ABSTRACT

To reduce the wastage of power across a load we need efficient switching. This efficient

switching can be done by using SCR. Because of the bistable characteristics of semiconductor

devices, whereby they can be switched on and off, and the efficiency of gate control to trigger

such devices, the SCRs are ideally suited for many industrial applications. SCRs have got

specific advantages over saturable core reactors and gas tubes owing to their compactness,

reliability, low losses, and speedy turn-on and turn-off. The bistable states (conducting and non-

conducting) of the SCR and the property that enables fast transition from one state to the other

are made use of in the control of power in both ac and dc circuits. In ac circuits the SCR can be

turned-on by the gate at any angle α with respect to applied voltage. This angle α is called the

firing angle and power control is obtained by varying the firing angle. This is known as phase

control. Power control in dc circuits is obtained by varying the duration of on-time and off-time

of the device and such a mode of operation is called on-off control or chopper control.

SCOPE

This mini project report documents the analysis and simulation of power controlling by u

the conversion scheme (AC/DC/AC converter) using MULTISIM simulation software.

This report basically separated into several parts. Firstly, a general introduction of

different types of switches and characteristics of SCR. Secondly, brief introduction of methods

of power control. Thirdly, the designing of power circuit and control circuit and the AC/DC/AC

converter is analyzed.

INDEX

Contents: PAGE NO.

CHAPTER: 1

1.0 INTRODUCTION 1

CHAPTER: 2

2.0 About power electronics 2

2.1 Applications of power electronics 3

2.2 Classification of switches 3

2.3 Different types of power electronic switches 4

CHAPTER: 3

3.0 Introduction of SCR 9

3.1 Characteristics of SCR 10

3.2 Advantages of SCR among other switches 11

CHAPTER: 4

4.0 Triggering methods of SCR 16

4.1 Why pulse triggering is preferred? 17

4.2 Ratings of SCR 18

CHAPTER: 5

5.0 Resistance triggering circuit 20

5.1 Resistance triggering circuit 21

CHAPTER:6

6.0 Power electronic converters 22

6.1 Performance factors 24

6.2 AC- voltage controller 29

6.3 Design procedure 30

CHAPTER: 7

7.0 MULTISIM simulation circuit 32

7.1 Running simulation 33

7.2 Results 34

Conclusion 35

ABBRIVATIONS

SCR Silicon Controlled Rectifier

GTO Gate Turn-Off thyristor

IGBT Integrated Gate Bipolar Transistor

BJT Bipolar junction transistor

MOSFET Metal Oxide Semiconductor Field Effect Transistor

LIST OF TABLES

Table: 6.2-Data sheet of SCR 2N2577

Table :7.2-Data sheet of TYN 612

LIST OF FIGURES

Page no

Fig : 3.0: V-I characteristics of SCR 13

Figs: 3.1: Comparison of different switches 15

Fig: 3.2: Resistance triggering circuit 20

Fig: 3.3: Resistance capacitance triggering circuit 21

Fig 4.1: Full wave AC voltage controller 29

Fig.7.1: MULTISIM simulation circuit 32

CHAPTER: 1

1.0 Introduction

Switch has great importance in any electronic circuit. A power electronic switch integrates a

combination of power electronic components or power semiconductors and a driver for the

actively switchable power semiconductors. The operation of switch will effect of entire operation

of circuit. The first fundamental principle which you should remember forever is that power

efficiency dictates that switches be used as control devices. The second fundamental principle,

which is of equal importance, is that all high-power controls or converters are simply switching

matrices. The switch should consume less power during turn on and turn off. And it should be

controlled easily. By switching we can get converter and inverter operations. The switching will

affect the efficiency of output .Hence selection of switch is very important.

CHAPTER: 2 2.0 About Power Electronics

As the technology for the power semiconductor devices and integrated circuit develops, the

potential for applications of power electronics become wider. There are already many power

semiconductor devices that are commercially available, however the development in this

direction is continuing.

Power Electronics defined as the application of solid state (devices) electronics for the control and conversion of electric power. Power electronics and converters utilizing them made a head start when the first device the Silicon Controlled Rectifier was proposed by Bell Labs and commercially produced by General Electric in the earlier fifties. The first very high power electronic devices were mercury arc valves. The Mercury Arc Rectifiers were well in use by that time and the robust and compact SCR first started replacing it in the rectifiers and cycloconverters. All of the important

parameters of the electrical waveform are subject to regulation or conversion by solid-state

power devices, including effective voltage, effective current, frequency, and/or power factor.

