What are Diodes?

Diodes are semiconductor devices which might be described as passing current in one direction only. Diodes however are far more versatile devices than that. They are extremely versatile in fact. 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. Let's look at the different types of diodes.

• The first diode is a semiconductor diode which could be a small signal diode sometimes used in switching applications, a rectifying diode or even a diode of the high power, high current stud mounting type. 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 of several very important properties of diodes.


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


• The next of the diodes in the schematic is a varactor or tuning diode. Which 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 capacitance will vary in accordance with the applied voltage. An example of a varactor diode would be tuning diodes of which there are two enscapsulated in a transistor like package. with a reverse voltage (cathode to anode) of 20V each diode has a capacitance of about 16 pF and at 3V this capacitance has altered to about 36 pF. Being economically priced, tuning diodes have for the most part replaced air variable capacitors in radio type 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. But are also found in Vacuum Tube Amplifiers.


• 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. Light emitting diodes are also used in todays Super Bright LED Falshlights. Special circuitry along with the light emitting diodes make these Flashlights Brighter than ordinary Flashlights.


• Rectifying Diodes The early application of diodes in this type of array  was to convert AC to DC, which could be smoothed sometimes using capacitors as well. This procedure is still carried out today and a number of rectifying schemes.


• Voltage Regulating Diodes For relatively light current loads zener diodes are a economical way to create voltage regulation. Zener diodes work on the principle of a constant voltage drop at a predetermined voltage. 


• Varactor or Tuning Diodes These types of diodes work on the principle that all diodes exhibit some capacitance. Indeed a zener diode has some capacitance. For AM Radio band applications some specific diodes have been devised. The These diodes typically have a capacitance of 500 - 620 pF at a reverse bias of 0.5V and 22 pF at 8V. This diode's capacitance ratio is such that it could not be achieved easily if at all with an air variable capacitor. There are several advantages that come to mind: a small transistor type package, very low cost, ease of construction on a circuit board, can be mounted away from heat generating devices, frequency determining circuitry entirely dependent upon resistor values and ratios, DC voltage control can be from a potentiometer.


• The only real limitation is your imagination and the calculations involved in making such a circuit. Now let's take a small break to absorb some of this information you have been reading about. The key to learning Electronics is having a strong background in theory. Take time to absorb all of this information and realize it can't all be learned in one sitting. When it comes to Electronics, there is nothing like hands on experience to go along with the theory. Thats why we offer you a wide v ariety of educational learning labs and electronic kits that will help you better understand what you have just got done reading as well as giving you some hands on experience.

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Mark Boehm