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# ELECTRONIC DESIGN NOTES - DIODES

There are very many types of diodes, and each is designed to perform specific functions. Diodes are presented in this page based on functionality, according to the following simplified structure:

1. Rectifier Diodes
2. Other types of diodes and their functions

NOTE

The basic notions highlighted in this page are related to a few electronic design topics presented in the first part, Hardware Design, of
LEARN HARDWARE FIRMWARE AND SOFTWARE DESIGN.

1. RECTIFIER DIODES

The diode is a PN (positive-negative) semiconductor junction, meaning, the current may flow in a single direction. In our day-to-day activity, there is little confusion generated by the fact that:

1. physically, the current flows from cathode (-) to anode (+)
2. conventionally, the current flows from anode (+) to cathode (-)

The following electrical schematics are presented next:

1. Biasing Diodes
2. AC Rectification
3. Overvoltage Protection
4. Voltage Clamping

1. A PN diode junction is biased (polarized) directly or in reverse, as follows.

 Fig 1: Directly biased diode (the voltage is measured in point A) The current passing through may be as high as to destroy the diode. Therefore, it needs to be limited by a current limiting resistor (R). Fig 2: Reverse biased diode (the voltage is measured in point A) The current passing through or stopped may be as high as to destroy the diode. Therefore, it needs to be limited by a current limiting resistor (R).

2. Most important, diodes are used to rectify AC currents in order to obtain DC currents. Following are a few of the most common schematics employed.

 AC TO DC DIODE RECTIFICATION Schematic Wave Fig Fig 3: Half-Wave          IA = IT / PI IT = the current supplied by the transformer Fig 4: Simple Full-Wave          IA = 2*IT / PI IT = the current supplied by the transformer Fig 5: Bridged Full-Wave          IA = 2*IT / PI IT = the current supplied by the transformer

All diodes have some small internal resistance which develops, accordingly, a small voltage drop as follows:

1. for germanium diodes the voltage drop is 0.2 to 0.3 V; the average is 0.25 V
2. for silicon diodes the voltage drop is 0.6 to 0.7 V; the average is 0.7 V

3. An important function the diodes exhibit is overvoltage protection. Please compare Figs 6 and 7. You should note that there is no difference in the graphs of the two protection schematics employed. However, the schematic in Fig 7 is cheaper, it handles way greater currents, and heat dissipation is drastically reduced when compared to employing a Zener diode. Fig 7 is our recommendation.

 OVERVOLTAGE DIODE PROTECTION Schematic Graph Fig Fig 6: Overvoltage protection using a Zener The Zener diode works in reverse bias. On high currents a Zener heats a lot, and it may be easily destroyed. Fig 7: Overvoltage protection using 2 ordinary rectifiers (type 1N4007) This schematic is named a  "Totem Pole" configuration. Both diodes work directly biased, and heat dissipation is at a minimum. This circuit is way more reliable.

An interesting article named "Driving Automotive Injectors" (A25 in "Amazing Articles") presents a few more schematics of Zener versus ordinary diodes.

4. Another important function diodes have is voltage clamping. That may be achieved in many types of circuits; following are presented two of the most common.

 VOLTAGE CLAMPING Schematic Graph Fig Fig 8: Positive voltage clamping in point A Diode D is inversely biased, therefore it clamps positive voltages. Fig 9: Negative voltage clamping in point A Diode D is directly biased, therefore it clamps negative voltages.

Additional details about processing input field signals using diodes may be found in
LEARN HARDWARE FIRMWARE AND SOFTWARE DESIGN.

2. OTHER TYPES OF DIODES AND THEIR FUNCTIONS

Diodes are designed to handle specific functions. For example, diodes are of type:

1. Led
2. Shockley
3. SCR
4. Diac
5. Triac
6. Zener
7. Tunnel
8. Varactor

9. Laser: used in very many application, from laser pointers to DVD reading/writing.
10. Infrared: used, for example, in TV remote controls.
11. Microwave: well, we all use one of those.
12. PIN diode: is a three layer diode working as a variable resistor when forward biased, and as a variable capacitance in reverse biased schematics.
13. Back diode: works better in reverse bias than in direct one. Because it is very stable over large temperature variations it is used in frequency control.
14. Step recovery diode: is a Shockley diode realized out of an ordinary PN junction.

Fig 10: A few common diode-schematic symbols

 Led diodes can have one, two, or three colors. They are used as visual indicators Shockley diode is a unidirectional thyristor working in its voltage breakover region to trigger other thyristors or diodes. Shockley diodes are N type junctions, and they develop about 0.2 V voltage drop. They are just a semiconductor layer over a thin metal foil (aluminum, silver, gold, platinum). Their switching speed in roughly 10 ns. SCR (Silicon Controlled Rectifier) is a diode of the thyristor family. SCRs handle the highest currents and voltages, and they are constructed as four layer PNPN junctions. Diac is similar in functionality to the Shockley thyristor, except it is bi-directional. It is used to trigger other thyristors into conduction. Triac acts a switch having a gate to control the switching state. Zenner is a voltage (overvoltage) clamping diode, working only reverse-biased. It plays the role of a voltage stabilizer. (Not good!) Tunnel diode is a very fast action (conducting) diode; sometimes it is also named Esaki diode. This is a negative resistance diode used in very high frequency oscillators. Fact is, they are the fastest switching diodes we can build, and their speed is limited only by the shunting capacitance of the connecting circuitry. Varactor: variable capacity diode, also named Varicap. It is used in RF and frequency control circuits. It works similar to a DC controlled variable capacitor.

This is all about diodes, in this page. Frankly, the best thing to do is to experiment with diodes.

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