Dual Converter using Thyristor and Its application

Dual converter- the name itself indicates that it has

two converters in it. It is an electric device mostly

found in variable speed drives. It is a power

electronics control system to get either polarity DC

from AC rectification by the forward converter and

reverse converter. In a dual converter, two converters

are connected together back to back.

One of the bridges works as a rectifier (converts AC to

DC), another half bridge works as an inverter

(converts DC to AC) and is connected commonly to a

DC load. Here two conversion processes take place

simultaneously, so it is called a dual converter. The

dual converter can provide four quadrant operations.

The four quadrant operation is shown below.

Dual Converter Four Quadrant Operations

Principle of Dual Converter

The dual converter basic principle of operation can be

explained with reference to the simplified equivalent

diagram of the DC circuit shown in the figure below. In

this simplified representation, two assumptions are

made.

● To understand the principle of the dual converter, we assume that both converters are ideal. It means that they produce pure DC output

voltage, there is no ripple at the output terminals.

● Each two-quadrant converter is assumed to be a controllable direct voltage source, connected in series with a diode.

Here Diode D1 and D2 represent the unidirectional

current flow characteristics of the converters. Let us

assume, the average output voltage of the converter 1

is V01 and converter 2 is V02. To make the output

voltage of the two converters in same polarity and

magnitude, the firing angles of the thyristors have to

be controlled.

Ideal Dual Converter Simplified Representation

Average output voltage of Single-phase converter =

2Vm COSα/ π

For converter 1, the average output voltage, V01=

2Vmax COSα1/π

For converter 2, the average output voltage, V02=2

Vmax COSα2/π

The Output voltage is given by,

The firing angle can never be greater than 180. So, α1+

α2= 180

Modes of Operation of Dual Converter

There are two functional modes: Non-circulating

current mode and circulating mode.

1) Dual Converter Operation without Circulating

Current

In this type of dual converter, only one converter is in

conduction and another converter is temporarily

blocked. So, at a time one converter operates and the

reactor is not required between the converters. At a

particular instant, let say converter-1 acts as a rectifier

and supplying the load current. At this instant,

converter-2 is blocked by removing the firing angle.

For inversion operation, converter-1 is blocked and

converter-2 is supplying the load current.

The pulses to the converter-2 are applied after a delay

time. The delay time is around 10 to 20 msec. Why we

apply delay time between the change of operation? It

ensures reliable operation of thyristors. If converter-2

trigger before the converter-1 has completely turned

off, It may cause short circuit on the i/p.

Thus in Noncirculating Dual Converter:

● One converter will perform at a time. There is no circulating current between the converters.

● During the converter 1 operation, the firing angle (α1) will be 0<α1< 90 (Vdc and Idc are positive)

● During the converter 2 operation, firing angle (α2) will be 0<α2< 90 (Vdc and Idc are negative)

● The delay time is around 10 to 20 msec to ensure reliable operation of thyristors.

Circulating Current Modeload current.

● In this mode, both converters will be in the ON condition at the same time. So circulating current is present.

● The firing angles are adjusted such that α1+ α2=180. Firing angle of converter 1 is α1 and firing angle of converter 2 is α2.

● In this mode, the Converter 1 works as a controlled rectifier when the firing angle is 0<α1< 90 and Converter 2 works as an inverter when the firing angle is 90 <α2< 180. In this condition, Vdc and Idc are positive.

● Converter 1 works as an inverter when firing angle be 90 <α1< 180and Converter 2 works as a controlled rectifier when the firing angle is 0<α2< 90 in this condition, Vdc and Idc are negative.

Single Phase Dual Converter

The below shown figure shows single phase dual

converter using thyristors. As explained above, in

single phase dual converter we use single phase

rectifier circuit for converting single phase AC into

steady DC.

The Converter 1 consists of Rectifier. Then the

rectified DC fed to a filter which removes pulses from

rectified DC and converts it to a pure DC by filtering.