Often the control of electrical power is desired simply as a means for controlling some non-

electrical parameter. For example, drives for controlling the speed of a motor. In other

applications, power electronics is used to control the temperature of an oven, the rate of an

electrochemical refining process, the intensity of lighting, etc. The design of power electronics

equipment involves interactions with the source and the load, and utilizes small-signal electronic

control circuits as well as power devices. Therefore power electronics draws upon, and indeed

depends upon all of the other areas of electrical engineering.

Power electronics have already found an important place in modern technology and are now used

in a great variety of high-power product, including heat controls, light controls, electric motor

control, power supplies, vehicle propulsion system and high voltage direct current (HVDC)

systems.

2.1Applications of Power Electronics:

Switch mode power supplies (SMPS).

Uninterruptible power supply (UPS) systems.

Photo-voltaic and fuel-cell power conversion systems

Photo-voltaic and fuel-cell power conversion systems

Heating and lighting, including high frequency illumination control. Induction heating.

DC and AC servo drives.

High efficiency industrial/commercial drives.

Electric vehicle applications.

Electric traction.

Renewable Energy systems.

2.2Classification of switches: 1) Un controlled switches 2) Semi controlled switches 3) Fully controlled switches Un controlled switches: Switches always conduct whenever a forward polarity of voltage is applied to their terminals.

Type 1 switches are known as rectifiers or diodes.

Semi controlled switches:

Switches do not conduct forward current until commanded to do so by a control signal.

Therefore Type 2 switches behave as Type 1 switches and continue to conduct as long as

forward current flows. Type 2 switches are known as thyristors (the full official name is reverse

blocking triode thyristor) or silicon controlled rectifiers (SCR’s).

Fully controlled switches:

Switches can not only turn on forward conduction when commanded by the control signal, but

can also interrupt conduction upon command without waiting for reverse polarity to be applied.

Power transistors and gate controlled switches exhibit Type 3 behavior. By artificial circuit

means, thyristors can also be “force commutated” to operate as Type 3 switches.

2.3Different types of Electronic Switches:

DIODE:

Uni-polar, uncontrolled, switching device used in applications such as rectification and circuit

directional current control. Reverse voltage blocking device, commonly modeled as a switch in

series with a voltage source, usually 0.7 VDC. The model can be enhanced to include a junction

resistance, in order to accurately predict the diode voltage drop across the diode with respect to

current flow.

SCR:

This semi-controlled device turns on when a gate pulse is present and the anode is positive

compared to the cathode. When a gate pulse is present, the device operates like a standard diode.

When the anode is negative compared to the cathode, the device turns off and blocks positive or

negative voltages present. The gate voltage does not allow the device to turn off.

GTO:

The gate turn-off thyristor, unlike an SCR, can be turned on and off with a gate pulse. One issue

with the device is that turn off gate voltages are usually larger and require more current than turn

on levels. This turn off voltage is a negative voltage from gate to source, usually it only needs to

be present for a short time, but the magnitude s on the order of 1/3 of the anode current. A

snubber circuit is required in order to provide a usable switching curve for this device. Without

the snubber circuit, the GTO cannot be used for turning inductive loads off. These devices,

because of developments in IGCT technology are not very popular in the power electronics

realm. They are considered controlled, uni-polar and bi-polar voltage blocking.

TRIAC:

The triac is a device that is essentially an integrated pair of phase-controlled thyristors connected

in inverse-parallel on the same chip. Like an SCR, when a voltage pulse is present on the gate

terminal, the device turns on. The main difference between an SCR and a Triac is that both the

positive and negative cycle can be turned on independently of each other, using a positive or

negative gate pulse. Similar to an SCR, once the device is turned on, the device cannot be turned

off. This device is considered bi-polar and reverse voltage blocking.

