Unit_1a

Jabatan Kejuruteraan Elektrik - Politeknik Kota Kinabalu___________________________________________________________ 1

Basic Electronic Devices and Their Application E4802/Unit 1

UNIT 1 BASIC ELECTRONIC DEVICES AND

THEIR APPLICATION

OBJECTIVES

General Objective: Explain the operation and use of basic electronic devices

Specific Objectives: At the end of this chapter, you should be able to

1.1 Explain the operation and use of a silicon controlled rectifier (SCR).

1.1.1 Explain the physical structure of SCR.

1.1.2 Draw the symbol for SCR and identifying the three terminals of an SCR.

1.1.3 Explain the operation of SCR during forward bias and reverse bias conditions.

1.1.4 Draw and label the static characteristic curve for an SCR.

1.1.5 Explain the meaning of Regenerative Action for an SCR.

1.1.6 Define the terms Break over Voltage, Latching Current, Holding Current and

Reverse Breakdown Voltage.

1.1.7 Explain the difference between trigger angle and conduction angle in the phase

control usage.

1.1.8 State the advantages and disadvantages of SCR compared to Magnetic Contactors.



INPUT

1.0 Introduction

1. Electrical Engineering field may be divided into three areas: Electronics, Power, and Control1.

2. Basically, the Electronics field is more on the study of semiconductor devices and circuits in

processing the lower power levels’ information. While the Power field deals with both the

rotating and static equipments for the generation, transmission and distribution system, and

the Control field deals with the stability and response characteristics of closed loop systems

using feedback on either a continuous or sampled-data basis.

3. On the other hand, Power Electronics deals with the use of electronics for the control and

conversion of large amounts electrical power*. The design of power electronics equipment

involves interactions between the source and the load and utilises small signal electronic control

circuit as well as power semiconductor devices.

4. And the widely utilized power semiconductor devices is the thyristor family – Silicone

controlled rectifier (SCR), Triode for alternating current (TRIAC), Diode for alternating

Current (DIAC), Gate turn-off thyristor (GTO) and Programmable unijunction transistor (PUT)2.

1 Santhosh,C. and Shanthi,M. (2007). Power Electronics. Petaling Jaya, Malaysia. ibs

2 Wikipedia. (2010). Thyristor, Retrieved on 21

st June 2010 from http://en.wikipedia.org/wiki/Thyristor

http://en.wikipedia.org/wiki/Thyristor



5. Apart from the thyristor family, there are also other families that are made to be the signal

control devices such as Field Effect Transistor (FET) family – Junction FET (JFET) and Metal

oxide semiconductor FET (MOSFET), Photo family – Photo transistor, Photo thyristor and Photo

diode; and there also individual devices that are opt for the challenge such as the Optocoupler,

Zener diode, Operational amplifier (OP-AMP).

* The examples of involving a controlling and conversioning large amounts electrical power

can be seen in a AC and DC speed control motor, Lighting controls, AC static circuit

breaker, DC static circuit breaker, Variable voltage converter, and Variable frequency

inverter.

1.1 Silicone Controlled Rectifier (SCR)

1. Silicone Controlled Rectifier (SCR) is a solid state semiconductor device with four layers of

alternating N and P* type material which happens to be the most distinctive criteria of a

thyristor family3. The SCR can be identified as seen in Figure 1. In a glimpse, an SCR can be

mistaken for a transistor. This can be further explained later on after you have understood the

material construction of an SCR.

2. SCR has three terminals – Gate (G), Anode (A) and Cathode (C) as shown in Figure 2. Figure 2

shows the material construction alongside with SCR symbols.

3. SCR can also be constructed through a PNP and NPN transistor connected back to back as

shown in Figure 3 as this type of connection can also creates a PNPN material (SCR) and this

explained the similarities with a transistor component.