After that, this pure DC is fed to load and from the

load, it is given to inverter circuit which converts this

DC to AC and finally this AC of inverter taken as the

output.

Single Phase Dual Converter

Applications of Dual Converter

● Direction and Speed control of DC motors. ● Applicable wherever, the reversible DC is

required. ● Industrial variable speed DC drives.

Cycloconverters – Types, Working and Application

What is Cycloconverter?

Cycloconverter is a frequency converter from one level to another, that can change AC power from one frequency to AC power at another frequency. Here, an AC-AC conversion process is done with a frequency change. Hence it is also referred to as frequency changer. Normally, the output frequency is less than the input frequency.

A cyclo-converter can achieve frequency conversion in one stage and ensures that voltage and the frequencies are controllable. One particular property of Cycloconverters is that it does not use a DC link in the conversion process thus making it highly efficient. The conversion is done by using power

electronic switches like thyristors and switching them in a logical manner. Normally these Thyristors will be separated into two groups, the positive group and the negative group. Postive GROUP SCRs are called so because when triggered they produce positive polarity o/p voltage and similarly when Negative group SCR’s are triggered they produce negative polarity o/p voltage. Each half will be made to conduct by turning them during each half cycle of the AC form thus enabling bi-directional power flow.

Why do we need Cycloconverters?

Okay, now we know that Cycloconveters convert AC power of fixed frequency to AC Power of variable Frequency. But why do we need to do that? What is the advantage of having an AC power supply which has a variable Frequency?

The answer to this question is Speed Control. Cycloconverters are extensively used for driving large motors like the one used in Rolling mills, Ball mills, Cement kilns etc. The out frequency of a Cycloconverters can be reduced upto to zero which helps us to start very large motors with full load at minimum speed and then gradually increase the speed of the motor by increasing the output Frequency. Before the invention of

Cycloconverters, these large motors had to be unloaded completely and then after starting the motor it has to be loaded gradually which results in time and man power consumption.

Types of Cycloconveters:

Based on the output frequency and number of phase in the input AC power source the Cycloconverters can be classified as below

1. Step-Up Cycloconverters

2. Step-Down Cycloconverters

I. Single-Phase to Single-Phase Cycloconverter II. Three-Phase to Single-Phase Cycloconverter III. Three-Phase to Three-Phase Cycloconverter

Step-Up Cycloconverters: Step-Up Cycloconverter as the name suggests this type of CCV provides output frequency greater than that of input frequency. But it is not widely used. it

will require forced commutation which increases the complexity of the circuit.

Step-Down Cycloconverters: Step-Down CCV provides an output frequency which is lesser than the input frequency. These are most commonly used and work with help of natural commutation hence comparatively easy to build and operate.

Basic Principle behind Cycloconverters:

It will have a Switching circuit on either side of the Load, one circuit will function during the positive half cycle of the AC power source and the other circuit will function during the negative half cycle.

The switching circuits will also need a control circuit, which instructs the Power electronic device when to conduct and when to turn off. The diodes in the above diagram are used to represent the direction of flow of current. The positive switching

circuit always sources current into the load and the negative switching circuit always sinks current from the load.

Single Phase to Single Phase Cycloconverters:

This Cycloconverter has two full wave converters connected back to back. If one converter is operating the other one is disabled, no current passes through it. The Single Phase to Single Phase CCV has two pairs of full wave rectifier circuits, each consisting of four SCR. One set is placed straight while the other is placed in an anti-parallel direction as shown on the picture below.