BJT:

N-P-N P-N-P

The BJT cannot be used at high power; they are slower and have more resistive losses when

compared to MOSFET type devices. In order to carry high current,

BJTs must have relatively large base currents, thus these devices have high power losses when

compared to MOSFET devices. BJTs along with MOSFETs are also considered unipolar and do

not block reverse voltage very well, unless installed in pairs with protection diodes. Generally,

BJTs are not utilized in power

Electronics switching circuits because of the I2R losses associated with on resistance and base

current requirements. BJTs have lower current gains in high power packages, thus requiring

them to be setup in Darlington configurations in order to handle the currents required by power

electronic circuits. Because of these multiple transistor configurations, switching times are in the

hundreds of nanoseconds to microseconds. Devices have voltage ratings which max out around

1500 V and fairly high current ratings. They can also be paralleled in order to increase power

handling, but must be limited to around 5 devices for current sharing.

POWER MOSFET:

The main benefit of the power MOSFET is that the base current for BJT is large compared to

almost zero for MOSFET gate current. Since the MOSFET is a depletion channel device,

voltage, not current is necessary to create a conduction path from drain to source. The gate does

not contribute to either drain or source current. Turn on gate current is essentially zero with the

only power dissipated at the gate coming during switching. Losses in MOSFETs are largely

attributed to on-resistance. The calculations show a direct correlation to drain source on-

resistance and the device blocking voltage rating, BVdss.

Switching times range from tens of nanoseconds to a few hundred microseconds, depending on

the device. MOSFET drain source resistances increase as more current flows through the device.

As frequencies increase the losses increase as well, making BJTs more attractive. Power

MOSFETs can be paralleled in order to increase switching current and therefore overall

switching power. Nominal voltages for MOSFET switching devices range from a few volts to a

little over 1000 V, with currents up to about 100 A or so. Newer devices may have higher

operational characteristics. MOSFET devices are not bi-directional, nor are they reverse voltage

blocking.

IGBT:

These devices have the best characteristics of MOSFETs and BJTs. Like MOSFET devices, the

insulated gate bipolar transistor has high gate impedance, thus low gate current requirements.

Like BJTs, this device has low on state voltage drop, thus low power loss across the switch in

operating mode. Similar to the GTO, the IGBT can be used to block both positive and negative

voltages. Operating currents are fairly high, in excess of 1500 A and switching voltage up to

3000 V. The IGBT has reduced input capacitance compared to MOSFET devices which

improves the Miller feedback effect during high dv/dt turn on and turn off.

IGBT

CHAPTER: 3

3.0Introduction of SCR

Shockley diodes are curious devices, but rather limited in application. Their usefulness may be

expanded, however, by equipping them with another means of latching. In doing so, each

becomes true amplifying devices (if only in an on/off mode), and we refer to these as silicon-

controlled rectifiers, or SCRs.

If an SCR's gate is left floating (disconnected), it behaves exactly as a Shockley diode. It may be

latched by breakover voltage or by exceeding the critical rate of voltage rise between anode and

cathode, just as with the Shockley diode. Dropout is accomplished by reducing current until one

or both internal transistors fall into cutoff mode, also like the Shockley diode. However, because

the gate terminal connects directly to the base of the lower transistor, it may be used as an

alternative means to latch the SCR. By applying a small voltage between gate and cathode, the

lower transistor will be forced on by the resulting base current, which will cause the upper

transistor to conduct, which then supplies the lower transistor's base with current so that it no

longer needs to be activated by a gate voltage. The necessary gate current to initiate latch-

up, of course, will be much lower than the current through the SCR from cathode to anode, so

the SCR does achieve a measure of amplification.

This method of securing SCR conduction is called triggering, and it is by far the most common

way that SCRs are latched in actual practice. In fact, SCRs are usually chosen so that their

breakover voltage is far beyond the greatest voltage expected to be experienced from the power

source, so that it can be turned on only by an intentional voltage pulse applied to the gate.

It should be mentioned that SCRs may sometimes be turned off by directly shorting their gate

and cathode terminals together, or by "reverse-triggering" the gate with a negative voltage (in

reference to the cathode), so that the lower transistor is forced into cutoff. I say this is

"sometimes" possible because it involves shunting all of the upper transistor's collector current

past the lower transistor's base. This current may be substantial, making triggered shut-off of

an SCR difficult at best.

3.1Comparison of different switches:

3.2Advantages of SCR among other switches:

It has high power ratings. The ability of an SCR to control large currents to a load by means of

small gate current makes the device very useful in switching and control applications. Because of

the bistable characteristics of semiconductor devices, whereby they can be switched on and off,

and the efficiency of gate control to trigger such devices, the SCRs are ideally suited for many

industrial applications. SCRs have got specific advantages over saturable core reactors and gas

tubes owing to their compactness, reliability, low losses, and speedy turn-on and turn-off.

