1 CHAPTER 2 Diode Applications By: Nur Baya Binti Mohd Hashim School of Computer and Communication Engineering

11

CHAPTER 2CHAPTER 2

Diode ApplicationsDiode ApplicationsBy:

Nur Baya Binti Mohd HashimSchool of Computer and Communication Engineering

22

ObjectivesObjectives Explain and analyze the operation of both half and full wave rectifiers

Explain and analyze filters and regulators and their characteristics

Explain and analyze the operation of diode limiting and clamping circuits

Explain and analyze the operation of diode voltage multipliers Interpret and use a diode data sheet

Troubleshoot simple diode circuits

33

Lecture’s contentsLecture’s contents 2-1 The Basic DC Power Supply 2.2 Half-wave rectifier 2-3 Full-wave rectifier 2-4 Power supply filters and regulators 2-5 Diode limiting and clamping circuits 2-6 Voltage multipliers 2-7 Diode data sheet 2-8 Troubleshooting Summary

44

Power supply is a group of circuits that convert the standard ac voltage (120 V, 60 Hz) provided by the wall outlet to constant dc voltage.The voltage produced is used to power all types of electronic circuits including:

Consumer electronics (ex: radio, television, DVD, etc.) Computers Industrial controllers Most laboratory instrumentation systems and equipment

The dc voltage level required depends on the application, but most applications require relatively low voltage.There are two basic types of power supplies: a linear power supply and a switching power supply. These components are described as follows:1. A linear power supply is one that provides a constant current

path between its input and its load.2. A switching power supply provides an intermittent current

path between its input and its output.

2-1 The Basic DC Power Supply2-1 The Basic DC Power Supply

55

The basic function of a DC power supply is to convert an AC voltage to a constant DC voltage (AC DC)

2-1 The Basic DC Power 2-1 The Basic DC Power Supply(cont.)Supply(cont.)

Fig. 2-1: Block diagram of a dc power supply with a load and rectifier.

Eliminates the fluctuations- produce smooth dc voltage

(a) Complete power supply with transformer, rectifier, filter, and regulator

Either half or full-waverectifier convert ac input voltageto a pulsating dc voltage.

Maintains a constant dcvoltage

(b) Half-wave rectifier

66

TransformerA transformer is a device that changes ac electric power at one voltage level to ac electric power at another voltage level through the action of a magnetic field.

Simple transformer consist of: 1. Primary winding (input winding) 2. Secondary winding (output winding) 3. Magnetic core

If the secondary has more turns than the primary, the output voltage across the secondary will be higher and the current will be smaller. If the secondary has fewer turns than the primary, the output voltage across the secondary will be lower and the current will be higher.

The core has a function to concentrate the magnetic flux.

Fig.2-2: The general arrangement of a transformer


77

There are three types of transformers: step-up, step-down, and isolation.These components are described as follows: 1. The step-up transformer provides a secondary voltage that is greater than

the primary voltage. Ex: a step-up transformer may provides a 240 Vac output with a 120 Vac input.

2. The step-down transformer provides a secondary voltage that is less than the primary voltage. Ex: a step-down transformer may provides a 30 Vac output with a 120 Vac input.

3. An isolation transformer provides an output voltage that is equal to the input voltage. This type of transformer is used to isolate the power supply electrically from the ac power line.+

-

N

P+

-

N

S

120 Vac

240 Vac

+

-

N

P+

-

N

S

120 Vac

30 Vac

+

-

N

P+

-

N

S

120 Vac

120 Vac

(a) (b) (c)Fig.2-3


Step-upStep-down Isolation

88

The turns ratio of a transformer is equal to the voltage ratio of the component and since, the voltage ratio is the inverse of the current ratio. By formula:

sec

secsec

I

I

V

V

N

N pri

pripri

whereNSec = the number of turns in the secondary NPri = the number of turns in the primaryVSec = the secondary voltage VPri = the primary voltage ISec = the secondary current IPri = the primary current

By the equation (2-1) can be stated that: Step-down transformer secondary current is greater than its primary

current (ISec > IPri).

Step-up transformer secondary current is less than its primary current (IPri > ISec).

(2-1)


99

2-2 Half-Wave Rectifiers2-2 Half-Wave Rectifiers

Diode – ability to conduct current in one direction Diode – ability to conduct current in one direction and block current in other directionand block current in other direction

used in circuit called used in circuit called RECTIFIER (ac RECTIFIER (ac dc)dc)

Objective:Objective: Discuss the operation of half-wave rectifiersDiscuss the operation of half-wave rectifiers Describe a basic dc power supply & half-wave rectificationsDescribe a basic dc power supply & half-wave rectifications Determine the average value, VDetermine the average value, VAVGAVG of half-waves rectified of half-waves rectified

voltagevoltage Discuss the effect of barrier potential, VDiscuss the effect of barrier potential, VPP on a half-wave on a half-wave

rectifier outputrectifier output Define Define Peak Inverse VoltagePeak Inverse Voltage (PIV) (PIV) Describe Transformer-couple half-wave rectifierDescribe Transformer-couple half-wave rectifier

1010

•A half wave rectifier(ideal) allows conduction for only 180° or half of a complete cycle..•During first one cycle:

-Vin goes positive – diode FB – conduct current

-Vin goes negative – diode RB – no current- 0V

•The output frequency is the same as the input (same shape).