All the gate terminals of the SCR’s will be connected to a control circuit which is not shown in the circuit above. This control circuit will be responsible for triggering the SCRs. To understand the working of the circuit let us assume that the input AC supply is of 50Hz frequency and the Load to be a pure resistive load and the firing angle of the SCR (α) to be 0°. Since the firing angle is at 0° the SCR when turned on will act like a diode in forward direction and when turned off will act like a diode in reverse direction. Let us analyse the wave form below to understand how frequency is stepped down using a CCV

The waveform of the supply voltage frequency is denoted by Vs and the wave form of the output voltage frequency is denoted by Vo. Here we are trying to convert the supply voltage frequency to 1/4th of its value. So to do that for the first two cycles of the supply voltage we will use the positive Bridge rectifier and for the following next two cycles we will use the negative bridge rectifier. Thus we have four positive pulses in the positive region and then four in the negative region as shown in the output frequency waveform Vo. The current waveform for this circuit will be the same as voltage waveform since the load is assumed to be purely resistive. Although the magnitude of the waveform will change based on the value of resistance of the load.

The output average voltage is represented using the dotted line on the Vo waveform, since it changes polarity only for every two cycles of the input waveform the output frequency with 1/4th of the input frequency, in our case for an input frequency of 50Hz the output frequency will be (1/4 * 50) around 12.5Hz. This output frequency can be controlled by varying the triggering mechanism in the control circuit.

Applications:

Cycloconverters have a large set of industrial application, the following are the few

● Electric traction applications to for example produce 25 Hz power in the U.S. and 16 2/3 Hz power in Europe

● Induction heating ● Gearless cement ● Ball mill drive ● wound-rotor induction motors (i.e., Scherbius drives)

● Aircraft Power supply ● SVG (Static VAR Generators) ● Ship Propulsion system

Power Electronics Questions & Answers

What are the different methods to turn on the

thyristor?

1. Forward voltage triggering

2. Gate triggering

3. dv/dt triggering

4. Temperature triggering

5. Light triggering

6. How can a thyristor turned off?

A thyristor can be turned off by making the current flowing

through it to zero.

Define latching current.

The latching current is defined as the minimum value of

anode current which it must attain during turn on process

to maintain conduction when gate signal is removed.

Define holding current.

The holding current is defined as the minimum value of

anode current below which it must fall to for turning off the

thyristor.

Q In THYRISTOR ratio of latching to holding current is

2.5.

What is a snubber circuit?

It consists of a series combination of a resistor and a

capacitor in parallel with the thyristors. It is mainly used for

dv / dt protection.

What losses occur in a thyristor during working

conditions?

1. Forward conduction losses

2. Loss due to leakage current during forward and

reverse blocking.

3. Switching losses at turn-on and turn-off.

4. Gate triggering loss.

What are the advantages of GTO over SCR?

1. Elimination of commutation of commutating

components in forced commutation, resulting in

reduction in cost, weight and volume.

2. Reduction in acoustic noise and electromagnetic

noise due to elimination of commutation chokes.

3. Faster turn-off, permitting high switching

frequencies.

4. Improved efficiency of the converters.

What is meant by phase controlled rectifier?

It converts fixed ac voltage into variable dc voltage.

Mention some of the applications of controlled

rectifier.

1. Steel rolling mills, printing press, textile mills and

paper mills employing dc motor drives.

2. DC traction

3. Electro chemical and electro-metallurgical process

4. Portable hand tool drives

5. HVDC transmission system

What is the function of freewheeling diodes in

controlled rectifier? It serves two process.

1. It prevents the output voltage from becoming

negative.

2. The load current is transferred from the main

thyristors to the freewheeling diode, thereby

allowing all of its thyristors to regain their blocking

states.

What are the advantages of freewheeling diodes in a

controlled in a controlled rectifier?

1. Input power factor is improved.

2. Load current waveform is improved and thus the

load performance is better.

What is meant by delay angle?

The delay angle is defined as the angle between the zero

crossing of the input voltage and the instant the thyristor is

fired.

Give an expression for average voltage of single

phase semiconverters or half control rectifier

Average output voltage Vdc = (Vm / π) (1 + cos α ).

What is meant by commutation?

This process is used in thyristors for turning it off.

What are the types of commutation?

1. Natural commutation

2. Forced commutation

What is meant by natural commutation?

Here the current flowing through the thyristor goes through

a natural zero and enable the thyristor to turn off.

What is meant by forced commutation?

In this commutation, the current flowing through the

thyristor is forced to become zero by external circuitry.

What is meant by dc chopper?

A dc chopper is a high speed static switch used to obtain

variable dc voltage from a constant dc voltage.

What are the applications of dc chopper?

1. Battery operated vehicles

2. Traction motor control in electric traction

3. Trolly cars

4. Marine hoists

5. Mine haulers

6. Electric braking.

What are the advantage of dc chopper?

Chopper provides

1. High efficiency

2. Smooth acceleration

3. Fast dynamic response

4. Regeneration

What is meant by step-up and step-down chopper?

In a step- down chopper or Buck converter, the average

output voltage is less than the input voltage. In a step- up

chopper or Boost converter, the average output voltage is

more than the input voltage.

Write down the expression for average output voltage

for step down chopper.

Average output voltage for step down chopper V0 = α Vs,

α is the duty cycle

Write down the expression for average output voltage

for step up chopper

Average output voltage for step up chopper V0 = Vs/(1-α)

here α is the duty cycle

What is meant by duty-cycle?

Duty cycle is defined as the ratio of the on time of the

chopper to the total time period of the chopper. It is

denoted by α.

What are the two types of control strategies?

1. Time Ratio Control (TRC)

2. Current Limit Control method (CLC)

What is meant by TRC?

In TRC, the value of Ton / T is varied in order to change

the average output voltage.

What are the two types of TRC?

1. Constant frequency control

2. Variable frequency control

What is meant by FM control in a dc chopper?

In frequency modulation control, the chopping frequency f

(or the chopping period T) is varied. Here two controls are

possible.

1. On-time Ton is kept constant

2. Off period Toff is kept constant.

What is meant by PWM control in dc chopper?

In this control method, the on time Ton is varied but

chopping frequency is kept constant. The width of the

pulse is varied and hence this type of control is known as

Pulse Width Modulation (PWM).

Write down the expression for the average output

voltage for step down and step up chopper.

Average output voltage for step down chopper is VO = α

VS. Average output voltage for step up chopper is

VO = α VS x [1/ ( 1- α )].

What is meant by inverter?

A device that converts dc power into ac power at desired

output voltage and frequency is called an inverter.

What are the applications of an inverter?

1. Adjustable speed drives

2. Induction heating

3. Stand-by aircraft power supplies

4. UPS

5. HVDC transmission

What are the main classification of inverter?

1. Voltage Source Inverter

2. Current Source Inverter

How output frequency is varied in case of a thyristor?

The output frequency is varied by varying the turn off time

of the thyristors in the inverter circuit, i.e. the delay angle

of the thyristors is varied.

Give two advantages of CSI.

1. CSI does not require any feedback diodes.

2. Commutation circuit is simple as it involves only

thy

What is meant a series inverter?

An inverter in which the commutating elements are

connected in series with the load is called a series

inverter.

What is the condition to be satisfied in the selection of

L and C in a series inverter?

R <√ 4L/C

What is meant a parallel inverter?

An inverter in which the commutating elements are

connected in parallel with the load is called a parallel

inverter.

What are the applications of a series inverter?

The thyristorised series inverter produces an

approximately sinusoidal waveform at a high output

frequency, ranging from 200 Hz to 100kHz. It is commonly

used for fixed output applications such as

1. Ultrasonic generator.

2. Induction heating.

3. Sonar Transmitter

4. Fluorescent lighting.

How is the inverter circuit classified based on

commutation circuitry?

1. Line commutated inverters.

2. Load commutated inverters.

3. Self commutated inverters.

4. Forced commutated inverters.

What are the applications of a CSI?

1. Induction heating

2. Lagging VAR compensation

3. Speed control of ac motors

4. Synchronous motor starting.

Compare CSI and VSI.

What is meant by cyclo-converter?

It converts input power at one frequency to output power

at another frequency with one-stage conversion.

Cycloconverter is also known as frequency changer.

What are the two types of cyclo-converters?

1. Step-up cyclo-converters

2. Step-down cyclo-converters

What is meant by step-up cyclo-converters?

In these converters, the output frequency is more than the

supply frequency.

What is meant by step-down cyclo-converters?

In these converters, the output frequency is less than the

supply frequency.

What are the applications of cyclo-converter?

1. Induction heating

2. Speed control of high power ac drives

3. Static VAR generation

4. Power supply in aircraft or ship boards

What is meant by positive converter group in a

cycloconverter?

The part of the cycloconverter circuit that permits the flow

of current during positive half cycle of output current is

called positive converter group.

What is meant by negative converter group in a

cycloconverter?

The part of the cycloconverter circuit that permits the flow

of current during negative half cycle of output current is

called negative converter group.POWER ELECTRONICS

Short Answer type question ( about 30-40 words)-2

1. Define SCR or Thyristor and draw its symbol.

Ans1: SCR stands for Silicon Controlled Rectifier. It is a four layer (p-n-p-n) semiconductor and three terminals (anode,cathode and gate) devices.

Symbol:

2. Define Latching current.

Ans2: Latching Current (IL): It is the minimum anode current value to keep the thyristor in conducting state after the gate pulse is removed.

3. Define holding current.

Ans3: Holding Current (IH): It is the minimum anode current at which thyristor continue to conduct. If the anode current is less than IH , the thyristor will be turned off.

4. What is thyristor commutation?

Ans4: It is the process to turn – off the thyristor. There are TWO ways to turn- off the thyristor . natural commutation, forced commutation

5. Define UJT.

Ans5: UJT stands for Uni- Junction Transistor. It is a two layer (PN) and three terminal solid state device. The pulse obtained by UJT relaxation oscillator is used to trigger the thyristor.

6. Why a heat sink is required with a thyristor?

Ans6: To keep the junction temperature constant, thyristor requires a heat sink to transfer the heat energy from thyristor to atmosphere.

7. What is Triac?

Ans7: It is referred as Triod for AC.Triac is a bidirectional device that conduct in both direction. It can

be used for speed control of small single phase series & induction motors.

8. SCR can be used as unidirectional switch.

9. What is dv/dt triggering of SCR?

Ans9: dv/dt triggering:- If the rate of increase of voltage dv/dt is made more than the value of critical rate of increase of voltage, then the SCR starts conducting. This method is known as dv/dt triggering, but it is not desirable because a high charging current may damage the thyristor.

10. What do you mean by ‘firing’ of SCR?

Ans10: Firing means the turning – ON of an SCR. The output voltage of SCR is controlled by controlling the firing angle (α) of SCR.

11. What do you mean by gate triggering method of SCR?

Ans11: When the SCR is turned – ON by applying the positive signal to the gate of a forward biased SCR . This type of triggering is known as Gate Triggering. As the gate current is increased, the forward blocking voltage is reduced.

12. What is forced commutation of thyristor?

And12: When the SCR has to be turned – off by a special commutation circuit using some extra components. This technique is known as forced commutation. Inductor & capacitor are known as commutating components.

13. Define GTO.

Ans13: GTO stands for Gate turn OFF thyristor. The GTO can br turned – ON by a positive gate signal and can also be turned off by a negative gate signal.

Symbol of GTO:

14. What are controlled rectifier?

Ans14: When the rectifier are built with SCRs or the combination of SCRs and Diodes is known as controlled rectifier which convert a.c. input voltage in to a controllable dc output voltage by controlling firing angle of SCR.

15. What are the effects of firing angle on an converter?

Ans15: When the firing angle is in between o˚ to 90˚, then a converter is acts as rectifier and when firing

angle is greater than 90˚ but less than 180˚, then converter acts as an Inverter.

16. Why 3- phase controlled rectifiers are preferred over single phase rectifier? OR what is the main advantages of 3- phase controlled rectifier than single phase rectifier?

Ans16: The main advantage of 3- phase controlled rectifier is that they produce less ripple voltage than a single phase controlled rectifier.

17. What is inverter?

Ans17: Inverter is an electronic circuit that converts direct current (D.C) to alternating current (A.C).

18. Define chopper?

Ans18: Chopper is an electronics circuit that converts a fixed DC voltage to a variable DC voltage, it is also known as DC-DC converter.

19. What is current source inverter(CSI) & voltage source inverter(VSI)?

Ans19: Current Source Inverter(CSI): When the inverter is fed from a DC source voltage with large inductance in series so that the current remains constant.

Voltage Source Inverter(VSI):- When the inverter is fed from a DC source voltage with negligible internal resistance , then it is called Voltage Source Inverter.

20. Define Duty Cycle.

Ans20: It is the ratio of Turn on time of chopper (Ton) to the sum of turn on & turn off (Toff) of chopper.It is denoted by α

α = Ton ___

Ton+Toff

21. What are cycloconverter?

Ans21: It converts AC at one frequency to AC at another frequency. It consists of positive group & negative group. The positive group provides positive half cycles and negative group provides negative half cycles of output.

22. Define Dual Converter?

Ans23: When two full converter in antiparallel connected to the same load, such arrangement is known as Dual Converter. It is generally used in the application where reversible DC is required.

23. A class – D chopper operates in 1st and 4th quadrant.

24. Writes the application of inverter?

Ans24: i) Inverter can be designed to provide 115VAC from 12VDC source provided in automobile.

ii) It is used in UPS.

iii) It is used in variable frequency drives and electric vehicle drives.

iv) It is used in HVDC power transmission.

25. What is electric drive?

Ans25: An electric motor together with its control equipment and energy-transmitting mechanism is called “electric drive”.For example, a celling – fan motor with its speed regulator is called electric drive.

26. Name two methods of speed control of D.C. motor.

Ans26:

Chopper Drive and converter drive

Armature voltage control method and field control method

27. What are the advantages of A.C. drves over D.C. drives?

Ans27: Advantages of AC drives:- 1. A.C. motor requires less maintenance than D.C. motors.

2. For the same power rating , ac motors are lighter in weights than dc motors.

3. Cost of a.c. motors is less as compared to equivalent d.c. motors.

4. A.C motors can be used in hazardous environments like chemicals & petrochemicals etc. While d.c motors are not suitable for such environments because of sparking.

28. Name the methods of speed control of 3- phase induction motor?

Ans28: i) Stator Voltage Control method

ii) Variable – Frequency control method

iii) Cycloconverter controlled A.C. Drive

29. What is VFD?

Ans29; Variable Freuency Drive - It is a system for controlling the rotational speed of an alternating current(AC) electric motor by controlling the frequency of the electrical power supplied to the motor.

30. What is UPS?

Ans30: UPS stands for Uninterrupted Power Supply. The system which provide an interruption free supply of a.c is called UPS.

31. What are the types of UPS?

Ans31: UPS are of three types; OFF-Line UPS, ON-Line UPS and Line interactive UPS.

32. What is SMPS?

33. SMPS satands for Switched Mode Power Supply. In low to medium power requirement (more than 50W), a dc powr supply is required which contains negligible ac ripples. SMPS is a multistage dc supply and is used for higher ratings.

34. Write the application of UPS?

Ans34; UPS are used in : computers

Data processors, data transmitters,

Microwave relay station,

Nuclear reactor control and communication etc.

35. What are the types of storage devices of an UPS?

Ans35: Battery are the main storage devices of an UPS. Battery may be of wet type and dry type, such as Lead Acid Battery(wet type), Nickel – Cadmium battery(dry type) and Sealed Maintenance Free battery.

36. In a dual converter, one converter operates as rectifier and other as Inverter.

37. What is parallel inverter?

Ans37: In parallel inverter, commutating components i.e. capacitor is connected parallel across the load through transformer.

38. What is series inverter?

Ans38: In Series inverter, commutating components i.e. inductors and capacitors are connected in series with the load, forming the under damped circuit.

POWER ELECTRONICS Questions and Answers 3| MCQs

POWER ELECTRONICS Questions with Answers :-

1. A Triac has three terminals viz ……………… 1. Drain, source, gate 2. Two main terminal and a gate terminal

3. Cathode, anode, gate 4. None of the above

Ans : 2 2. A triac is equivalent to two SCRs …………..

1. In parallel 2. In series 3. In inverse-parallel 4. None of the above

Ans : 3 3. A triac is a …………. switch

1. Bidirectional 2. Unidirectional 3. Mechanical 4. None of the above

Ans : 1 4. The V-I characteristics for a triac in the first and third quadrants are essentially identical to those of ………………. in its first quadrant

1. Transistor 2. SCR 3. UJT 4. none of the above

Ans : 2 5. A triac can pass a portion of …………… half-cycle through the load

1. Only positive 2. Only negative 3. Both positive and negative

4. None of the above Ans : 3

6. A diac has ………….. terminals 1. Two 2. Three 3. Four 4. None of the above

Ans : 1 8. A diac has …………… pn junctions

1. Four 2. Two 3. Three 4. None of the above

Ans : 2 9. The device that does not have the gate terminal is ……………….

1. Triac 2. FET 3. SCR 4. Diac

Ans : 4 11. A UJT has ……………….

1. Two pn junctions 2. One pn junction 3. Three pn junctions 4. None of the above

Ans : 2 12. The normal way to turn on a diac is by ………………..

1. Gate current 2. Gate voltage 3. Breakover voltage 4. None of the above

Ans : 3 13. A diac is …………………. switch

1. A a c. 2. A d.c. 3. A mechanical 4. None of the above

Ans : 1 14. In a UJT, the p-type emitter is ……………. doped

1. Lightly 2. Heavily 3. Moderately 4. None of the above

Ans : 2 15. Power electronics essentially deals with control of a.c. power at …………

1. Frequencies above 20 kHz 2. Frequencies above 1000 kHz 3. Frequencies less than 10 Hz 4. 50 Hz frequency

Ans : 4 16. When the emitter terminal of a UJT is open, the resistance between the base terminal is generally

……………….. 1. High 2. Low 3. Extremely low 4. None of the above

Ans : 1 17. When a UJT is turned ON, the resistance between emitter terminal and lower base terminal …………….

1. Remains the same 2. Is decreased 3. Is increased 4. None of the above

Ans : 2 18. To turn on UJT, the forward bias on the emitter diode should be …………… the peak point voltage

1. Less than 2. Equal to 3. More than 4. None of the above

Ans : 3 19. A UJT is sometimes called …………. diode

1. Low resistance 2. High resistance 3. Single-base 4. Double-base

Ans : 4 22. Between the peak point and the valley point of UJT emitter characteristics we have ………….. region

1. Saturation 2. Negative resistance 3. Cut-off 4. None of the above

Ans : 2 24. A diac is turned on by …………………

1. A breakover voltage 2. Gate voltage 3. Gate current 4. None of the above

Ans : 1 25. The device that exhibits negative resistance region is ………………..

1. Diac 2. Triac 3. Transistor 4. UJT

Ans : 4 26. The UJT may be used as ……………….

1. Am amplifier 2. A sawtooth generator 3. A rectifier 4. None of the above

Ans : 2 27. A diac is simply ………………

1. A single junction device 2. A three junction device 3. A triac without gate terminal 4. None of the above

Ans : 3 28. After peak point, the UJT operates in the ……………. region

1. Cut-off 2. Saturation 3. Negative resistance 4. None of the above

Ans : 3 29. Which of the following is not a characteristic of UJT?

1. Intrinsic stand off ratio 2. Negative resistance 3. Peak-point voltage 4. Bilateral conduction

Ans : 4 30. The triac is …………….

1. Like a bidirectional SCR 2. A four-terminal device 3. Not a thyristor 4. Answers (1) and (2)

Ans : 1