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SCR Characteristics TrainerNV6530

Learning MaterialVer 1.1

141-B, Electronic Complex,

Pardeshipura, Indore- 452 010 India

Tel.: 91-731- 4211500

Toll-free:1800-103-5050

Email: [email protected]

Website: www.nvistech.com

mailto:[email protected]

NV6530

Nvis Technologies Pvt. Ltd. 2


Table of Contents1. Introduction 3

2. Features 4

3. Technical Specification 5

4. Theory 6

5. Experiment

Experiment 1 14To Study and plot V-I Characteristic of SCR

Experiment 2 19To study the phenomenon of holding current and latching current.

6. Data Sheet 21

7. Warranty 23

8. List of Accessories 23

www.nvistech.com

NV6530


Introduction

SCR Characteristic Trainer NV6530 is a compact, ready to use experiment board todemonstrate the fundamental firing concept and operating characteristic of SCR. Thisis useful for students to plot V-I characteristics of SCR. It can be used as stand aloneunit with inbuilt DC power supply hence no external components required. It hasinbuilt voltmeter and ammeter to measure the SCR parameters of the circuit likevoltage and current. Student can observe and plot a graph between voltage and currentin a very simple manner.

NV6530


Features

Unique and Compact design

Stand alone operation

Simple representation of SCR characteristic

Ammeter and voltmeter to measure parameters

Designed by considering all the safety standards

Learning Material

2 Year Warranty

NV6530


Technical Specification

Mains Supply : 230 V AC ±10%, 50 Hz

DC Power Supply : +35V, +15V, 250mA

Voltmeter : 0-35V (max.)

Ammeter : 0-50 mA (max.)

Weight : 2.15Kg

Dimensions (mm) : 345×240×110

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TheoryPower Electronics:

Introduction:

Power electronic converters can be found wherever there is a need to modify theelectrical energy form (i.e. modify its voltage, current or frequency).Therefore theirpower range from some milliwatts (as in a mobile phone) to hundreds of megawatts(e.g. in a HVDC transmission system). With "classical" electronics, electrical currentsand voltage are used to carry information, whereas with power electronics, they carrypower.

The first very high power electronic devices were mercury arc valves. In modernsystems the conversion is performed with semiconductor switching devices such asdiodes, thyristors and transistors. In contrast to electronic systems concerned withtransmission and processing of signals and data, in power electronics substantialamounts of electrical energy are processed. An AC/DC converter (rectifier) is themost typical power electronics device found in many consumer electronic devices,e.g., television sets, personal computers, battery chargers, etc. The power range istypically from tens of watts to several hundred watts. In industry the most commonapplication is the variable speed drive (VSD) that is used to control an inductionmotor. The power range of VSDs starts from a few hundred watts and end at tens ofmegawatts.

The power conversion systems can be classified according to the type of the input andoutput power

AC to DC (rectification)

DC to AC (inversion)

DC to DC (chopping)

AC to AC

Principle:

As efficiency is at a premium in a power electronic converter, the losses that a powerelectronic device generates should be as low as possible. The instantaneous dissipatedpower of a device is equal to the product of the voltage across the device and thecurrent through it (P = VI). From this, one can see that the losses of a power deviceare at a minimum when the voltage across it is zero (the device is in the On-State) orwhen no current flows through it (Off-State). Therefore, a power electronic converteris built around one (or more) device operating in switching mode (either On or Off).With such a structure, the energy is transferred from the input of the converter to itsoutput by bursts.

Applications:

Power electronic systems are virtually in every electronic device. For example,around us:

DC/DC converters are used in most mobile devices (mobile phone, pda.) tomaintain the voltage at a fixed value whatever the charge level of the batteryis. These converters are also used for electronic isolation and power factor

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correction.

AC/DC converters (rectifiers) are used every time an electronic device isconnected to the mains (computer, television,...)

AC/AC converters are used to change either the voltage level or the frequency(international power adapters, light dimmer). In power distribution networksAC/AC converters may be used to exchange power between utility frequency50 Hz and 60 Hz power grids.

DC/AC converters (inverters) are used primarily in UPS or emergency light.During normal electricity condition, the electricity will charge the DC battery.During blackout time, the DC battery will be used to produce AC electricity atits output to power up the appliances.

SCR Theory

Introduction:-

The Silicon Controlled Rectifier (SCR) is a semiconductor device that is a member ofa family of control devices known as Thyristors. The SCR has become the workhouse of the industrial control industry. Its evolution over the year has yielded adevice that is less expensive , more reliable and smaller in size than ever beforeTypical applications include : DC motor control , generator field regulation ,variablefrequency drive (VFD) DC bus voltage control solid state Relays and lighting systemcontrol .

