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Electronics
Principles & Applications
Eighth Edition
Charles A. Schuler
Chapter 3
Diodes
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INTRODUCTION
• The PN Junction
• Characteristic Curves of Diodes
• Diode Lead Identification
• Diode Types and Applications
• Photovoltaic Energy Sources
Diodes are
two-terminal
workhorses.
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The P-side of a junction diode is doped with acceptor atoms.
The N-side of a junction diode is doped with donor atoms.
junction
P
Anode
N
Cathode
A silicon crystal
Schematic Symbol
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Zero Bias
The electrons near the junction cross
over and fill the holes near the junction.
Depletion region
Having no carriers, the depletion region is an insulator.
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Forward Bias
The carriers move toward the junction
and collapse the depletion region.
The diode is on (conducting).
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Reverse Bias
The carriers move away from the junction.
The depletion region is reestablished
and the diode is off.
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Diode quiz
At zero bias, diodes show a depletion region
which acts as an________.
insulator
The depletion region is collapsed by applying
_________ bias.
forward
The depletion region is made wider by applying
________ bias.
reverse
A forward-biased diode has its anode________
with respect to its cathode.
positive
Diode forward voltage drop decreases as
temperature ________.
increases
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5 ohms
200
175
150
10 ohms
mA
125
100
75
20 ohms
50
25
0
0
0.5
1.0
1.5
Volts
Resistor volt-ampere characteristic curves
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Forward current in mA
200
175
150
125
100
75
knee
50
25
0
0
0.5
1.0
1.5
Forward bias in volts
Silicon diode volt-ampere characteristic curve
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Linearity
• The volt-ampere characteristic curve for
a resistor is a straight line (linear).
• A diode has a non-linear characteristic
curve.
• The barrier potential produces a knee in
the diode curve.
• The knee voltage is approximately 0.6 to
0.7 volts for silicon diodes.
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Forward current in mA
200
175
100 oC
150
125
25 oC
100
75
-50 oC
50
25
0
0
0.5
1.0
1.5
Forward bias in volts
The effect of temperature
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breakdown
600
Reverse bias in Volts
400
200
0
20
40
60
80
Reverse
current
in mA
100
120
140
Silicon diode reverse bias characteristic curve
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Cathode lead
Anode lead
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Cathode lead
V
mA
Anode lead
The diode is forward biased by the ohmmeter.
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Cathode lead
V
mA
Anode lead
The diode is reverse biased by the ohmmeter.
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Silicon diode ohmmeter testing
• Low resistance in both directions: the
diode is shorted.
• High resistance in both directions: the
diode is open.
• Relatively low resistance in the reverse
direction: the diode is leaky.
• The ratio of reverse resistance to forward
resistance is > 1000: the diode is good.
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Reverse bias in Volts
6
4
2
0
20
40
60
80
Reverse
current
in mA
100
120
140
A zener diode is designed to break down and
conduct backwards at lower voltages.
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Reverse bias in Volts
4
6
2
0
20
40
60
V
I
80
Reverse
current
in mA
100
120
140
V
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The voltage across a conducting
zener is relatively constant.
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Using a zener diode as a voltage regulator
Unregulated Supply
Load
The load is in parallel with the zener and will see a
relatively constant voltage as long as the zener is conducting.
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This circuit is called a clipper or limiter.
When VIN < 1.2 VPP
VIN
VOUT
VOUT is not clipped
The diodes do not conduct.
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When VIN > 1.2 VPP
VIN
VOUT
+ 0.6 V
VOUT is clipped
The diodes conduct.
- 0.6 V
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This circuit is called a clamp or dc restorer.
When VIN > 1.2 VPP
C is charged.
VIN
C
VOUT
dc
VOUT is clamped.
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0.6 V
VPP
- 0.6 V
VOUT(dc) =
2
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CEMF
The inductive kick
can cause damage.
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Transient suppression diodes can be used with inductive loads.
CEMF
The coil discharges
through the diode and
there is no arc.
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LED
As the electrons cross the junction, they
lose energy in the form of photons.
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LED circuit
RS
Power
supply
LED
VS
VD
IS =
VS - VD
RS
The typical voltage drop for most LEDs is from 1.5 to 2.5 V.
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VISIBLE AND/OR
INVISIBLE
LASER DIODE
VISIBLE AND/OR INVISIBLE
LASER RADIATION-AVOID
DIRECT EXPOSURE TO BEAM
AVOID EXPOSURE
visible and/or invisible
Laser Radiation is
emitted from this
aperture
Ultraviolet LEDs are also dangerous and invisible!
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RS
Power
supply
VS
Photodiode
Photodiodes are reverse biased and
conduct in the presence of light.
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Input
Output
Input
Optocoupler
Output
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Step-index multimode fiber
Input pulse
Output pulse
The combined shorter and longer path lengths
act to stretch the output pulse.
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Step-index multimode fiber
Due to pulse stretching, high speed data transmission is not possible.
Input
Output
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Single mode fiber
Input
A single path means no pulse stretching and
high speed data transmission is possible.
Output
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Tuning diode
L
C
ZERO BIAS
C is maximum and fR is minimum.
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Tuning diode
L
C
REVERSE BIAS
C is less and fR increases.
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Tuning diode
L
C
MAX. REVERSE BIAS
C is minimum and fR is maximum.
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Diode applications quiz
A circuit used to control the amplitude of a
signal is the ________.
clipper
A circuit used to add a dc component to a
signal is the ________.
clamp
A device containing an LED and a photodiode is the ________.
optocoupler
A tuning diode shows less capacitance as
reverse bias ________.
increases
The device that is often used to regulate
voltage is the ________ diode. zener
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Here we see a photon
enter a PV cell and
liberate an electron.
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Then we see the
electron move to the
front contact and
through the load.
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For a given level of illumination, there is one and
only one load that will result in maximum power
transfer to the load.
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The MPPT controller maintains system efficiency
for varying levels of illumination.
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Measuring the current output from a PV source is a
common troubleshooting task.
A camp type current probe is a safer and more
convenient method of measuring output current.
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Photovoltaic quiz
Photovoltaic devices directly convert
light energy to ________ energy. electrical
A photon entering a PV cell might liberate a
free ________.
electron
The ideal (best power transfer) load on a PV
source varies with the level of_______. illumination
MPPT improves PV system efficiency by
adjusting the ________ resistance.
load
PV troubleshooting might involve measuring
the voltage and/or the ____ output.
current
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REVIEW
• The PN Junction
• Characteristic Curves of Diodes
• Diode Lead Identification
• Diode Types and Applications
• Photovoltaic Energy Sources
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