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Jurusan Teknik ElektroUGM

2007

SPECIAL DIODES

Bahan Kuliah minggu ke 5Elektronika Dasar

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Zener Diodes

A zener is used in reverse breakdown mode

The voltage across a zener is more orless independent of the current through

it

The function of a zener is to provide avoltage reference in a circuit

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ZENER CHARACTERISTIC

A zener diode is much like a normal diode. Theexception being is that it is placed in the circuit inreverse bias and operates in reverse breakdown.

Operationregion

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Some important characteristics: Nominal Zener Voltage :

5.1V zener,12V zener, etc.

Nominal Bias Current:the I z to get the nominal V z

Tolerance on zener voltage, e.g. :12V 5%,

Maximum Power:1Watt zener, 5 Watt zener, etc.

Temperature coefficient:by what % does zener voltage change as diode temp. changes1OC

Dynamic Resistance (Rd):Rd = V/ I

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Zener Operation Region Zeners are available

with voltagebreakdowns of 1.8 Vto 200 V.

This curve illustratesthe minimum andmaximum ranges ofcurrent operation thatthe zener caneffectively maintainits voltage.

V

I

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Basic Zener Circuit

Key points:Vin > VzIT = (Vin

Vz)/Rs IL+IZ

ITIZ IL

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Calculation: Find RSuppose a 5.1 Volt zener is connected to a 12 Volt

supply through a resistor. The zener requires a15 mA bias, and the load is 510 Ohms. Find therequired resistor value.

load current: I = 5.1V / 510 = 10 mAtotal current: I T = I + IZ = (10 + 15) = 25mA

drop across R: V = 12V 5.1V = 6.9 VR value : R = V / IT = 6.9 V / 25 mA =276 OhmsSelect standard value resistor: R = 270Ohms

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DATA SHEET

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Zener Diode ApplicationsRegulation

In this simple illustration of zener regulation circuit, the zenerdiode will adjust its impedance based on varying inputvoltages and loads (R L) to be able to maintain its designatedzener voltage. Zener current will increase or decrease directly

with voltage input changes. The zener current will increase ordecrease inversely with varying loads . Again, the zener has afinite range of operation.

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A 10 V zener has 20 mA of bias current.The load resistor across the zener is 20Ohms. What power rating should thezener have ? Remember: if the load is

removed, all current is in the zener.Find total current: IT = I BIAS + I LOAD = 20mA + 50mA = 70 mAFind power in zener (Pz) at a current (Iz)

= 70 mA: Pz = V z I z = 10V 70ma = 700 mWDouble value for reliability:

Use a zener rated for 1.5 Watts or higher

Calculation: Find PMAX

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Troubleshooting Although precise power supplies typically use IC typeregulators, zener diodes can be used alone as a voltageregulator.

A properly functioning zener will work to maintain the output voltage withincertain limits despite changes in load.

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Zener Limiting

Zener diodes can be used as limiters . Thedifference to consider for a zener limiter is aits zener breakdown characteristics.

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Voltage Surge Protectors

Fast, high- voltage transients, called spikes,on AC power lines can damage electronicequipment.Back-to-back zeners can clip off the spikes.

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Fig 3.12 Capacitance range: from 50 pF to 500 pF

Varactor Capacitance

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Varactor Diodes

A varactor diode is best explained as a variable capacitor.Think of the depletion region a variable dielectric. Thediode is placed in reverse bias. The dielectric is adjustedby bias changes.

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Calculation: C & fRIf the varactor of figure 3.12 isbiased at V R =5 V.

1. Find the capacitance from the graph.

2. Find the resonant frequency with a253 uH inductor.

From the graph, C = 100 pF.Resonant frequency f R = 1/(2 LC)= 1.0 MHz

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Varactor Tuner

Similar tuners are used in TVs, cell-phones, etc.

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Varactor Diodes

The varactor diode can be useful in filter circuits as theadjustable component.

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The PIN Diode

Usable at high-frequencies

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PIN DIODE The pin diode is also used in mostly microwave frequencyapplications . Its variable forward series resistance characteristicis used for attenuation, modulation, and switching. In reversebias exhibits a nearly constant capacitance.

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Not a PN junctionFast, b ut reverse breakdown voltageless than 50 V

Schottky Diodes

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Schottky diode

The Schottky diodes significant characteristic is itsfast switching speed. This is useful for highfrequencies and digital applications. It is not a typicaldiode in the fact that it does not have a p-n junction,

instead it consists of a heavily doped n-material andmetal bound together.

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TUNNEL Diode

The tunnel diode has negativeresistance . It will actually conduct well withlow forward bias.

With further increases in bias it reachesthe negative resistance range where

current will actually go down . This isachieved by heavily doped p and nmaterials that creates a very thin depletion

region.

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TUNNEL DIODE CHARACTERISTIC

The step-recovery diode is also used for fastswitching applications.

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LASER DIODE

The laser diode ( light a mplification by s timulated e missionof r adiation) produces a monochromatic (single color) light.Laser diodes in conjunction with photodiodes are used toretrieve data from compact discs.

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Laser is an abbreviation of ...