The average value VDC or VAVG :

Ideal diode model

pAVG

VV

-Measure on dc voltmeter

2-2 Half-Wave Rectifiers 2-2 Half-Wave Rectifiers (cont.)(cont.)

(The Half-Wave Rectifier)(The Half-Wave Rectifier)ac source load resistor

1111

Practical Diode – barrier potential of 0.7V (Si) taken into account.Practical Diode – barrier potential of 0.7V (Si) taken into account. During +ve half-cycle – VDuring +ve half-cycle – Vinin must overcome V must overcome Vpotentialpotential for forward bias. for forward bias.

Example 1: Calculate the peak o/p voltage, VExample 1: Calculate the peak o/p voltage, Vp(oup(out)t)??

The peak o/p voltage:The peak o/p voltage: VVV inpoutp 7.0)()(


(Effect of the Barrier Potential on the Half-Wave (Effect of the Barrier Potential on the Half-Wave Rectifier Output)Rectifier Output)

V

VV

VVV inpoutp

30.4

7.05

7.0)()(

1212


(Effect of the Barrier Potential on the Half-Wave (Effect of the Barrier Potential on the Half-Wave Rectifier Output)Rectifier Output)

Example 2:

Sketch the output V0 and determine the output level voltage for thenetwork in above figure.

1313

- Peak inverse voltage (PIV) is the maximum voltage across the diode when it is in reverse bias.

The diode must be capable of withstanding this amount of voltage.

)(inpVPIV

2-2Half-Wave Rectifiers (cont.)2-2Half-Wave Rectifiers (cont.) [Peak Inverse Voltage (PIV)][Peak Inverse Voltage (PIV)]

1414

Transformers are often used for voltage change and isolation.The turns ratio, n of the primary to secondary determines the output versus the input.

The advantages of transformer coupling: 1) allows the source voltage to be stepped up or down 2) the ac source is electrically isolated from the rectifier, thus prevents shock hazards in the secondary circuit.

to couple ac input to the rectifier

VVV poutp 7.0(sec))(

(sec)pVPIV priN

Nn sec

prinVV sec


(Half-Wave Rectifier with Transformer-Coupled Input (Half-Wave Rectifier with Transformer-Coupled Input Voltage)Voltage)

If n>1, Vsec is greater than Vpri.If n<1, Vsec is less than Vpri.

If n=1, Vsec= Vpri.

1515

Example 3:Example 3:

Determine the peak value of output voltage as shown in below Figure.

1616

Objective:Objective: Explain & Analyze the operation of Full-Wave Explain & Analyze the operation of Full-Wave

Rectifier.Rectifier. Discuss how full wave rectifier differs from half-wave Discuss how full wave rectifier differs from half-wave

rectifier rectifier Determine the average valueDetermine the average value Describe the operation of center-tapped & bridge.Describe the operation of center-tapped & bridge. Explain effects of the transformers turns ratioExplain effects of the transformers turns ratio PIVPIV Comparison between center-tapped & bridge.Comparison between center-tapped & bridge.

2-3 Full-Wave Rectifiers2-3 Full-Wave Rectifiers (Introduction)(Introduction)

1717

Twice output

p

AVG

VV

2

2-3 Full-Wave Rectifiers (cont.)2-3 Full-Wave Rectifiers (cont.) (Introduction)(Introduction)

A full-wave rectifier allows current to flow during both the positive and negative half cycles or the full 360º whereas half-wave rectifier allows only during one-half of the cycle.

The no. of +ve alternations is twice the half wave for the same time interval

The output frequency is twice the input frequency.

The average value – the value measured on a dc voltmeter

63.7% of Vp

1818

Coupled inputvoltage

•This method of rectification employs two diodes connected to a secondary center-tapped transformer.

•The i/p voltage is coupled through the transformer to the center-tapped secondary.

2-3 Full-Wave Rectifiers2-3 Full-Wave Rectifiers (i - The Center-Tapped Full-Wave Rectifier)(i - The Center-Tapped Full-Wave Rectifier)

1919

•+ve half-cycle input voltage (forward-bias D1 & reverse-bias D2)-the current pass through the D1 and RL

•-ve half-cycle input voltage (reverse-bias D1 & forward-bias D2)-the current pass through D2 and RL

•The output current on both portions of the input cycle – same direction through the load.

•The o/p voltage across the load resistors – full-wave rectifiers

2-3 Full-Wave Rectifiers (cont.)2-3 Full-Wave Rectifiers (cont.) (i - The Center-Tapped Full-Wave Rectifier)(i - The Center-Tapped Full-Wave Rectifier)

2020

priVV 2sec


If n=1, Vp(out)=Vp(pri) - 0.7V 2 Vp(sec)=Vp(pri)

If n=2,

7.0)()( pripoutp VV

-Effect of the Turns Ratio on the Output Voltage--Effect of the Turns Ratio on the Output Voltage-

priVV 2sec

• In any case, the o/p voltage is always one-half of the total secondary voltageminus the diode drop (barrier potential), no matter what the turns ratio.