The SCR is a three – lead device with an anode and a cathode ( us with astandard diode) plus a third control lead or gate . As the name implies, it is arectifier which can be controlled – or more correctly one hat can be triggeredto the “ON” state by applying a small positive voltage (VTM) to the gate lead.

Once gated ON, the trigger signal may be removed and the scr will remainconducting as long as current flows through the device.

The load to be controlled by the SCR is normally placed in the anode circuit.

Commutation:-

For the SCR to turn OFF current flow through the device must be interrupted , or dropbelow the maximum holding current(IH) , for a short period of time (typically 10-20microseconds) which is known as the commutated turn – off time (tq).

When applied to alternating current circuits or pulsating DC system, thedevice will self – commutate at the end of every half – cycle when the currentgoes through zero.

When applied to pure DC circuits, in applications such as alarm or trip circuitlatching , the SCR can be reset manually by interrupting the current with apush button. When used in VFD's or inverters ,SCRs are electronically forcedOFF using additional Commutating circuitry, Such as smaller SCRs andcapacitors, which momentarily apply an opposing reverse bias voltage acrossthe SCR.(This is complicated everything has to be exactly right).

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SCR is one of the most important types of power semiconductor device. They areoperated as bitable switches, operating from non conducting state to conducting state.

Figure l shows the schematic symbol of SCR and its internal layer diagram.

Symbol and Internal LayerFigure 1

Basic three modes of operation of SCR are

Reverse blocking mode

Cathode is positive with respect to anode with gate open.

SCR is in reverse bias i.e., junction J1 & J3 in reverse bias J2 is in forward bias.

The device act as two PN diode connected in series with reverse voltage appliedacross it. Small leakage current of the order of a few milliampere ormicroampere flows, this is off state of SCR.

If reverse voltage increases, then at critical breakdown level or reversebreakdown voltage (VBR) an avalanche occurs at J1 & J3 & reverse currentincrease rapidly, so more loss in SCR. This may lead to SCR damage becausejunction temperature is increasing.

Maximum working reverse voltage across SCR does not exceed VBR. If appliedreverse voltage across SCR < VBR, then the device offers high impedance inreverse direction. SCR is treated as open switch.

Forward blocking mode (off state mode)Anode is positive with respect to cathode with gate open.

SCR is forward bias, junction J1 & J3 is forward bias and J2 is reverse bias.Here small forward leakage current flow.

If forward voltage increases then J2 junction (rev. bias) will have avalanchebreakdown called forward break over voltage (VBO)

Maximum working forward voltage across SCR does not exceed VBO. Ifforward voltage < VBO; SCR offers high impedance. Hence SCR is treated asopen switch even though it is forward blocking mode.

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3. Forward conduction mode (on state mode)If we want to bring SCR from forward blocking mode to forward conductionmode there are two modes:

By exceeding the forward break over voltage (VBO).

By applying gate pulse between gate and cathode.

Then SCR is in on state and behave as closed switch.

V-1 CharacteristicsFigure 2

Terminal Characteristic of Thyristor:

Thyristor is a four layers, three junction, p-n-p-n semiconductor switching device. Ithas three terminals; anode, cathode and gate. Fig. 3 gives constructional details of atypical Thyristor. Basically, a Thyristor consists of four layer of alternate p-type andn-type silicon semiconductors forming three junctions j1, j2, j3 as shown in Fig. 3.The threaded portion is for the purpose of tightening the Thyristor to the frame orheat sink with the help of a nut .Gate terminal is usually kept near the cathodeterminal , Fig. 1 .Schematic diagram and circuit symbol for a Thyristor are shownrespectively in Fig. 4 and Fig. 5.The terminal connected to outer p region is calledanode(A) ,the terminal connected to outer n region is called cathode and thatconnected to inner better p region is called the gate (G).

For large current application, Thyristor need better cooling; this is achieved to a greatextent by mounting them onto heat sinks. SCR rating has improved considerably sinceits introduction in 1957. Now SCR of voltage rating 10kV and an rams current ratingof 3000 A with corresponding power-handling capacity of 30MW are available. Sucha high power Thyristor can be switched amplification capability (=3*10power 6) ofthis device. As SCR are solid state devices, they are compact, posses high reliabilityand have low loss. Because of these useful features, SCR is almost universallyemployed these days for all high power-controlled devices.

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Th re a d e d s tu d

A n o d e (A lu m in iu m )

G a te te rm in a lw e ld e d to P re g io n

C a th o d e

J1

J2

J3

N

P

N

P

Constructional details Figure 3

Schematic diagram

Figure 4

Circuit symbol of a thyristor

Figure 5

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An SCR is so called because silicon is used for its construction and its operation as arectifier (very low resistance in forward conduction and very high resistance in thereverse direction) can be controlled. Like the diode, an SCR is an unidirectionaldevice that blocks the current flow from cathode to anode. Unlike the diode, athyristor also blocks the current flow from anode to until it is triggered intoconduction by a proper gate signal between gate and cathode terminals.Forengineering applications of thyristors, their terminal characteristics must be known.

