ELECTRONIC DESIGN NOTES #6

Diodes
 
 

 
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RECTIFIER DIODES

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

We use diodes to rectify AC currents in order to obtain DC currents. Following are few of the most common schematics used.
 

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

All diodes have some small internal resistance which develops a small voltage drop accordingly, as follows:
1. for germanium diodes the voltage drop is 0.2..0.3 V; the average is 0.25 V
2. for silicon diodes the voltage drop is 0.6..0.7 V; the average is 0.7 V

An important function the diodes have is overvoltage protection. Please compare Figs 6 and 7. You should notice 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 works reverse biased. On high currents it heats a lot and it may be easily destroyed.
		Fig 7: Overvoltage protection using 2 ordinary rectifiers (type 1N4007) This schematic is named  "Totem Pole" configuration. Both diodes work directly biased and heat dissipation is minimum. This circuit is way more reliable.

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

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.

OTHER TYPES OF DIODES AND THEIR FUNCTIONS

	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 on 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. Plays the role of a voltage stabilizer. (Not good!)
	Tunnel diode is very fast action (conducting) diode, it is sometimes named Esaki diode. This is a negative resistance diode used in very high frequency oscillators. In fact, 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. The best thing to do is to experiment with diodes.