VV

Vout 7.02sec

2121


-Peak Inverse Voltage (PIV)-

Maximum anode voltage:Maximum anode voltage:2(sec)

1pV

D 2(sec)

2pV

D

D1: forward-bias – its cathode is at the same voltage of its anode minus diode drop;

This is also the voltage on the cathode of D2. PIV across D2 :

VV

VV

VPIV

p

pp

7.0

27.0

2

(sec)

(sec)(sec)

VVV

VV

V

outpp

poutp

4.12

7.02

)((sec)

(sec))(

We know thatWe know that

Thus;Thus;

VVPIV outp 7.02 )(

2222

It employs four diodes arranged such that current flows in the direction through the load during each half of the cycle.

When Vin +ve, D1 and D2 FB and conduct current. A voltage across RL

looks like +ve half of the input cycle. During this time, D3 and D4 are RB.

When Vin –ve, D3 and D4 are FB and conduct current. D1 and D2 are RB.Used 4 diode:2 diode in forward2 diode in reverse

2 diode always in series with load resistor during +ve and –ve half cycle .

Without diode drop (ideal diode):

With diode drop (practical diode):

(sec))( poutp VV

VVV poutp 4.1(sec))(

2-2 Full-Wave Rectifiers (cont.)2-2 Full-Wave Rectifiers (cont.) (ii - The Bridge Full-Wave Rectifier)(ii - The Bridge Full-Wave Rectifier)

2323

)(outpVPIV

VVPIV outp 7.0)(

Note that in most cases we take the diode drop into account.

0V (ideal diode)

For each diode, VVPIV outp 7.0)(

2-2 Full-Wave Rectifiers (cont.)2-2 Full-Wave Rectifiers (cont.) (ii - The Bridge Full-Wave Rectifier)(ii - The Bridge Full-Wave Rectifier)

For ideal diode, PIV = Vp(out)

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2-3 Power Supply Filters And 2-3 Power Supply Filters And RegulatorsRegulators

(introduction)(introduction) ObjectiveObjective::

Explain & Analyze the operation & characteristic of Explain & Analyze the operation & characteristic of power supply filters & Regulatorspower supply filters & Regulators

Explain the purpose of a filterExplain the purpose of a filter Describe the capacitor-input filterDescribe the capacitor-input filter Define ripple voltage & calculate the ripple voltageDefine ripple voltage & calculate the ripple voltage Discuss surge current in capacitor-input filterDiscuss surge current in capacitor-input filter Discuss voltage regulation & integrated circuit Discuss voltage regulation & integrated circuit

regulatorregulator

2525

Power Supply FiltersPower Supply Filters

To reduce the fluctuations in the output voltage of half / full-To reduce the fluctuations in the output voltage of half / full-wave rectifier – wave rectifier – produces constant-level dc voltage..

It is necessary – electronic circuits require a constant source to It is necessary – electronic circuits require a constant source to provide power & biasing for proper operation.provide power & biasing for proper operation.

Filters are implemented with Filters are implemented with capacitorscapacitors..

RegulatorsRegulators Voltage regulation in power supply done using integrated Voltage regulation in power supply done using integrated

circuit voltage regulators.circuit voltage regulators. To To prevent changes in the filtered dc voltage/ / to fix output dc

voltage due to variations in input voltage or load.

2-4 Power Supply Filters And 2-4 Power Supply Filters And Regulators (cont.)Regulators (cont.)

(introduction) (introduction)

2626

•In most power supply – 60 Hz ac power line voltage is converted to constant dc voltage.

•60Hz pulsating dc output must be filtered to reduce the large voltage variation.

•Small amount of fluctuation in the filter o/p voltage - ripple

ripple


(introduction) (introduction)

2727


(Capacitor-Input Filter) (Capacitor-Input Filter) load

capacitor

Capacitive FilterCapacitive filter is simply a capacitor

connected in parallel with the load resistance or connected from the rectifier output to ground, as shown in Fig.2-39.

During the positive first quarter-cycle of the input, the diode is forward-biased, allowing the capacitor charges rapidly, as illustrated in Fig.2-39(a).

When the input begins to go negative, the diode is reverse-biased, and the capacitor slowly discharges through the load resistance (Fig.2-39(b)). As the output from the rectifier drops below the charged voltage of the capacitor, the capacitor acts as the voltage source for the load.

During first quarter of the next cycle, as illustrated in part (c), the diode will again become forward-biased when the input voltage exceeds the capacitor voltage.