~ Even if I don't reach all my goals, I've gone higher than I would have if I hadn't set any~

Danielle Fotopoulis

3 Wikipedia. (2010). Thyristor, Retrieved on 21

st June 2010 from http://en.wikipedia.org/wiki/Thyristor

http://en.wikipedia.org/wiki/Thyristor



*P type material is a semiconductor that has more holes than electrons as its free

charge carriers4, for example silicone (Si) (four covalent bonds) + gallium (Ga) boron

(three covalent bonds) = seven covalent bonds material with one hole5 (refer Figure 4).

And to a P-type material this hole has to outnumber the electrons thus more gallium

should be added to the silicone. Vice versa process is implemented in getting an N type material.

Figure 1: Silicone Controlled Rectifier

~ Enjoy your life today because yesterday has gone and tomorrow may never come ~

Alan Coren

4 Wikipedia. (2010). P-type Semiconductor, Retrieved on 18

th November 2010 from http://en.wikipedia.org/wiki/P-

type_semiconductor

5 Dziekan,M. (2009). The Citizen Scientist, Retrieved on 19

th November 2010 from

http://www.sas.org/tcs/weeklyIssues_2009/2009-01-02/feature1/index.html.

Anode

Gate

Cathode

http://en.wikipedia.org/wiki/P-type_semiconductor

http://en.wikipedia.org/wiki/P-type_semiconductor

http://www.sas.org/tcs/weeklyIssues_2009/2009-01-02/feature1/index.html



Figure 2: SCR symbol (right) and SCR constructions (left)

~ The secret of a good memory is attention, and attention to a subject depends upon our interest in it.

We rarely forget that which has made a deep impression on our minds. ~

Tryon Edwards

Figure 3: A PNP and NPN transistor can create SCR2

Anode

Cathode

Gate



4. As the name implies, SCR can rectified an Alternating Current (AC) Voltage to a Direct

Current (DC) Voltage if certain rules are fulfilled.

5. Among the rules are6 (refer to Figure 5)

a. Positive supply from the AC voltage is connected to the SCR’s Anode terminal.

b. Negative supply from the same supply voltage is connected to the SCR’s Cathode

terminal.

c. A positive pulse generator is connected to the SCR Gate terminal.

~ Wear a smile and have friends; wear a scowl and have wrinkles. What do we live for if not to make the

world less difficult for each other? ~

George Elliot

6 Maharashtra State Board of Technical Education

Figure 4: A silicon crystal doped with an impurity atom of gallium5



Figure 5: SCR circuit configuration6

6. After these rules are met, you can experiment with the circuit and obtained few SCR’s

operation modes. The modes can be categorized as6

a. Forward Blocking Mode can be obtained during the Forward break over Voltage (VFBO)

determination – This voltage is the forward anode voltage at which SCR switches ON and

begins to conduct, when gate current is zero (the gate has not yet be triggered By VGG).

SCR is in the Forward Blocking Mode when the forward anode voltage (VA) is less than

VFBO and small forward leakage current flows through SCR. Figure 6 shows the VFBO

concept structure.

~ Making mistakes simply means you are learning faster. ~



Figure 6: VFBO concept structure

b. Forward Conducting Mode can be obtained once the gate terminal was triggered hence

taking over the controlled of the value for the forward break over voltage and the

forward bias level at which the SCR is turned ON. The higher the value of the gate

current, the lower the anode voltage requires to turn SCR ON. Once the SCR is turned

ON, gate loses control and its removal does not affect the conduction of the SCR. The

anode voltage and the external load then determine the anode current, solely. Figure 7

shows the Ig concept structure.

Figure 7: Ig concept structure

c. Latching Current (IL) is said to be the value of the forward anode current (IA) requires

in turning the SCR ON and if that forward anode current is less than the latching

current the SCR shall remains OFF.



Figure 8: Latching Current concept structure

d. Once the SCR has been ON through the value of Latching Current, it need to be

maintained ON through a current namely Holding Current. Holding Current (IH) is the

minimum forward anode current required to be maintained to keep the SCR ON, once it

was triggered. And if during the conduction of the SCR, the anode current (IA) falls

below the level, it will automatically turns OFF.