Light Amplification by S timulatedEmission of Radiation Stimulated emission

antonym of spontaneous emission optical transition stimulated by the effect o

electric field of light waveon the contrary usually emission occurspontaneously without help of electric field

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What is the difference between LED and LD?

LED is light emitting diodeLD is laser diode

Diode is a semiconductor device which hasan effect of rectification Both LED and LD are semiconductor diode

with a forward bias. Both emit light

LED emits light by spontaneous emission mechanism, while LD has an optical cavity which enables multiplication of photon bystimulated emission

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Explain how the light is transmittedthrough optical fiber.

Light is transmitted along the core bytotal reflection mechanism at theboundary with the cladding layer

N=1.46N=1.48

cladding

core

Light Ray EnteringCore from Air

Light is propagated by Total internal reflection

CROSS SECTION

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LEDs: Light Emitting Diodes

Brightness proportional to currentColors: red, white, blue, green, orange, yellowDrop across an LED is about 1.5 Volts

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Optical Diodes

The light-emitting diode (LED) emits photons asvisible light. Its purpose is for indication and otherintelligible displays. Various impurities are addedduring the doping process to vary the color output.

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How much power does an LEDconsume if it requires 25 mA and

has a forward drop of 2.0 Volts?

P = V I = 2V .025A = 50mW

Calculation: Power in an LED

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7 SEGMENT DISPLAYThe seven segment display is an example of LEDs use fordisplay of decimal digits.

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How much power would a 4-digit 7-segment LED display consume if each LEDrequired 10 mA and had a forward drop of1.5 Volts?

Power in one LED : P LED = V I = 2V .01A = 20 mW Assume all segments are lit, then:

Power in a Digit: PD = 7 PLED = 7 20mW = 140 mW

Total Power: PT = 4 PD = 4 140 mW = 560 mW

Thats over half a Watt!

Power in a 7-Segment Display

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Multiplexing to Reduce Power

Suppose a 4-digit display requires400 mW if all segments are lit. If thedisplay is multiplexed so that eachdigit is lit in a continuous sequence(1,2,3,4,1,2,3,4...) how much powerwould the display use?

Since each digit is on for only 25% ofthe time,

P = 0.25 400 mW = 100 mW

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Troubleshooting Although precise power supplies typically use IC type

regulators, zener diodes can be used alone as a voltageregulator.

A properly functioning zener will work to maintain the output voltage withincertain limits despite changes in load.

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DIODA FOTO

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Fig. Description Harga

A Photo Transistor $0.70 $0.55

A Photo Transistor 0.40 0.30B Photo Transistor 0.45 0.40C Photo Transistor 0.40 0.30

A Photo Diode 0.75 0.65 A Photo Diode 0.50 0.40H Photo Diode 0.50 0.40D Photo Darlington/Motorola 2.95 2.50D Photo Darlington 2.25 1.95D Photo Transistor GE 1.85 1.65

D Photo Transistor GE 1.85 1.65E Photo Diode 1.20 0.90D Photo Transistor/Motorola 1.90 1.60F Photo Darlington 2.50 1.95F Photo Diode 1.60 1.40G Solar Cell 2.10 1.95

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FOTODIODA INFRA MERAH

http://www.roithner-laser.com/MID_IR_PD.html

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FOTODIODA InGaAs

http://www.roithner-laser.com/ingaas_photodiodes.html

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FOTODIODA ULTRAVIOLET

http://www.roithner-laser.com/UV-PD.htmlhttp://www.roithner-laser.com/UV-PD.html

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photodiode The photodiode is used to vary current by the amount oflight that strikes it. It is placed in the circuit in reverse bias.

As with most diodes when in reverse bias, no current flowswhen in reverse bias, but when light strikes the exposed

junction through a tiny window, reverse current increasesproportional to light intensity.

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Fundamentals of photodiode

Illuminate the pn junctionElectrons and holes

are generated by anexcitation across thegap

Generated electronsand holes areseparated and driftto electrodes by

-type -type

Depletion layer

++++

----

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PENGIRIM DAN PENERIMA

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LIGHT DETECTOR CIRCUIT

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Symbols for Special Diodes

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Silicon diodes can be checked for opensand shorts by measuring their resistancewith a DMM or a VOMZener diodes are checked by measuringtheir voltage either in-circuit or in a test fixture.LEDs can be checked out of circuit with aDC voltage source and a resistor. Put 10 to20 milliamps through the LED and see if itlights.Other special diode require special testfixtures, such as an oscillator circuit and

frequency counter for a varactor.

Troubleshooting

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Summary

The zener diode operates in reverse breakdown .

A zener diode maintains a nearly constant voltage acrossits terminals over a specified range of currents.

Line regulation is the maintenance of a specific voltagewith changing input voltages.

Load regulation is the maintenance of a specific voltagefor different loads.

There are other diode types used for specific RF purposes

such as varactor diodes (variable capacitance), Schottkydiodes ( high speed switching), and PIN diodes ( microwaveattenuation and switching ).

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Summary

Light emitting diodes (LED) emit eitherinfrared or visible light when forward biased.

Photodiodes exhibit an increase in reversecurrent with light intensity.

The laser diode emits a monochromaticlight