2828

Ripple Voltage

Ripple voltage is the fluctuation in the capacitor voltage due to the difference between the charge and discharge times.The difference between the charge and discharge times is caused by two distinct RC time constant in the circuit. One time constant is found as:

RCwhere R and C are the total circuit resistance and capacitance, respectively.Since it takes five time constants for a capacitor to charge or discharge fully, this time period (T) can be found as:

55 RCT

2929

The discharge path for the capacitor is through the resistor as shown in Fig. 2-40(b). For this circuit, the time constant is found as:

msFk 100)100)(1(

For example, refer to Fig. 2-40(a), the capacitor charges through the diode. Assuming that diode has a forward resistance of 5 Ω, so the time constant for the circuit is found as:

sF 500)100)(5(

and the total capacitor charge time is found as:

mssT 5.2)500)(5(

msmsT 500)100)(5(

and the total capacitor discharge time is found as:

3030

(a) Charge circuit

(b) Discharge circuit

Fig.2-40: The basic capacitive filter.

3131

Ripple Voltage: the variation in capacitor voltage due to the charging and discharging times.

The advantage of a full-wave rectifier over a half-wave is quite clear. The capacitor can more effectively reduce the ripple when the time between peaks is shorter.

Easier to filter-shorted time between peaks.-smaller ripple.

2-4Power Supply Filters And 2-4Power Supply Filters And Regulators (cont.)Regulators (cont.)

(Capacitor-Input Filter) (Capacitor-Input Filter)

3232

DC

ppr

V

Vr )(

Ripple factor: indication of the effectiveness of the filter

Vr(pp) = peak to peak ripple voltage; VDC = VAVG = average value of filter’s output voltage.•Lower ripple factor better filter

[can be lowered by increasing the value of filter capacitor or increasing the load resistance]

[half-wave rectifier]

•For the full-wave rectifier:

)(

)()(

2

11

1

rectpL

AVGDC

rectpL

ppr

VCfR

VV

VCfR

V

Vp(rect) = unfiltered

peak.



3333

Surge Current in the Capacitor-Input Filter:

When you first turn on power supply, the filter capacitor has no accumulated charge to oppose Vp(sec). For first instant, the capacitor appears as a short circuit, thus the current through the diodes can momentarily be quite high. To reduce risk of damaging the diodes, a surge current limiting resistor is placed in series with the filter and load.

FSM

psurge I

VVR

4.1(sec) IFSM = forward surge current rating specified on diode data sheet.



The min. surgeResistor values:

3434

Connected to the output of a filtered & maintains a constant output voltage (or current) despite changes in the input, load current or temperature.

Combination of a large capacitor & an IC regulator – inexpensive & produce excellent small power supply

Popular IC regulators have 3 terminals:(i) input terminal

(ii) output terminal(iii) reference (or adjust) terminal

Type number: 78xx (xx –refer to output voltage) i.e 7805 (output voltage +5.0V); 7824 (output voltage +24V)


(IC Regulators) (IC Regulators)

3535

Regulation is the last step in eliminating the remaining ripple and maintaining the output voltage to a specific value. Typically this regulation is performed by an integrated circuit regulator. There are many different types used based on the voltage and current requirements.

Fig. 2-23 : A basic +5.0V regulated power supply

Gnd

output

Connected to the outputof filtered rectifier

Bridge-full waverectifier filter regulators


(IC Regulators) (IC Regulators)

3636

Percent Regulation

The regulation can be stated in a percentage in terms of input (line) regulation or

load regulation.

Line regulation specifies how much change occurs in the output voltage for a

given change in the input voltage. It is mathematically defined as a ratio of a

change in output voltage for a corresponding change in the input voltage

expressed as a percentage.

%100xV

VregulationLine

IN

OUT

Load regulation specifies how much change occurs in the output voltage over a certain range of load current values, usually from minimum current (no load, NL) to maximum current (full load, FL). It can be mathematically determined with the following formula:

%100xV

VVregulationLoad

FL

FLNL


(Percent Regulations) (Percent Regulations)

3737

2-5 Diode Limiting & Clamping Circuits2-5 Diode Limiting & Clamping Circuits

(Introduction) (Introduction) Objectives:Objectives:Analyze the operation of diode limiting, clamping circuit, Analyze the operation of diode limiting, clamping circuit, voltage multipliers and interpret and use diode data sheet.voltage multipliers and interpret and use diode data sheet.Determine V of biased limiter & used voltage-divider bias to Determine V of biased limiter & used voltage-divider bias to set limiting level.set limiting level.Discuss voltage doublers, triplers & quadruples.Discuss voltage doublers, triplers & quadruples.Identify V & current ratings.Identify V & current ratings.Determine the electrical characteristics of a diode.Determine the electrical characteristics of a diode.Analyze graphical data Analyze graphical data Select an appropriate diode for a given set of specifications.Select an appropriate diode for a given set of specifications.

2-5 Diode Limiting & Clamping Circuits 2-5 Diode Limiting & Clamping Circuits

(Diode Limiters) (Diode Limiters)

• Diode limiters/clippers – that limits/clips the portion of signal Diode limiters/clippers – that limits/clips the portion of signal voltage above or below certain level.voltage above or below certain level.

• Limiting circuits limit the positive or negative amount of an Limiting circuits limit the positive or negative amount of an input voltage to a specific value. input voltage to a specific value.