Figure 9: Holding Current concept structure

e. Reverse Blocking Mode can be obtained if the polarities of the supply voltage were

applied in the reverse connection from previous forcing the SCR to block any current

that wants to pass through it. By increasing that voltage little by little, there will be

small reverse leakage current flows through the SCR till it reached its limit of blocking

the reverse voltage and breakdown. This limit is called reverse breakdown voltage.



7. These modes if were plotted together in the same graph, it will looked like Figure 107. This

kind of graph is known as the V-I static characteristic of the SCR.

Figure 10: SCR V-I static characteristic7

8. And if you remember that in the beginning of our discussion, there was a mention about the

possibility of constructing an SCR from a PNP and NPN transistor. Despite that, this does not

alter the shape of the SCR V-I static characteristic as the original terminal (base, collector and

emitter) has now been modified to accommodate the requirement in being an anode, cathode and

gate.

7 Bright Hub. (2010). VI Characteristic, Retrieved on 21

st November 2010 from

http://www.brighthub.com/engineering/electrical/articles/58051.aspx?image=34602



Figure 11: Regenerative Action by two transistor

9. Thus, the emitter of both the PNP and NPN transistor are now the Anode and Cathode of the

SCR, respectively, while the connection between the PNP collector and the NPN base is now the

Gate terminal.

10. By referring to Figure 118, you can see the new labels to the transistors.

11. If these transistors were made to operate, the collector of Q2 drives the base of Q1, while

the collector of Q1 feeds back to the base of Q2. β1 is the current gain of Q1, and β2 is the

current gain of Q2. The gain of this positive feedback loop is their product, 1 times 2.

12. When the product is less than one, the circuit is stable; if the product is greater than unity,

the circuit is regenerative.

13. And when a small negative current is applied to Gate terminal will bias the NPN transistor

into cut off, and the loop gain is less than unity. Under these conditions, the only current that

can exist between output terminals Anode and Cathode is a very small cut off collector current

of the two transistors. For this reason the impedance between A and C is very high and the

‘SCR’ is OFF.

8 Electrical Engineering Training Series. (2009). SCR Structure, Retrieved on 21

st June 2010 from

http://www.tpub.com/neets/book7/26d.htm

http://www.tpub.com/neets/book7/26d.htm



14. But, when a positive current is applied to Gate terminal, transistor

Q2 is biased into conduction, causing its collector current to rise. Since

the current gain of Q2 increases with increased collector current, a

point (called the break over point) is reached where the loop gain equals

unity and the circuit becomes regenerative.

15. At this point, collector current of the two transistors rapidly

increases to a value limited only by the external circuit. And both the

transistors are driven into saturation, and the impedance between A and

C is very low; the ‘SCR’ is OFF.

16. The positive current applied to Gate terminal, which served to trigger

the self-regenerative action, is no

longer required since the collector of

PNP transistor Q1 now supplies more than enough current to

drive Q2. The circuit will remain ON until it is turned OFF by

a reduction in the collector current to a value below that

necessary to maintain conduction. (Similar as you reduce the

forward anode current below the value of the holding

current).

17. By examining the graph, you discovered that the SCR can

be turned ON by either breaking its forward voltage limit or

by giving its Gate terminal a short positive pulse. By giving

the Gate terminal a pulse, you had shortened the time for the SCR to be switched ON. But what

the graph failed to illustrate was what kind of pulse has to be given to that Gate terminal.

18. The kind of pulse that can be given to the Gate terminal are either from a DC gate signal

triggering as seen in Figure 5, an AC gate signal triggering (Figure 129) or a Pulsed gate signal

triggering (Figure 1310).

9 Jojo. (2009). SCR control circuit, Retrieved on 21

st November 2010 from http://www.circuitstoday.com/scr-control-

circuits

http://www.circuitstoday.com/scr-control-circuits

http://www.circuitstoday.com/scr-control-circuits



Figure 12: AC gate signal triggering9

Figure 13: Pulsed gate signal triggering10

19. If the Gate was triggered from a DC signal, the DC supply should be properly connected

between the Gate and the Cathode.