In some ac circuits it is necessary to apply the voltage to the load when the instantaneous value

of this voltage is going through the zero value. This is to avoid a high rate of increase of current

in case of purely resistive loads such as lighting and furnace loads, and thereby reduce the

generation of radio noise and hot-spot temperatures in the device carrying the load current. By

using SCR we can do this.

SCRs can be employed for protecting other equipment from over-voltages owing to their fast

switching action. The SCR employed for protection is connected in parallel with the load.

Whenever the voltage exceeds a specified limit, the gate of the SCR will get energized and

trigger the SCR. A large current will be drawn from the supply mains and voltage across the load

will be reduced.

4.0 Triggering methods of SCR

Forward Voltage Triggering: In this method when anode to cathode forward voltage is increased with gate circuit open, then

the reverse bias junction J2 will have a avalanche breakdown at a voltage called forward break

over voltage VBO. At this voltage thyristor or SCR changes from OFF state to ON state. The

forward voltage drop across the SCR during ON state is of the order of 1 to 1.5V and increases

slightly with increase in the load current.

Thermal Triggering (Temperature Triggering): Width of the depletion layer of the thyristor decreases on increasing the junction temperature.

Thus in the SCR when the voltage applied is very near to the breakdown voltage, the device can

be triggered by increasing its junction temperature. By applying the temperature to certain

extent, a situation comes when the reverse biased junction collapse making the device to

conduct. This method of triggering the thyristor by heating is known as the Thermal Triggering

process

Radiation Triggering (Light Triggering): Thyristors are bombarded with energy particles such as neutrons and protons. Light energy is

focused on the depletion region results in the formation of charge carriers. This lead to

instantaneous flow of current with in the device and the triggering of the device.

dv/dt Triggering: In this method of triggering if the applied rate of change of voltage is large, then the device will

turn on even though the voltage appearing across the device is small. We know that when SCR is

applied with forward voltage across the anode and cathode, junctions j1 and j3 will be in forward

bias and junction j2 will be in reverse bias. This reverse biased junction j2 will have the

characteristics of the capacitor due to the charges exist across the junction. If the forward voltage

is suddenly applied a charging current will flow tending to turn on the SCR. This magnitude of

the charging current depends on the rate of change of applied voltage.

Gate Triggering: This is the most commonly used method for triggering the SCR or thyristor. For gate triggering a

signal is applied across the gate and cathode of the device. By applying a positive signal at the

gate terminal of the SCR it will be triggered much before the specified break over voltage. Three

types of signals can be used for triggering the SCR. They are either dc signal, ac signal or pulse

signal.

DC Gate triggering: In this type of triggering a dc voltage of proper magnitude and polarity is applied between the

gate and cathode such that gate becomes positive with respect to the cathode. When the applied

voltage is sufficient to produce required gate current the device starts conducting

CHAPTER: 4

AC Gate triggering: ac source is most commonly used triggering source for thyristor for ac applications.

Advantages:

Have the advantages than dc source such as power isolation between the power and the control

circuits and firing angle can be controlled by changing the phase angle of the control signal.

Drawback:

Gate drive is maintained for one half cycle of the device is turned ON.

Severe reverse voltage is applied across gate and cathode during negative half cycle.

The drawback of this scheme is that a separate transformer is required to step down the ac supply

increasing the cost

4.1Why Pulse Gate Triggering?: This is the most popular method for triggering the SCR. In this method gate drive consists of

single pulse appearing periodically or sequence of high frequency pulses. This is known as

carrier frequency gating. A pulse transformer is used for isolation. The main advantage of this

method is there is no need to apply continuous gate signal and hence gate losses are very much

reduced. Electrical isolation is also provided between the main device supply and its gating

signals.

4.2Ratings of SCR:

Current ratings of an SCR:

The current carrying capability of an SCR is solely determined by the junction temperature.

Except in case of surge currents, in no other case the junction temperature is permitted to exceed

the permissible value. Some of the current ratings used in industry to specify the device are given

below.

i) Forward Current Rating:

The maximum value of anode current, that an SCR can handle safely (without any damage), is

called the forward current rating. The usual current rating of SCRs is from about 30 A to 100 A.

