diode applications 3

8/3/2019 Diode Applications 3

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Diode Applications


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LoadLoad--Line AnalysisLine Analysis

22

The load line plots all possible

combinations of diode current (ID)

and voltage (VD) for a given circuit.

The maximumID equalsE/R, and

the maximumV

D equalsE

.

The point where the load line and

the characteristic curve intersect is

the Q-point, which identifiesID and

VD

for a particular diode in a given

circuit.


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Series Diode ConfigurationsSeries Diode Configurations

Constants

Silicon Diode: VD = 0.7 V

Germanium Diode: VD = 0.3 V

Analysis (for silicon)

VD = 0.7 V (or VD =EifE< 0.7 V)

VR = E VD

ID = IR = IT= VR/ R

33

Forward BiasForward Bias


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Series Diode ConfigurationsSeries Diode Configurations

Diodes ideally behave as open circuits

Analysis

VD = E

VR = 0 V

ID = 0 A

44

Reverse BiasReverse Bias


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Parallel ConfigurationsParallel Configurations

55

mA142

mA28

D2I

D1I

mA28.33k

V.7V10

R

DVE

RI

V9.3

R

V

V0.7O

VD2

VD1

V

V0.7D

V

!!!

!

!

!

!

!!!

!


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A. Rectifiers


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- converts the alternating current input signalto a pulsating direct current


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ACSignal

+Vp

-Vp

Vp-p

1 cycle

Frequency the no.of cycles in a second;

Hz

Period the time it

would take to

complete one cycle; s Wavelength

distance covered by

one cycle; m

NOTE:

2 P rmsV V! 2 P rmsV V!


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HalfHalf--Wave RectificationWave Rectification

99

The diode only

conducts when it is

forward biased,

therefore only halfof the AC cycle

passes through the

diode to the

output.


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Derivation (half-wave rectifier)

The DC output voltage is 0.318Vm, where Vm = the peak AC voltage.


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PIV (PRV)PIV (PRV)

1111

Because the diode is only forward biased for one-half of the AC cycle, it is

also reverse biased for one-half cycle.

It is important that the reverse breakdown voltage rating of the diode be

high enough to withstand the peak, reverse-biasing AC voltage.

PIV (or PRV) > Vm

PIV =Peak inverse voltage

PRV =Peak reverse voltage

Vm =Peak AC voltage


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FullFull--Wave RectificationWave Rectification

1212

Half-wave: VVdcdc = 0.318= 0.318VVmm Full-wave: VVdcdc = 0.636= 0.636VVmm

The rectification process can be improved by

using a full-wave rectifier circuit.

Full-wave rectification produces a greaterDC output:


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1313

Bridge RectifierBridge Rectifier

Four diodes are connected ina bridge configuration

VDC = 0.636Vm


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1414

CenterCenter--Tapped TransformerTapped Transformer

RectifierRectifier

Requires

Two diodes

Center-tapped transformer

VDC = 0.636Vm


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Derivation (full-wave rectifier)

The DC output voltage is 0.636Vm, where Vm = the peak AC voltage.


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Summary ofRectifier CircuitsSummary ofRectifier Circuits

1616

Vm = peak of the AC voltage.

IIn the center tapped transformer rectifier circuit, the peak AC voltageis the transformer secondary voltage to the tap.

RectifierRectifier IdealIdeal VVDCDC RealisticRealistic VVDCDC

Half Wave Rectifier VDCDC = 0.318Vm VDCDC = 0.318Vmm 0.7

Bridge Rectifier VDCDC = 0.636Vm VDCDC = 0.636Vm 2(0.7 V)

Center-Tapped Transformer

RectifierVDCDC = 0.636Vm VDCDC = 0.636Vm 0.7 V


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B. WaveshapingCircuits


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Diode Clippers / LimitersDiode Clippers / Limiters

used to clip-off portions of an ACsignal voltages above or below acertain kind.

1818

The diode in a series clipperseries clipper clipsany voltage that does not forward

bias it:

A reverse-biasing polarity

A forward-biasing polarity less than0.7 V (for a silicon diode)


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Biased ClippersBiased Clippers

1919

Adding a DC source in

series with the clipping

diode changes the

effective forward bias of

the diode.


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Parallel ClippersParallel Clippers

2020

The diode in a parallel clipperparallel clipper

circuit clips any voltage that

forward bias it.

DC biasing can be added in

series with the diode to change

the clipping level.


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Summary of Clipper CircuitsSummary of Clipper Circuits

2121

moremore


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Clampers / DC restorersClampers / DC restorers

2222

A diode and capacitor can be

combined to clamp an AC

signal to a specific DC level.


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Summary of Clamper CircuitsSummary of Clamper Circuits

2424


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Zener DiodesZener Diodes

2525

The Zener is a diode operated

in reverse bias at the Zener

Voltage (Vz).

When Viu VZ The Zener is on

Voltage across the Zener is VZ Zener current:IZ=IRIRL The Zener Power: P

Z

=VZ

IZ

When Vi< VZ The Zener is off

The Zener acts as an open circuit


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Zener Resistor ValuesZener Resistor Values

2626

ZKRLIII !

min

min

max

L

Z

LI

VR !

min

max

L

Z

L

L

L

R

V

R

VI !!

Zi

Z

L

VV

RVR

!min

IfR is too large, the Zener diode cannot conduct because the available amount of

current is less than the minimum current rating,IZK. The minimum current isgiven by:

The maximum value of resistance is:

IfR is too small, the Zener current exceeds the maximum current

rating,IZM. The maximum current for the circuit is given by:

The minimum value of resistance is:


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VoltageVoltage--Multiplier CircuitsMultiplier Circuits

Voltage Doubler

Voltage Tripler Voltage Quadrupler

2727

Voltage multiplier circuits use a combination of diodes andcapacitors to step up the output voltage of rectifier circuits.


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Voltage DoublerVoltage Doubler

2828

This half-wave voltage doublers output can be calculated by:

Vout = VC2 = 2Vm

where Vm = peak secondary voltage of the transformer


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Voltage DoublerVoltage Doubler

2929

Positive Half-Cycle

o D1 conducts

o D2 is switched off

o Capacitor C1 charges to Vm

Negative Half-Cycle

o D1 is switched off

o D2 conducts

o Capacitor C2 charges to Vm

Vout= VC2 = 2Vm


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Voltage Tripler and QuadruplerVoltage Tripler and Quadrupler

3030


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Practical ApplicationsPractical Applications

Rectifier Circuits

Conversions of AC to DC for DC operated circuits

Battery Charging Circuits

Simple Diode Circuits

Protective Circuits against

Overcurrent

Polarity Reversal

Currents caused by an inductive kick in a relay circuit

Zener Circuits

Overvoltage Protection

Setting Reference Voltages

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diode applications 3

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