20. By referring to Figure 5, you can see that the Gate received a positive supply from the DC

voltage while the negative was connected to Cathode as is why, you can understand by linking

that fact to the material construction of the Gate and Cathode which is from a P-type material

and N-type material, respectively (Figure 2).

10

Kuphaldt, T.R. (2003). Lessons in Electric Circuits, Retrieved on 21st November 2010 from

http://www.faqs.org/docs/electric/Semi/SEMI_7.html



21. But the drawback of this method is that the gate’s

signal has to be continuously applied resulting in increase in

internal power dissipation and there is no isolation of gate-

control circuit from the main power circuit.

22. While, if the Gate was triggered from an AC input

(Figure 12), the gate-to-cathode voltage is obtained from a

phase-shifted AC voltage derived from the main supply.

23. The main advantage of this method is that proper isolation of power and control circuits can

be provided. The firing angle control is obtained very conveniently by changing the phase angle

of the control signal.

24. However, the gate drive is maintained for one half-cycle after the device is turned ON and

a reverse voltage is applied between the Gate and the Cathode during the negative half-cycle.

25. And in Figure 13, the Gate was triggered from a single pulse appearing periodically or in a

sequence of high frequency pulses. A pulse transformer is used for isolation10 seen here in

Figure 14. The gate losses are very much reduced since the triggering is discontinuous.

Figure 14: A pulse transformer providing isolation to the Gate terminal10

26. The end results of these different types of Gate triggering methods and if were made to

compare were not that much in difference.

~ Adopt the pace of nature, her secret is patience. ~



27. For example in Figure 1511, the gate was triggered from DC gate signal while in Figure 1612

was the result from an AC gate signal triggering. And by comparing these two figures with

Figure 13, you can see that in terms of the waveform shape it is more or less the same.

Figure 15: The output waveform from a DC gate signal triggering11

Figure 16: The output waveform from an AC gate signal triggering12 11

Joint Apprenticeship Training Committee. (2010). Power Supply Rectification and Control Circuits, Retrived on 21st

November 2010 from http://www.ibewlocal625jatc.ca/technical/power.html

12 Hobby Projects Electronic Circuits Tutorials & Videos. (2010). Thyrostor, Diac and Triac – Electronic Tutorial: Phase

Triggering The Thyristor Tutorial, Retrived on 21st November 2010 from

http://www.hobbyprojects.com/thyristor_triac_and_diac/phase_triggering_the_thyristor.html



28. But what important is when or at what phase the SCR was

triggered because this in a way influenced the value of the load’s

power being delivered as seen in Figure 1713. By delaying the

triggering phase or sometimes being called as triggering angle will

cause the conduction phase (or conduction angle) be only half (50%)

or one-fourth (25%) from load’s full capacity.

29. However, the delayed in triggering the phase is sometimes necessary for example in

controlling a bulk of dimmers in stage lighting, or in welding power supplies where it is used to

maintain a constant output current or voltage14.

Figure 17: Triggering Points for various power output13

13

Powtran Technology Co. (2009). DC Drives – Principles of operation, Retrieved on 21st November 2010 from

http://www.powtran.net/faq.asp?Page=3

14 Wikipedia. (2010). Silicone Controlled Rectifier, Retrieved on 21

st June 2010 from http://en.wikipedia.org/wiki/Silicone-

controlled_rectifier



30. By analyzing the previous discussion, you will come up with a conclusion that this SCR

operates more or less like a switch, an electronic switch to be exact. And as a switch, it should

be able to be turn OFF after being turned ON.

31. To turn OFF an electronic switch such as the SCR it is not as easy as shutting down the

supply. Shutting down the supply means that you have permanently stops the machine and halts

any ongoing operation, and that is not the intention of turning OFF the SCR.

32. So how can you turn OFF the electronic switch but not the machine?

Turning OFF the SCR is much easier than you think. Remember how you

turned ON the SCR – it’s either by breaking the SCR’s forward voltage or

giving a pulse to the Gate and a certain value of forward anode current (IA)

must be given in maintaining the SCR operation.

