Transcript Zener Diodes

Bahan Kuliah minggu ke 5
Elektronika Dasar
SPECIAL DIODES
Jurusan Teknik Elektro
UGM
2007
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Zener Diodes
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A zener is used in reverse breakdown
mode
The voltage across a zener is more or
less independent of the current through
it
The function of a zener is to provide a
voltage reference in a circuit
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ZENER CHARACTERISTIC
A zener diode is much like a normal diode. The
exception being is that it is placed in the circuit in
reverse bias and operates in reverse breakdown.
Operation
region
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Some important characteristics:
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Nominal Zener Voltage :
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Nominal Bias Current:
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1Watt zener, 5 Watt zener, etc.
Temperature coefficient:
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12V  5%,
Maximum Power:
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the Iz to get the nominal Vz
Tolerance on zener voltage, e.g. :
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5.1V zener,
12V zener, etc.
by what % does zener voltage change as diode temp. changes
1OC
Dynamic Resistance (Rd):
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Rd = V/ I
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Zener Operation Region
• Zeners are available
with voltage
breakdowns of 1.8 V
to 200 V.
• This curve illustrates
the minimum and
maximum ranges of
current operation that
the zener can
effectively maintain
it’s voltage.
∆I
∆V
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Basic Zener Circuit
IT
IZ
IL
Key points:
 Vin > Vz
 IT = (Vin – Vz)/Rs  IL+IZ
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Calculation: Find R
Suppose a 5.1 Volt zener is connected to a 12 Volt
supply through a resistor. The zener requires a
15 mA bias, and the load is 510 Ohms. Find the
required resistor value.
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load current: IL = 5.1V / 510 = 10 mA
total current: IT = IL + IZ = (10 + 15) = 25
mA
drop across R: VR = 12V – 5.1V = 6.9 V
R value : R = VR / IT = 6.9 V / 25 mA =
276 Ohms
Select standard value resistor: R = 270
Ohms
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DATA SHEET
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Zener Diode Applications
Regulation
In this simple illustration of zener regulation circuit, the zener
diode will “adjust” it’s impedance based on varying input
voltages and loads (RL) to be able to maintain it’s designated
zener voltage. Zener current will increase or decrease directly
with voltage input changes. The zener current will increase or
decrease inversely with varying loads. Again, the zener has a
finite range of operation.
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Calculation: Find PMAX
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A 10 V zener has 20 mA of bias current.
The load resistor across the zener is 20
Ohms. What power rating should the
zener have? Remember: if the load is
removed, all current is in the zener.
Find total current:
•
IT = IBIAS + ILOAD = 20mA + 50mA = 70 mA
•
Pz = Vz Iz = 10V  70ma = 700 mW
•
Use a zener rated for 1.5 Watts or higher
Find power in zener (Pz) at a current (Iz)
= 70 mA:
Double value for reliability:
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Troubleshooting
Although precise power supplies typically use IC type
regulators, zener diodes can be used alone as a voltage
regulator.
A properly functioning zener will work to maintain the output voltage within
certain limits despite changes in load.
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Zener Limiting
Zener diodes can be used as limiters. The
difference to consider for a zener limiter is a
it’s zener breakdown characteristics.
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Voltage Surge Protectors
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Fast, high-voltage transients, called “spikes”,
on AC power lines can damage electronic
equipment.
Back-to-back zeners can clip off the spikes.
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Varactor Diodes
A reverse-biased PN junction makes a
voltage-controlled capacitor
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Varactor Capacitance
Fig 3.12 Capacitance range: from 50 pF to 500 pF
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Varactor Diodes
A varactor diode is best explained as a variable capacitor.
Think of the depletion region a variable dielectric. The
diode is placed in reverse bias. The dielectric is “adjusted”
by bias changes.
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Calculation: C & fR
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If the varactor of figure 3.12 is
biased at VR =5 V.
1. Find the capacitance from the graph.
2. Find the resonant frequency with a
253 uH inductor.
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From the graph, C = 100 pF.
Resonant frequency fR = 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 the
adjustable 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 frequency
applications. It’s variable forward series resistance characteristic
is used for attenuation, modulation, and switching. In reverse
bias exhibits a nearly constant capacitance.
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Schottky Diodes
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Not a PN junction
Fast, but reverse breakdown voltage
less than 50 V
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Schottky diode
The Schottky diode’s significant characteristic is it’s
fast switching speed. This is useful for high
frequencies and digital applications. It is not a typical
diode in the fact that it does not have a p-n junction,
instead it consists of a heavily doped n-material and
metal bound together.
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TUNNEL Diode
The tunnel diode has negative
resistance. It will actually conduct well with
low forward bias.
With further increases in bias it reaches
the negative resistance range where
current will actually go down. This is
achieved by heavily doped p and n
materials that creates a very thin depletion
region.
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TUNNEL DIODE CHARACTERISTIC
The step-recovery diode is also used for fast
switching applications.