Static I-V Characteristics of a Thyristor:An elementary circuit diagram for obtaining static I-V characteristics of a thyristor isshown in Fig. 6. The anode and cathode are connected to main source through theload .The gate and cathode are fed from a source Es which provides positive gatecurrent from gate to cathode.

Fig. 7 shows static I-V characteristics of thyristor. Here Va is the anode voltage acrossthyristor terminal A, K and Ia is the anode current. Typical SCR, I-V characteristicshown in Fig. 7 reveals that a thyristor has three basic modes of operation ; namelyreverse blocking mode, forward blocking (off-state) mode and forward conduction (on –state ) mode. These three modes of operation are now discussed below:

Reverse Blocking Mode:When cathode is made positive with respect to anode with switch S open , Fig. 6,thyristor is reverse biased as shown in Fig.8 .Junctions j1,j3 are seen to be reversebiased whereas junction j2 is forward biased. The device behaves as if two diodes areconnected in series with reverse voltage applied across them. A small leakage currentof the order a few milliamperes (or a few microamperes depending upon the SCRrating) flows. This is reverse blocking mode, called the off-state,of the thyristor. InFig 7, reverse blocking mode is shown by OP. If the reverse voltage is increased, thenat a critical breakdown level, called reverse breakdown voltage VBR , an avalancheoccurs at J1 and J3 and the reverse current increases rapidly. A large currentassociated with VBR gives rise to more losses in the SCR. This may lead to thyristordamage as the junction temperature rise. It should, therefore, be ensured thatmaximum working reverse voltage, across a thyristor does not exceed VBR. In Fig.7,reverse avalanche region is shown by PQ. When reverse voltage applied across a SCRis less than VBR, the device offers high impedance in the reverse direction. The SCRin the reverse blocking mode may therefore be treated as an open switch.

L o a d+

_

Va

E s

A

K

S

GE

Elementary circuit for obtaining thyristor I-V characteristics

Figure 6

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Static I-V characteristics of a thyristor

Figure 7

VBO = Forward breakover voltage

VBR = Reverse Breakdown voltage

Ig = Gate Current

Note that I-V characteristic after avalanche breakdown during reverse blocking modeis applicable only when load resistance is present, a large anode current associatedwith avalanche breakdown at VBR would cause substantial voltage drop across loadand as a result , I-V characteristic in third quadrant would bend to the of vertical lineat VBR .

J2 forward biased and J1, J3 reverse biased

Figure 8

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J2 reverse biased and J1, J3 forward biased

Figure 9

Forward blocking mode:

When anode is positive with respect to the cathode, with gate circuit open , thyristor issaid to be forward biased as shown in Fig.9. It is seen from this figure that junctionsJ1,J3 are forward biased but junction J2 is reverse biased. In this mode, a smallcurrent, called forward leakage current, flows as shown in Figs.7 and 9 . In Fig.7, OMrepresents the forward blocking mode of SCR. AS the forward leakage current issmall, SCR offers high impedance. Therefore, a SCR can be treated as an open switcheven in the forward blocking mode.

Forward conduction mode:

When anode to cathode forward voltage is increased with gate circuit open, reversebiased junction J2 will have an avalanche breakdown at a voltage called forwardbreak over voltage VBO. After this breakdown, thyristor gets turned on with point Mat once shifting to N and then to a point anywhere between N and K. Here NKrepresents the forward conduction mode. A SCR can be brought from forwardblocking mode to forward conduction mode by turning it on by applying (1) a positivegate pulse between gate and cathode or (2) a forward break over voltage across anodecathode.

Forward conduction mode NK shows that voltage drop across thyristor is of the orderof 1 to 2 V depending upon the rating of SCR. It may also be seen from NK thatvoltage drop across SCR increase slightly with an increase in anode as voltage dropacross SCR is quit small. This small voltage drop VT across the device is due toohmic drop in four layers. In forward conduction mode, thyristor is treated as a closedswitch.

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Experiment 1

Objective:Study and plot V-I Characteristic of SCR

Equipments Needed:

1. NV6530 SCR Characteristic Trainer

2. 2mm Patch cords

Circuit diagram:

Circuit used to plot different characteristics of SCR is shown in figure 10.

+35V

1

Gnd

2

+15V

3

+35V

4

P1

R1

5 6 7

A

KG

8

9 10 11 R2 P2

13

Power


12

+15V

Voltmeter Ammeter

+ - + -

V mA

SCR

Figure 10

Procedure:

1. Connect terminal1 to terminal 4, terminal 2 to terminal 8 and terminal 3 toterminal 12 as shown in figure 11.

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+35V

1

Gnd

2

+15V

3

+35V

4

P1

R1

5 6 7

A

KG

8

9 10 11 R2 P2

13

Power


12

+15V

Voltmeter Ammeter

+ - + -

V mA

SCR

Figure 11

Figure 12

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2. Connect Voltmeter across terminal 7 and 8 and Ammeter across terminal 9 and10 as shown in figure 12.