• 4 basic clipper configuration: 4 basic clipper configuration: Negative series clipperNegative series clipper diode is in series with its loaddiode is in series with its load

Positive series clipperPositive series clipper

Negative shunt clipperNegative shunt clipper diode is in parallel with its loaddiode is in parallel with its load

Positive shunt clipperPositive shunt clipper

3838


(Diode Limiters) (Diode Limiters)

3939

Clipper configuration

2-5 Diode Limiters2-5 Diode Limiters(series clipper/limiter)(series clipper/limiter)

Positive series clipperPositive series clipper Diode is reverse-biased during Diode is reverse-biased during

+ve alternation of i/p signal.+ve alternation of i/p signal. Diode is forward-biased when Diode is forward-biased when

i/p signal is –ve.i/p signal is –ve. Eliminates positive alternation Eliminates positive alternation

of its input.of its input.

4040

Negative series clipperNegative series clipper Diode is forward-biased during Diode is forward-biased during

+ve alternation of i/p signal.+ve alternation of i/p signal. Diode is reverse-biased when Diode is reverse-biased when

i/p signal is –ve. i/p signal is –ve. Eliminates negatives alternation Eliminates negatives alternation

of its i/p.of its i/p.

2-5Diode Limiters2-5Diode Limiters(shunt clipper/limiter)(shunt clipper/limiter)

Negative shunt clipperNegative shunt clipper Reverse-biased diode act as Reverse-biased diode act as

open cct during +ve cycle.open cct during +ve cycle. Forward-biased diode act as Forward-biased diode act as

short cct during –ve cycle.short cct during –ve cycle. o/p signal is limit/clip to -0.7V o/p signal is limit/clip to -0.7V

during –ve cycle of i/p signal.during –ve cycle of i/p signal.

Positive shunt clipperPositive shunt clipper Forward-biased diode when i/p Forward-biased diode when i/p

is +ve cycle.is +ve cycle. Reverse-biased diode when i/p Reverse-biased diode when i/p

is in –ve cycle.is in –ve cycle. o/p signal is limit/clip to +0.7V o/p signal is limit/clip to +0.7V

during +ve cycle of i/p signal.during +ve cycle of i/p signal.

4141


(Diode Limiters) (Diode Limiters)Series clipperSeries clipper

When diode in –ve series When diode in –ve series clipper is FB, load voltage clipper is FB, load voltage is: Vis: VLL=V=Vinin – 0.7V – 0.7V

When diode is RB, doesn’t When diode is RB, doesn’t conduct, so:conduct, so: VVLL = 0V = 0V

+ve series clipper operates +ve series clipper operates the same. The only the same. The only differences are:differences are:

O/p voltage polarities are O/p voltage polarities are reversed. Vreversed. VLL=-V=-Vinin + 0.7V + 0.7V

Current direction through cct Current direction through cct are reversed.are reversed.

Shunt clipperShunt clipper For –ve shunt, when i/p For –ve shunt, when i/p

signal +ve cycle, diode is signal +ve cycle, diode is RB (open circuit), thus:RB (open circuit), thus:

During –ve cycle, diode is During –ve cycle, diode is FB, load voltage is equal to FB, load voltage is equal to diode forward voltage.diode forward voltage.

VVLL = -V = -VFF = -0.7V = -0.7V

For +ve shunt, o/p voltage For +ve shunt, o/p voltage and current direction are and current direction are reversed. reversed.

4242

inL

Lout V

RR

RV

1

4343

Question 4:What would you expect to see displayed on an oscilloscope connectedacross RL in the limiter shown below.

4444

Solution question 4Solution question 4 The diode is forward biased and conducts when input The diode is forward biased and conducts when input

voltage goes below -0.7V. So, for –ve limiter, the voltage goes below -0.7V. So, for –ve limiter, the peak output voltage across RL is:peak output voltage across RL is:

The waveform is shown below:The waveform is shown below:

VVk

kV

RR

RV inp

L

Loutp 09.910

1.1

0.1)(

1)(

4545

2.5 Diode Limiting & Clamping Circuits (cont.) 2.5 Diode Limiting & Clamping Circuits (cont.)

(Diode Limiters) (Diode Limiters) Biased Limiters :• Use dc biasing source, VBIAS to set limit on the circuit output voltage. • This allow the circuit to clip input waveform at values other than diode forward voltage, 0.7V.• In each circuit, bias voltage is in series with shunt diode. As a result, the diode conducts and clips the i/p waveform when signal voltage equals sum of VF and VBIAS. • 2 type of biased limiter: positive-biased limiter negative-biased limiter• Positive limiter• The voltage at point A must equal VBIAS+0.7V before diode become FB and conduct.• Once diode begin to conduct, voltage at point A is limited to VBIAS+0.7V, so all i/p

voltage above this level is clipped off.• Negative limiter• Voltage at point A must go below –VBIAS - 0.7V to forward-bias the diode and initiate limiting action.

A positive limiter A negative limiter

4646

2-5 Diode Limiting & Clamping Circuits (cont.) 2-5 Diode Limiting & Clamping Circuits (cont.)