In case the current exceeds the forward current rating, the SCR may get damaged due to

intensive heating at the junctions.

ii) On-state Current:

When the device is in conduction, it carries a load current determined by the supply voltage and

the load. On-state current is defined in terms of average and rms values.

ITav is the average value of maximum continuous sinusoidal on-state current (frequency 40-60

Hz, conduction angle 180°) which should not be exceeded even with intensive cooling. The

temperature at which the current is permissible has to be mentioned. It is this current which

determines the application of device.

ITrms is the rms value of maximum continuous sinusoidal on-state current (frequency 40-60 Hz,

conduction angle 180°) which should not be exceeded even with intensive cooling.

Latching Current:

It is the minimum device current, which must be attained by the device, before the gate drive is

removed while turning-on, for maintaining it into conduc tion.

Holding Current:

It is the minimum on-state current required to keep the SCR in conducting state without

any gate drive. Its usual value is 5 m A.

(v) Surge Current:

It is the maximum admissible peak value of a sinusoidal half cycle of 10 ms duration at a

frequency of 50 Hz. The value is specified at a given junction temperature.

During maximum surge on-state current the junction temperature is exceeded though

temporarily and forward blocking capabilities are lost for a short period. The maximum

surge on-state current should only occur occasionally.

(vi) I2t Value:

I2t value is the time integral of the square of the maximum sinusiodal on-state current. This is

usually specified for 3 ms and 10 ms, and determines the thermal rating of the device.

(vii) Critical Rate of Rise of Current:

The maximum rate of increase of current during on-state which the SCR can tolerate is called the

critical rate of rise of current for the device. This is specified at maximum junction temperature.

During initial period of turning-on, only a small area near the gate conducts the anode current. If

the current increases too fast, localised overheating may take place. This is called the hole

storage effect. Due to localised heating the device may get permanently damaged. To-day

devices are available which can withstand rate of rise of current upto 200-250 A/microsecond,

however in application this rate is hardly allowed to exceed beyond 5-10 A/micro

second.Protection against dI/dt is provided by series inductor.

Voltage Ratings:

The device voltage rating is a measure of the maximum voltage which can be applied across the

device without causing a breakdown in the junction area. The different voltage ratings of SCR

are given as below:

(i)Peak Repetitive Forward Blocking Voltage:

This is the peak voltage that the thyristor can block in the forward direction. It is defined at the

maximum permissible junction temperature with gate circuit open or with a specified biasing

resistance between gate and cathode terminals.

(ii)Peak Repetitive Reverse Voltage:

This is the peak reverse voltage that the thyristor can bear without breakdown at the maximum

permissible junction temerature. When this rating is sustianlly exceeded,the device may be

destroyed by junction breakdown.

(iii)Non-Repetitive Peak Reverse Voltage:

This is the maximum transient reverse voltage that can be safely blocked by the

thyristor.Transient reverse voltage rating can be raised by inserting a didoe of equal current

rating in series with the thyristor.

(iv)Forward dv/dt Rating:

When the rate of increae of forward voltage is higher in comparison to the specified maximum

value,it can cause switching from the off-state to the on-state.Beacause gate pulse triggeringis

generally used for the turn on of a thyristor,the dv/dt results in unscheduled turn on of a

thyristor.This technique of switching is to be avoided beacuse this can lead to the destruction of

the thyristor through high local current density. Due to this, the dv/dt rating is specified either by

linear or exponential waveform.

CHAPTER:5

5.0Resistance Triggering circuit:

The above circuit is similar in design to the DC SCR circuit except for the omission of an

additional “OFF” switch and the inclusion of diode D1 which prevents reverse bias being applied

to the Gate. During the positive half-cycle of the sinusoidal waveform, the device is forward

biased but with switch S1 open, zero gate current is applied to the thyristor and it remains “OFF”.

On the negative half-cycle, the device is reverse biased and will remain “OFF” regardless of the

condition of switch S1.

If switch S1 is closed, at the beginning of each positive half-cycle the thyristor is fully “OFF” but

shortly after there will be sufficient positive trigger voltage and therefore current present at the

Gate to turn the thyristor and the lamp “ON”. The thyristor is now latched-”ON” for the duration

of the positive half-cycle and will automatically turn “OFF” again when the positive half-cycle

ends and the Anode current falls below the holding current value. During the next negative half-

cycle the device is fully “OFF” anyway until the following positive half-cycle when the process

repeats itself and the thyristor conducts again as long as the switch is closed. Then in this

condition the lamp will receive only half of the available power from the AC source as the

thyristor acts like a rectifying diode, and conducts current only during the positive half-cycles

when it is forward biased. The thyristor continues to supply half power to the lamp until the

switch is opened.