3. Make short terminals 5 and 6.

4. Rotate the knob P1 and P2 fully in counter clockwise.

5. Switch “ON” the power supply.

6. Set the value of Anode Voltage at 35V by using the knob P1.

7. Now Increases gate current Ig gradually by varying knob P2 and observe it.

8. At certain value of gate current, voltmeter reading falls down to almost zero.This action indicates the firing of SCR.

9. Note the gate current value at this position (firing of SCR).

10. Keep the gate current constant by shorting terminal 9 with 10 and connectAmmeter to the terminal 5 and 6 (as in figure 13).

+35V

1

Gnd

2

+15V

3

+35V

4

P1

R1

5 6 7

A

KG

8

9 10 11 R2 P2

13

Power


12

+15V

Voltmeter Ammeter

+ - + -

V mA

SCR

Figure 13

11. Rotate the potentiometer P1 fully in counter clockwise.

12. Rotate knob P1 (from initial position to its maximum limit) gradually and recordAnode current for respective value of anode voltages.

13. Plot the graph between anode voltage Va and anode current Ia.

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Observation Table:

Result Analysis:-

Anode Voltage (Va)

Anode current (Ia) Gate Current Ig = 7 mA

0 0

1.3 1.9

3.6 2

5.6 2.1

7.2 2.3

8.9 2.4

11.1 2.6

12.7 2.9

14.6 3.5

.8 15.1

.8 42.1

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Characteristic of SCR

Figure 14

Latching Current and Holding Current:

Latching Current:-Once the SCR is conducting a forward current, reverse biasedjunction J2 no longer exists. As such, no gate current is required for the device toremain in on state. Therefore, if the gate current is removed, the conduction of currentfrom anode to cathode remains unaffected. However, if gate current is reduced to zerobefore the rising anode current attains a value, called the latching current, the thyristorwill turn off again. The gate pulse width should therefore be judiciously chosen toensure that anode current rises above the latching current. “Thus latching currentmay be defined as the minimum value of anode current which it must attainduring turn-on process to maintain conduction when gate signal is removed”.

Holding Current:-Once the thyristor is conducting, gate loses control. The thyristorcan be turned-off (or the thyristor can be returned to forward blocking state) only ifthe forward current falls below a low – level current called the Holding current.“Thus holding current may be defined as the minimum value of anode currentbelow which it must fall for turning -off the thyristor. The latching current ishigher than the holding current”.

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Experiment 2Objective:To study the phenomenon of holding current and latching current.

Apparatus Required:

1. NV6530 SCR Characteristic Trainer

2. 2mm Patch cords

Circuit Diagram:

+35V

1

Gnd

2

+15V

3

+35V

4

P1

R1

5 6 7

A

KG

8

9 10 11 R2 P2

13

Power


12

+15V

Voltmeter Ammeter

+ - + -

V mA

SCR

Figure 15

Procedure:

1. Make the connections as shown in the above figure

2. Rotate both the potentiometer at fully anticlockwise direction.

3. Set the value of Anode Voltage at 35V using the potentiometer P1.

4. Increase the value of gate current as slow as you can using the potentiometerP2 and bring the SCR in Conduction band.

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5. Now remove the gate current by removing the patch cord between the point 9and point 10.

6. Here you will observe that the SCR is still in conduction band. It shows that,once the SCR is triggered no gate current is required for the device toremain in on state. That value of anode current is known as LatchingCurrent.

7. Now decrease the anode current very slowly using the potentiometer P1,here you will see that after a particular value of anode current the SCR willturn-off. That value of anode current is known as Holding Current.

Result Analysis:

Latching Current: 42mA (approximately)

Holding Current: 31 mA (approximately)

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Data Sheet

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Warranty

We guarantee the product against all manufacturing defects for 24 monthsfrom the date of sale by us or through our dealers. Consumables like dry celletc. are not covered under warranty.

The guarantee will become void, if

The product is not operated as per the instruction given in the learningmaterial.

The agreed payment terms and other conditions of sale are not followed.

The customer resells the instrument to another party.

Any attempt is made to service and modify the instrument.

The non-working of the product is to be communicated to us immediatelygiving full details of the complaints and defects noticed specificallymentioning the type, serial number of the product and date of purchase etc.

The repair work will be carried out, provided the product is dispatchedsecurely packed and insured. The transportation charges shall be borne by thecustomer.

List of Accessories

1. 2mm Patch Cord 12”.............................................................................10 Nos.

2. Mains Cord .............................................................................................1 No.

3. Learning Material CD…….……………………………………………1 No.

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