(Diode Limiters) (Diode Limiters) Voltage-Divider Bias:

• The bias voltage source – used to illustrate the basic operation of diode limiters can be replace by a resistive voltage divider that derives the desired bias voltage from dc Vsupply .• VBIAS – set by the resistor values according to the voltage-divider formula:

• The desired amount of limitation can be attained by a power supply or voltage divider. The amount clipped can be adjusted with different levels of VBIAS.

• The bias resistor << R1- the forward current through the diode will not effect VBIAS

SUPPLYBIAS VRR

RV

32

3

4747

2-5 Diode Limiting & Clamping Circuits 2-5 Diode Limiting & Clamping Circuits (cont.) (cont.)

(Diode Limiter Applications) (Diode Limiter Applications) Half-wave rectifierHalf-wave rectifier

Circuit alters the shape of ac signal and change it to pulsating Circuit alters the shape of ac signal and change it to pulsating dc.dc.

Transient protection circuit Transient protection circuit Transient Transient abrupt current or voltage spike in short duration. abrupt current or voltage spike in short duration. Many digital circuits have i/p that cannot tolerate voltage fall Many digital circuits have i/p that cannot tolerate voltage fall

outside a specified range which can cause serious damage. A outside a specified range which can cause serious damage. A clipper can be used to protect these circuits.clipper can be used to protect these circuits.

AM detectorAM detector Eliminate –ve portion of i/p waveform, so capacitor charges and Eliminate –ve portion of i/p waveform, so capacitor charges and

discharges at rate of peak i/p variations. This provides a signal discharges at rate of peak i/p variations. This provides a signal at load that is a reproduction of i/p signal. at load that is a reproduction of i/p signal.

4848

4949

Example 5:Example 5:

1. Sketch the output voltage waveform as shown in the circuit combining a positive limiter with negative limiter in Figure 5-1.

Figure 5-1

+15V

-15V6V 6V

5050

Example 5 (cont.):Example 5 (cont.):

2. A student construct the circuit as shown in Figure 5-2. Describe the output voltage waveform on oscilloscope CH2.

Figure 5-2

+20V

-20V

CH2

+15V

2.7 Clampers (DC Restorers)

Clamper is a diode circuit designed to shift a waveform either above or below a given

reference voltage without distorting the waveform.

There are two types of clampers: the positive clamper and the negative clamper.

1. A positive clamper shifts its input waveform so that the negative peak of the

waveform is equal to the clamper dc reference voltage.

For example: Fig. 2-54 shows what happens when a 20 Vpp sin wave is applied to a

positive clamper with a dc reference of 0 V. The input and output waveforms have the

value of 20 Vpp. However, the clamper output waveform has the positive peak of +20 V and

the negative peak of 0 V. The positive clamper has shifted the entire waveform so that

its negative peak is equal to the circuit’s dc reference voltage.

2-5 Diode Limiting & Clamping Circuits (cont.) 2-5 Diode Limiting & Clamping Circuits (cont.) (Diode Clampers) (Diode Clampers)

2-5 Diode Limiting & Clamping Circuits (cont.) 2-5 Diode Limiting & Clamping Circuits (cont.) (Diode Clampers) (Diode Clampers)

2. A negative clamper shifts its input waveform so that the positive peak of the waveform

is equal to the clamper dc reference voltage.

For example: Fig. 2-55 shows what happens when a 20 Vpp sin wave is applied to

a negative clamper with a dc reference of 0 V. In this case, The clamper output

waveform has the positive peak of 0 V and the negative peak of –20 V. The

negative clamper has shifted the entire waveform so that its negative peak is

equal to the circuit’s dc reference voltage.

Clamper OperationThe clamper is similar to a shunt clipper; the difference is added capacitor in the clamper, as illustrated in Fig. 2-56. For the circuit in Fig. 2-56(a), the diode is forward biased and it charges the capacitor. Thus, the charging time constant is found as:

CRDwhere RD is bulk resistance of the diode and C is capacitance of the capacitor. The total charge time is:

CRT DeCh 5arg

(2-39)

(2-40)

(a) Capacitor charge circuit

+

−

Reverse-biased

(b) Capacitor discharge circuit

Fig.2-56: Clamper charge and discharge

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When the diode is reverse biased, the capacitor starts to discharge through the resistor, as shown in Fig. 2-56(b). Therefore, the discharge time constant is found as:

CRL

CRT LeDisch 5arg

and the total discharge time is found as:

(2-41)

(2-42)

The effect of the clamping action is shown in Fig. 2-56. The capacitor retains a charge approximately equal to the input peak less the diode drop so that it acts as a battery.

Fig.2-57: Positive clamper operation

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If the diode is turned around, a negative dc voltage is added to the input voltage to produce the output voltage as shown in Fig. 2-57:

Fig.2-57: Negative clamper operation

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2-5 Diode Limiting & Clamping Circuits 2-5 Diode Limiting & Clamping Circuits (cont.)(cont.) (Diode Clampers) (Diode Clampers)

A Clamper Application:

A clamping circuit is often used in TV receivers as a dc restorer.