If it were possible to rapidly turn switch S1 ON and OFF, so that the thyristor received its Gate

signal at the “peak” (90o) point of each positive half-cycle, the device would only conduct for

one half of the positive half-cycle. In other words, conduction would only take place during one-

half of one-half of a sine wave and this condition would cause the lamp to receive “one-fourth”

or a quarter of the total power available from the AC source. By accurately varying the timing

relationship between the Gate pulse and the positive half-cycle, the Thyristor could be made to

supply any percentage of power desired to the load, between 0% and 50%. Obviously, using this

circuit configuration it cannot supply more than 50% power to the lamp, because it cannot

conduct during the negative half-cycles when it is reverse biased.

5.1 Resistance-Capacitance Triggering Circuit:

Phase control is the most common form of thyristor AC power control and a basic phase-control

circuit can be constructed as shown above. Here the thyristors Gate voltage is derived from the

RC charging circuit via the trigger diode, D1.

During the positive half-cycle when the thyristor is forward biased, capacitor, C charges up via

resistor R1 following the AC supply voltage. The Gate is activated only when the voltage at

point A has risen enough to cause the trigger diode D1, to conduct and the capacitor discharges

into the Gate of the thyristor turning it “ON”. The time duration in the positive half of the cycle

at which conduction starts is controlled by RC time constant set by the variable resistor, R1.

Increasing the value of R1 has the effect of delaying the triggering voltage and current supplied

to the thyristors Gate which in turn causes a lag in the devices conduction time. As a result, the

fraction of the cycle over which the device conducts can be controlled between 0 and 180o,

which means that the average power dissipated by lamp can be adjusted. However, the thyristor

is a unidirectional device so only a maximum of 50% power can be supplied.

There are a variety of ways to achieve 100% full-wave AC control using “thyristors”. One way is

to include a single thyristor within a diode bridge rectifier circuit which converts AC to a

unidirectional current through the thyristor while the more common method is to use two

thyristors connected in inverse parallel. A more practical approach is to use a single Triac as this

device can be triggered in both directions, therefore making them suitable for AC switching

applications.

CHAPTER: 6

6.0 Power electronic converters:

STATIC CONVERTERS

Static converter is power electronic converters that can conversion of electric power from one to another. The static power converters perform these function of power conversion.

The Power Electronic Converter can be classified into six types: 1. Diode Rectifier 2. AC to DC Converter (Controlled Rectifier) 3. DC to DC Converter (DC Chopper) 4. AC to AC Converter (AC voltage regulator) 5. DC to AC Converter (Inverter) 6. Static Switches

Static converter is power electronic converters that can conversion of electric power from one to another.

The static power converters perform these function of power conversion.

The Power Electronic Converter can be classified into six types: 1. Diode Rectifier 2. AC to DC Converter (Controlled Rectifier) 3. DC to DC Converter (DC Chopper) 4. AC to AC Converter (AC voltage regulator) 5. DC to AC Converter (Inverter) 6. Static Switches

Diode Rectifiers: A diode rectifier circuit converts AC voltage into a fixed DC voltage. The input voltage to

rectifier could be eithersingle phase or three phase.

AC to DC Converters: An AC to DC converter circuit can convert AC voltage into a DC

voltage. The DC output voltage can be controlled by varying the firing angle of the thyristors.

The AC input voltage could be a single phase or three phase.

AC to AC Converters

This converter can convert from a fixed ac input voltage into variable AC output voltage. The

output voltage is controlled by varying firing angle of TRIAC. These type converters are known

as AC voltage regulator.

DC to DC Converters These converters can converter a fixed DC input voltage into variable DC voltage or vice versa.

The DC output voltage is controlled by varying of duty cycle.

Static Switch: Because the power devices can be operated as static switches or contactors, the supply to these

switches could be either AC or DC and the switches are called as AC static switches or DC static

switches.