33. So, in short in turning OFF the SCR you can simply make IA less than the

value of the holding current (IA < IH) by shunting the SCR with anode device or you can applied a

reverse voltage across the anode-cathode circuit. But, naturally when an AC supply is used as

the main supply voltage to that particular machine, IA will goes through a natural zero value

whenever the cycle went from positive to negative15. This action is known as natural

commutation.

34. But then again, why goes to all the fuss in utilizing an electronic switch if a mechanical

switch can do the same function as the electronic

switch?

35. SCR is made with voltage ratings of up to 7.5kV

and current ratings up to 3kA per device. Some of

the larger ones can take over 50kA in single-pulse

operation14.

36. SCR is used in power switching, phase control, chopper, battery chargers and inverter

circuits. Industrially it is applied to produce variable DC voltages for motors (from a few to

15

Sen, P.C. (2008). Principles of Electric Machines and Power Electronics. Second Edition. Wiley – India Edition.



several thousand HP) from AC line voltage14. Figure 18 charts out the SCR applications in a more

convenient way.

Figure 18: SCR Applications6

37. Thus, by linking up the list of the SCR applications with the

harsh environment requirement in which the SCR was installed,

you can figure out why it was chosen at the first place rather

than any mechanical switch (such as relay).

38. If you carefully looked at the list of the SCR applications, you

can see that it mostly need to switch ON and OFF small or large amounts of power, so it needs

to perform this function successfully unless it has moving parts that wear out or require

replacing.

39. SCR can often replaced much slower and larger mechanical

switches as it is an extremely fast switch. In 1 second, the SCR can

be switched ON and OFF 25,000 times. It takes just microseconds

for it to turn ON and OFF.

40. Furthermore, nowadays most devices operate on pulses of

power (AC voltage is a special form of alternating positive and

negative pulse) it can be used readily in control application. Motor



speed controllers, inverters, remote switching units, controlled

rectifier, circuit overload protectors, latching relays and

computer logic circuits all use SCR.

41. It can also provide fast response and high resolution as well as

the ability to limit current and regulate load voltage, current or

power to all that applications.

42. By that, in terms of its reliability is quite high as there are no moving parts to wear out.

Simply keeping it clean and cool provides users with many years of reliable service. In other

words, no moving parts means low maintenance requirement.

43. In short, both the SCR and Magnetic Contactor can be compared in the same table as shown

in Figure 18.

Figure 19: SCR versus Magnetic Contactor

Types SCR Magnetic Contractor

Current Do not need high current to operate the

circuit need high current to operate the circuit

Time Faster to ‘on’ the device Take time to induce the loop before ‘on’ the

device.

Size Small Big

Cost Low High

Voltage

Drop Low High

Performance Long time Short Time

Direction Current flow one direction only Can flow more than one direction

Protection Can protect complex circuit Only protect simple circuit



Exercise:

1. Draw the SCR material (structure) construction and its symbol.

2. State the requirements in switching ON and OFF the SCR.

(July 2006, Jan 2007, July 2007, Jan 2008, July 2008, July

2009, Jan 2010, July 2010)

3. With the help of an equivalent circuit figure, explain the

regenerative action of an SCR.

(Jan 2007, July 2008, July 2009, Jan 2010)

4. Draw the SCR’s transistor equivalent circuit and explain its

operation. (Jan 2008)

5. What is Forward break over voltage?

6. What is Reverse break over voltage?

7. Differentiate between holding and latching current. (July 2008)

8. What are the working modes of an SCR?

9. Draw the V-I characteristics of SCR.

10. What happens when gate drive is applied?

11. Is gate signal necessary during reverse biased?

12. What type of triggering is used in SCR?

13. What are the advantages of SCR?

14. What is the disadvantage of an SCR?

15. What are SCR applications?

~ Books are the legacies that a great genius leaves to mankind, which are delivered down from

generation to generation as presents to the posterity of those who are yet unborn ~

Joseph Addison

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Unit_1a