The incoming composite video signal is normally processed through capacitively coupled amplifiers that eliminate the dc component, thus losing black and white reference levels and the blanking level. Before applied to the picture tube, these reference level must be restored.

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2-6 Voltage Multiplier 2-6 Voltage Multiplier (Introduction)(Introduction)

• Use clamping action to increase peak rectified voltages without necessary to increase input transformer’s voltage rating.• Multiplication factors: two, three or four.• Three types of voltage multipliers:

* Voltage doubler- Half – wave voltage doubler

- Full – wave voltage doubler* Voltage tripler* Voltage Quadrupler

• Voltage multipliers are used in high-voltage, low current applications, i.e TV receivers.

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2-6 Voltage Multiplier (cont.) 2-6 Voltage Multiplier (cont.) (Voltage Doubler)(Voltage Doubler)

Half-wave voltage Doubler:

Clamping action can be used to increase peak rectified voltage. Once C1 and C2 charges to the peak voltage they act like two batteries in series, effectively doubling the voltage output. The current capacity for voltage multipliers is low.

Half-wave voltage doubler operation. Vp is the peak secondary voltage.

By applying Kirchhoff’s Law at (b):

12 CpC VVV ~ approximately 2Vp (neglecting diode drop D2)

PIV = 2Vp

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2-6 Voltage Multiplier (cont.) 2-6 Voltage Multiplier (cont.) (Voltage Doubler)(Voltage Doubler)

Full-wave voltage doubler:

Arrangement of diodes and capacitors takes advantage of both positive and negative peaks to charge the capacitors giving it more current capacity. forward-bias

charges

Secondary voltage positive Secondary voltage negative

forward-bias

outputcharges

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2-6 Voltage Multiplier (cont.) 2-6 Voltage Multiplier (cont.) (Voltage Tripler & Voltage Quadrupler)(Voltage Tripler & Voltage Quadrupler)

Voltage triplersVoltage triplers and and quadruplersquadruplers utilize utilize threethree and and fourfour diode capacitor diode capacitor arrangements, respectively.arrangements, respectively.

Voltage triplerVoltage tripler and and quadrupler quadrupler gives output gives output 3V3Vp p and and 4V4Vpp, respectively., respectively.

Tripler output is taken across Tripler output is taken across CC11 and C and C33, thus , thus VVoutout = 3V = 3Vpp

Quadrupler output is taken across Quadrupler output is taken across CC22 and and CC44 , thus , thus VVoutout = 4V = 4Vpp

PIV for both cases: PIV for both cases: PIV = 2VPIV = 2Vpp

Voltage Triple Voltage Quadruple

6161

• The data sheet for diodes and other devices gives detailed information about specific characteristics such as the various maximum current and voltage ratings, temperature range, and voltage versus current curves (V-I characteristic).

• It is sometimes a very valuable piece of information, even for a technician. There are cases when you might have to select a replacement diode when the type of diode needed may no longer be available.

• These are the absolute max. values under which the diode can be operated without damage to the device.

2-7 The Diode Data Sheet 2-7 The Diode Data Sheet (Introduction)(Introduction)

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2-7 The Diode Data Sheet (cont.) 2-7 The Diode Data Sheet (cont.) (Maximum Rating) (Maximum Rating)

RatingRating SymbolSymbol 1N40011N4001 1N40021N4002 1N40031N4003 UNITUNIT

Peak repetitive reverse voltagePeak repetitive reverse voltage

Working peak reverse voltageWorking peak reverse voltage

DC blocking voltageDC blocking voltage

VVRRMRRM

VVRWMRWM

VVRR

5050 100100 200200 VV

Nonrepetitive peak reverse Nonrepetitive peak reverse voltagevoltage

VVRSMRSM 6060 120120 240240 VV

rms reverse voltagerms reverse voltage VVR(rms)R(rms) 3535 7070 140140 VV

Average rectified forward Average rectified forward current (single-phase, resistive current (single-phase, resistive load, 60Hz, Tload, 60Hz, TAA = 75 = 75ooCC

IIoo

11

AA

Nonrepetitive peak surge Nonrepetitive peak surge current (surge applied at rated current (surge applied at rated load conditions)load conditions)

IIFSMFSM

30 (for 1 30 (for 1 cycle)cycle)

AA

Operating and storage junction Operating and storage junction temperature rangetemperature range

TTjj, T, Tstgstg -65 to -65 to +175+175

ooCC

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FIGURE 2-56FIGURE 2-56 A selection of rectifier diodes based on maximum ratings of I A selection of rectifier diodes based on maximum ratings of IOO, I, IFSMFSM, ,

and Iand IRRMRRM..

2-7 The Diode Data Sheet (cont.) 2-7 The Diode Data Sheet (cont.) (Maximum Rating)(Maximum Rating)

6464

2-8Troubleshooting2-8Troubleshooting(introduction)(introduction)

Objective: Objective:

Troubleshoot diode circuits using accepted techniques.Troubleshoot diode circuits using accepted techniques. Discuss the relationship between symptom & cause, power check, Discuss the relationship between symptom & cause, power check,

sensory check, component replacement method and discuss the sensory check, component replacement method and discuss the signal tracing technique in the three variations.signal tracing technique in the three variations.

