhat different.

Switching Diodes in Logic Circuits

If you you completed the tutorial on digital basics you should be aware of binary numbers. There are a whole range of digital building blocks available and just by way of one illustration of using diodes we have presented the 74HC4040 twelve stage binary ripple counter (there are others with varying number of stages).

In the schematic of figure 7 we have this counter which divides by successive division of two for twelve stages. Initially because there is no voltage drop across the resistor a high appears on all anodes as well as on pin 4 the master reset causing the counter to reset forcing all outputs low and in turn a voltage drop across each diode and across the resistor and a low on reset.

Progressively each of the outputs change from low to high for a certain period of time and without unduly complicating matters when all outputs as selected by our diode combination (in this particular case 1 + 2 + 32 + 64 = 99) are simultaneously high the voltage drop across the resistor will cease and cause pin 11 (reset which was formerly low) to go high and reset all the internal ripple counters.

Rectifier

A rectifier is an electrical device that converts alternating current (AC) to direct current (DC), a process known as rectification. Rectifiers have many uses including as components of power supplies and as amplitude modulation detectors (envelope detectors) of radio signals. Rectifiers are most commonly made using solid state diodes but other type of components can be used when very high voltages or currents are involved. When only a single diode is used to rectify AC (by blocking the negative or positive portion of the waveform), the difference between the term diode and the term rectifier is simply one of usage. The term rectifier describes a diode that is being used to convert AC to DC. Most rectifier circuits contain a number of diodes in a specific arrangement to more efficiently convert AC power to DC power than is possible with only a single diode.

Half-wave rectification

In half wave rectification, either the positive or negative half of the AC wave is passed, while the other half is blocked. Because only one half of the input waveform reaches the output, it is only 50% efficient if used for power transfer. Half-wave rectification can be achieved with a single diode in a single phase supply as shown in figure 6.1, or with three diodes in a three-phase supply.

Full-wave rectification

A full-wave rectifier converts both the positive and negative halves of the input waveform to a single polarity (positive or negative) at its output. By using both halves of the AC waveform full-wave rectification is more efficient than half wave.

When a simple transformer with out a center tapped secondary is used, four diodes are required instead of the one needed for half-wave rectification. Four diodes arranged this way are called a diode bridge or bridge rectifier as shown in figure 6.2. The bridge rectifier can also be used for translating a DC input of unknown or arbitrary polarity into an output of known polarity. This is generally required in electronic telephones or other telephony devices where the DC polarity on the two phone wires is unknown. There are also applications for protecting against accidental battery reversal in battery-powered circuits.