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Special Purpose Diodes

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• Zener Diode• Schottky Diode• Light Emitting Diode(LED)• Photodiode

Contents

Types Example application

Standard Signal Diode Reverse polarity protection. Rectifier

Photo diode Light SensingZener Diode Voltage Regulation

Tunnel Diode Oscillators (microwaves)

Varicap Voltage-controlled oscillators

Schottky Diode Power ApplicationsLED-Light Emitting Diode

Light signalization, lighting

TVS-Transient Voltage Suppression Over voltage protection

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Zener Diode

A Zener diode is a type of diode that permits current not only in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the breakdown voltage known as "Zener knee voltage" or "Zener voltage."

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Basic Function

The basic function of zener diode is to maintain a specific voltage across its terminals within given limits of line or load change. Typically it is used for providing a stable reference voltage for use in power supplies and other equipment.

This particular zener circuit will work to maintain 10 V across the load.

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Equivalent Circuit

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Zener Diode applicationsA zener diode is used on reverse polarization, as a 1. voltage limiter 2. basic (shunt) regulator 3. For protection against voltage peaks.

Zener regulated power supply preliminary schematic.

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Example (Basic Regulator)

• Let's suppose that we have an input voltage of 12 Volt, and we want to limit the output voltage to 7.5 Volt. Then we choose a 1N47377.5V zener diode. On the datasheet given, we can see that IZT= 34mA, which is the minimum current needed for reaching Vz, and IZM=121 mA is the maximum admisible current.

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• A good working point is IZ=80 mA, near the middle of the range.

Note. A zener diode is used as a reference or regulator only on low current simple applications.

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Zener Limiting

Zener diodes can used for limiting just as normal diodes. Recall in previous chapter studies about limiters. The difference to consider for a zener limiter is its zener breakdown characteristics.

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Example(Zener Limiting)

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Schottky Diodes

• Low forward voltage drop• Fast switching action

Special Features• Switching Power Supply• Voltage clamping• Discharge protection

Applications

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Comparing Diodes

Reference: http://www.powerdesigners.com

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Light Emitting Diode

The light-emitting diode (LED) emits photons as visible light. Its purpose is for indication and other intelligible displays. Various impurities are added during the doping process to vary the color output.

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Powersupply

VS

RS

The typical voltage drop for most LEDs is from 1.5 to 3.2 V.

LED

IS =VS - VD

RS

LED driving circuit

VD

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The forward voltage in the table is value for a typical forward current of 20 mA.

LED driving circuit

Example:Source Voltage = 9 voltsVoltage Drop = 3.1 volts typical for a blue or white LEDDesired Current = 13 milliampsSo the resistor we need is: (9 – 3.1) / ( 13 / 1000 ) = 452 ohms so we will use a 470 Ω resistor.

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Example• Design the circuit to drive standard RED color LED from

5 Vdc supply. This LED requires 20 mA to generate proper brightness.

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7 Segment LED

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Photodiodes

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The photodiode is used to vary current by the amount of light that strikes it. It is placed in the circuit in reverse bias. As with most diodes when in reverse bias, no current flows, but when light strikes the exposed junction through a tiny window, reverse current increases proportional to light intensity.

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I-V characteristic

In the 1st quadrant, the device acts as a photovoltaic detector. It produces a voltage proportional to the incident light intensity. In the 3rd quadrant, when a reverse voltage is applied, it acts as a photoconductive detector. In the dark, the reverse current(Dark current, IDARK) is very small. When light strikes the diode, there is little increase in the forward current, but the reverse current can increase significantly. The current is proportional to the intensity of the incident light.

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Using a photodiode in photoconductive mode

Ai

Reverse Current

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Operation

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IR LED and receiver

IR, or infrared, communication is a common, inexpensive, and easy to use wireless communication technology. IR light is very similar to visible light, except that it has a slightlty longer wavelength. This means IR is undetectable to the human eye - perfect for wireless communication. An IR transmitter contains an LED that emits infrared light. Thus the name. The receiver contains either a photodiode or a phototransistor (usually the latter). The carrier frequency for the vast majority of IR remote controls is 38 KHz or something near.

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Basic schematics of the IR emitter and IR detector circuits.

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Counting System

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Symbols