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LASER DIODE
The laser diode (light amplification by stimulated emission
of radiation) produces a monochromatic (single color) light.
Laser diodes in conjunction with photodiodes are used to
retrieve data from compact discs.
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Laser is an abbreviation of …...
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Light Amplification by Stimulated
Emission of Radiation
• “Stimulated emission”
antonym of “spontaneous emission”
• optical transition stimulated by the effect of
electric field of light wave
on the contrary usually emission occur
spontaneously without help of electric field
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What is the difference between LED and LD?
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LED is light emitting diode
LD is laser diode
• Diode is a semiconductor device which has
an 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 by
stimulated emission
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Explain how the light is transmitted
through optical fiber.
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Light is transmitted along the core by
total reflection mechanism at the
boundary with the cladding layer
cladding
CROSS SECTION
N=1.46
N=1.48
Light Ray Entering
Core from Air
core
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Light is propagated by Total internal reflection
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LEDs: Light Emitting Diodes
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Brightness proportional to current
Colors: red, white, blue, green, orange, yellow
Drop across an LED is about 1.5 Volts
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Optical Diodes
The light-emitting diode (LED) emits photons as
visible light. It’s 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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Calculation: Power in an LED
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How much power does an LED
consume if it requires 25 mA and
has a forward drop of 2.0 Volts?
P = V  I = 2V  .025A = 50
mW
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THE 7-SEGMENT DISPLAY
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Bright, but consumes a lot of power
Typically multiplexed to conserve power
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7 SEGMENT DISPLAY
The seven segment display is an example of LEDs use for
display of decimal digits.
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Power in a 7-Segment Display
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How much power would a 4-digit 7segment LED display consume if each LED
required 10 mA and had a forward drop of
1.5 Volts?
Power in one LED:
• PLED = V  I = 2V  .01A = 20 mW
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• Assume all segments are lit, then:
Power in a Digit:
• PD = 7  PLED = 7  20mW = 140 mW
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Total Power:
• PT = 4  PD = 4  140 mW = 560 mW
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That’s over half a Watt!
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Multiplexing to Reduce Power
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Suppose a 4-digit display requires
400 mW if all segments are lit. If the
display is multiplexed so that each
digit is lit in a continuous sequence
(1,2,3,4,1,2,3,4...) how much power
would the display use?
Since each digit is on for only 25% of
the 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 voltage
regulator.
A properly functioning zener will work to maintain the output voltage within
certain limits despite changes in load.
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DIODA FOTO
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Fig.
Description
Harga
A
A
B
C
A
A
H
D
D
D
D
E
D
F
F
G
Photo Transistor
Photo Transistor
Photo Transistor
Photo Transistor
Photo Diode
Photo Diode
Photo Diode
Photo Darlington/Motorola
Photo Darlington
Photo Transistor GE
Photo Transistor GE
Photo Diode
Photo Transistor/Motorola
Photo Darlington
Photo Diode
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Solar Cell
$0.70
0.40
0.45
0.40
0.75
0.50
0.50
2.95
2.25
1.85
1.85
1.20
1.90
2.50
1.60
2.10
$0.55
0.30
0.40
0.30
0.65
0.40
0.40
2.50
1.95
1.65
1.65
0.90
1.60
1.95
1.40 39
1.95
FOTODIODA INFRA MERAH
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FOTODIODA InGaAs
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FOTODIODA ULTRAVIOLET
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photodiode
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
when in reverse bias, but when light strikes the exposed
junction through a tiny window, reverse current increases
proportional to light intensity.
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Fundamentals of photodiode
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Illuminate the pn
junction
Electrons and holes
are generated by an
excitation across the
gap
Generated electrons
and holes are
separated and drift
to electrodes by
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ｐ-type
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+
+
+
+
ｎ-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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Troubleshooting
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Silicon diodes can be checked for opens
and shorts by measuring their resistance
with a DMM or a VOM
Zener diodes are checked by measuring
their voltage either in-circuit or in a test
fixture.
LEDs can be checked out of circuit with a
DC voltage source and a resistor. Put 10 to
20 milliamps through the LED and see if it
lights.
Other special diode require special test
fixtures, such as an oscillator circuit and
frequency counter for a varactor.
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Summary
 The zener diode operates in reverse breakdown.
 A zener diode maintains a nearly constant voltage across
it’s terminals over a specified range of currents.
 Line regulation is the maintenance of a specific voltage
with changing input voltages.
 Load regulation is the maintenance of a specific voltage
for different loads.
 There are other diode types used for specific RF purposes
such as varactor diodes (variable capacitance), Schottky
diodes (high speed switching), and PIN diodes (microwave
attenuation and switching).
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Summary
Light emitting diodes (LED) emit either
infrared or visible light when forward biased.
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Photodiodes exhibit an increase in reverse
current with light intensity.
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The laser diode emits a monochromatic
light
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