Fault analysis.Fault analysis.

Our study of these devices and how they work leads more effective troubleshooting. Efficient troubleshooting requires us to take logical steps in sequence. Knowing how a device, circuit, or system works when operating properly must be known before any attempts are made to troubleshoot. The symptoms shown by a defective device often point directly to the point of failure. There are many different methods for troubleshooting. We will discuss a few.

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2-8 Troubleshooting (cont.)2-8 Troubleshooting (cont.)(Troubleshooting Techniques)(Troubleshooting Techniques)

Here are some helpful troubleshooting techniques:

Power Check: Sometimes the obvious eludes the most proficient troubleshooters. Check for fuses blown, power cords plugged in, and correct battery placement.

Sensory Check: What you see or smell may lead you directly to the failure or to a symptom of a failure.

Component Replacement: Educated guesswork in replacing components is sometimes effective.

Signal Tracing: Look at the point in the circuit or system where you first lose the signal or incorrect signal.

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2-8 Troubleshooting (cont.)2-8 Troubleshooting (cont.)(Troubleshooting Techniques)(Troubleshooting Techniques)

Signal tracing techniques:

Input to output

Output to input

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2-8 Troubleshooting (cont.)2-8 Troubleshooting (cont.)(Fault Analysis)(Fault Analysis)

Can be applied when you measure an incorrect voltage at a test point using signal tracing and isolate the fault to a specific circuit.Example 1:

Effect of an Open Diode in a Half-Wave Rectifier:

- Zero o/p voltage- Open diode breaks the current path from transformer secondary winding to thefilter and load resistor – no load current.Other faults: open transformer winding, open fuse, or no input voltage.

6868


Example 2:

Effect of an Open Diode in a Full-Wave Rectifier:

-The effect of either of two diodes is open diode, the o/p voltage will have large than normal ripple voltage at 60 Hz rather than at 120 Hz.

- Another fault – open in one of the halves of the transformer secondary winding.- Open diode give same symptom to bridge full-wave rectifier. (See Figure 2-63)

6969


Example 3:

Effect of a Shorted Diode in a Full-Wave Rectifier:

Fuse should blow – cause by short circuit

D1,D4 will probably burn open.

7070


Example 4:

Effect of a fault filter capacitor:

Open – o/p is full-wave rectified voltage

Shorted – the o/p is 0V

Leaky – increase the ripple voltage on the o/p

Example 5:

Effect of a Faulty Transformer:

Open primary/secondary winding of a

transformer – 0V o/p

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2-8 Troubleshooting (cont.)2-8 Troubleshooting (cont.) (The complete Troubleshooting Process) (The complete Troubleshooting Process)

The complete troubleshooting process:The complete troubleshooting process:

(i) Identify the symptom(s).(i) Identify the symptom(s).

(ii) Perform a power check(ii) Perform a power check

(iii) Perform a sensory check(iii) Perform a sensory check

(iv) Apply a signal tracing technique.(iv) Apply a signal tracing technique.

(v) Apply fault analysis(v) Apply fault analysis

(vi) Use component replacement to fix the (vi) Use component replacement to fix the problem.problem.

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The basic function of a power supply to give us a smooth ripple free DC voltage from an AC voltage.

Half-wave rectifiers only utilize half of the cycle to produce a DC voltage.

Transformer Coupling allows voltage manipulation through its windings ratio

Full-Wave rectifiers efficiently make use of the whole cycle. This makes it easier to filter.

The full-wave bridge rectifier allows use of the full secondary winding output whereas the center-tapped full wave uses only half.

Filtering and Regulating the output of a rectifier helps keep the DC voltage smooth and accurate

SummarySummary

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Limiters are used to set the output peak(s) to a given value.

Clampers are used to add a DC voltage to an AC voltage.

Voltage Multipliers allow a doubling, tripling, or quadrupling of rectified DC voltage for low current applications.

The Data Sheet gives us useful information and characteristics of device for use in replacement or designing circuits.

Troubleshooting requires use of common sense along with proper troubleshooting techniques to effectively determine the point of failure in a defective circuit or system.

SummarySummary

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Solution 2:

The peak output voltage for the circuit is:Vp(out) = Vp(in) – 0.7V = 5 – 0.7 = 4.30 V

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Solution 3:

Vp(pri) = Vp(in) = 156 V

The peak secondary voltage is:

Vp(sec) = nVp(pri) = 0.5 (156 V) = 78 V

The rectified peak output voltage is:

Vp(out) = Vp(sec) – 0.7V = 78 – 0.7 = 77.3 Vwhere Vp(sec) is the input to the rectifier.

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Solution 5:1.

+6.7V

-6.7V

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Solution 5 (cont.):2.

V

VVRR

RV SUPPLYBIAS

31.10

15220100

220

32

3

+10.31V

-10.31V

Documents

1 CHAPTER 2 Diode Applications By: Nur Baya Binti Mohd Hashim School of Computer and Communication Engineering