AC to DC Converters:

Single phase, half wave AC to DC converter:

As shown in Fig the single-phase half-wave rectifier uses a single thyristor to control the

load voltage. The thyristor will conduct, ON state, when the voltage vTis positive and a firing

current pulse iGis applied to the gate terminal. Delaying the firing pulse by an angle α does the

control of the load voltage. The firing angle α is measured from the position where a diode

would naturally conduct. In Fig.(1), the angle a is measured from the zero crossing point of the

supply voltage vs. The load is resistive and therefore current idhas the same waveform as the

load voltage. The thyristor goes to the non-conducting condition, OFF state, when the load

voltage and, consequently, the current try to reach a negative value.

The load average voltage is given by:

Where Vmax is the supply peak voltage.

AC to AC converter:

6.1 Performance Factors of AC voltage Controller: RMS Output (Load) Voltage

122

2 2

0

sin .2

mO RMS

nV V t d t

n m

2

mSO RMS i RMS

V nV V k V k

m n

SO RMS i RMSV V k V k

Where S i RMSV V = RMS value of input supply voltage.

Duty Cycle

ON ON

O ON OFF

t t nTk

T t t m n T

Where,

nk

m n

= duty cycle (d).

RMS Load Current

O RMS O RMS

O RMS

L

V VI

Z R ; for a resistive load LZ R .

Output AC (Load) Power

2

O LO RMSP I R

Input Power Factor

output load power

input supply volt amperes

O O

S S

P PPF

VA V I

2

LO RMS

i RMS in RMS

I RPF

V I

; S in RMS

I I RMS input supply current.

The input supply current is same as the load current in O LI I I

Hence, RMS supply current = RMS load current; in RMS O RMSI I .

2

LO RMS O RMS i RMS

i RMS in RMS i RMS i RMS

I R V V kPF k

V I V V

nPF k

m n

The Average Current of Thyristor T AvgI

0 2 3 t

Im

nmiT

Waveform of Thyristor Current

0

sin .2

mT Avg

nI I t d t

m n

0

sin .2

m

T Avg

nII t d t

m n

0

cos2

m

T Avg

nII t

m n

cos cos0

2

m

T Avg

nII

m n

1 1

2

m

T Avg

nII

m n

2

2mT Avg

nI I

m n

.m m

T Avg

I n k II

m n

duty cycle ON

ON OFF

t nk

t t n m

.m m

T Avg

I n k II

m n

,

Where mm

L

VI

R = maximum or peak thyristor current.

RMS Current of Thyristor T RMSI

12

2 2

0

sin .2

mT RMS

nI I t d t

n m

122

2

0

sin .2

m

T RMS

nII t d t

n m

122

0

1 cos 2

2 2

m

T RMS

tnII d t

n m

122

0 0

cos 2 .4

m

T RMS

nII d t t d t

n m

122

0 0

sin 2

24

m

T RMS

nI tI t

n m

122

sin 2 sin 00

4 2

m

T RMS

nII

n m

122

0 04

m

T RMS

nII

n m

1 12 22 2

4 4

m m

T RMS

nI nII

n m n m

2 2

m m

T RMS

I InI k

m n

2

m

T RMS

II k

6.2 Full wave AC voltage controller:

Circuit operation:

• Potentiometer R controls the angle of conduction of the two SCRs.

• The greater the resistance of the pot, lesser will be the voltage across capacitors C1 and

C2.

• During positive half cycle capacitor C2 gets charged through diode D1, pot R, and diode

D4.

• When the capacitor gets fully charged it discharges through Zener diode Z.

This gives a pulse to the primary and thereby secondary of the transformer T2.

• Thus SCR2, which is forward biased, is turned on and conducts through load RL.

• During negative half cycle similar action takes place due to charging of capacitor C1 and

SCR1 is triggered.

• Thus power to a load is controlled by using SCRs.

For SCR 2N2577

Vgt=3.5V Igt=40mA

6.3Design procedure:

RC>=50*(T/2)=157/W

R<<(Vs-Vc)/Igt

R<<(Vs-Vgt-Vd)/Igt

R<<(230-3.5)/40mA

R<<5.6625K ohms

R~=5Kohms

RC~=0.5

5kohm*C~=0.5 C~=0.1uF

Zener Diode:

It operates in reverse bias mode it maintain constant gate triggering voltage across SCR.

Pulse transformer:

It is the 1:1 transformer it isolates the control circuit from power circuit.

CHAPTER: 7

7.1MULTISIM simulation circuit:

7.2Results:

CONCLUSION: