1 POWER ELECTRONICS UNIT - I INTRODUCTION To AND OTHER POWER ELECTRONICS

UNIT - I Power Electronics & its Roll : Power electronics is a technology that deals with the conversion and control of electrical power with high-efficiency switching mode electronic devices for a wide range of applications.

Power electronics makes it possible to transport electricity over long distances with minimum losses, which is accomplished by Power Converters. Power Converters convert alternating current (AC) into high-voltage direct current (HVDC) and vice-versa Power Electronics Devices are also used many applications like speed control of AC / DC Motors, electrochemical process, heating and lighting control, electronic welding, power line volt–ampere reactive (VAR) and harmonic compensators, high-frequency (HF) heating, and motor drives.

Silicon Controlled Rectifier ( SCR ) / A silicon controlled rectifier is a four-layer solid state current-controlling device which control high Voltage and Power. A SCR is device / component have three terminals namely anode (A), cathode (K),

Gate (G), four Layers P-N-P-N, and three Junctions J 1, J 2, J 3 . It allows the current in one direction from Anode to Cathode only and the Gate controls the flow of current between the Anode and Cathode. The primary function of SCR / Thyristor is to control electric power and current by acting as a switch.

Figure shows the Construction & Symbol of SCR:

2 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

SCR V-I Characteristics

Forward Characteristics When Anode is positive with respect to Cathode, the curve between V and I is called the forward characteristics.

In figure, OMNK is the forward characteristics of SCR at IG=0. If the supply voltage is increased from zero, a point reached ( point M ) when the SCR starts conducting and the voltage across SCR suddenly drops as shown by dotted curve MN and then to any point in between N and K. Since the anode current in this mode will only be limited by the load, so based on the value of load the anode current will change and may at any point in between N and K. Thus NK represents the forward conduction of SCR and supply voltage appears across the load resistance RL . If proper gate current is made to flow, SCR can close at much smaller supply voltage. Reverse Characteristics

When Anode is negative with respect to Cathode, the curve between V and I is known as reverse characteristics. In figure, MPQ is the reverse characteristics of SCR. In reverse characteristics, Initially voltage increases from O and reached up to P Point, only negligible leakage current will flow through SCR. At Point P, SCR reached at reverse breakdown voltage ( VBR ) 3 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS represents this reverse breakdown voltage in the V-I characteristics. It can be seen that, there is a sharp increase in reverse current at this voltage. This increased reverse current may result in more losses in the SCR which in turn may damage the SCR. Therefore the reverse voltage across the SCR terminals should not exceed reverse breakdown voltage during its operation.

Important Terms in V-I Characteristics of SCR 1. Forward Break-Over Voltage :- Forward break over voltage of SCR is the minimum forward voltage at which SCR starts conducting. Thus, if the break-over voltage of an SCR is 200 V, it means that it can block a forward voltage (i.e. SCR remains open) as long as the supply voltage is less than 200 V. If the supply voltage is more than this value, then SCR will be turned on. In practice, the SCR is operated with supply voltage less than break-over voltage and it is then turned on by means of a small voltage applied to the gate.

2. Peak Reverse Voltage (PRV) / Reverse Breakdown Voltage: It is the maximum reverse voltage (cathode positive w.r.t. anode) applied to an SCR up-to which it remains safe i. e. without damaging in the reverse direction.” PRV is an important consideration while connecting an SCR in an a.c. circuit. During the negative half of A. C. supply, reverse voltage is applied across SCR. If PRV is exceeded, there may be avalanche breakdown and the SCR will be damaged if the external circuit does not limit the current. Commercially available SCRs have PRV ratings up-to 2.5 kV.

3. Forward Current Rating: It is the maximum anode current that an SCR is capable of passing without destruction. Every SCR has a safe value of forward current which it can conduct. If the value of current exceeds this value, the SCR may be destroyed due to intensive heating at the junction. For example, if an SCR has a forward current rating of 40 A, it means that the SCR can safely carry only 40 A. Any attempt to exceed this value will result in the destruction of the SCR.

4 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

4. Holding Current: It is the minimum anode current at which the thyrister / SCR continue to conduct. If the anode current less than Holding current, the thyrister will be turned off.

Holding current of SCR or thyrsistor is that minimum value of current below which anode current must fall to come in OFF state. This means if the value of holding current is 5 mA, then anode current of SCR must become less than 5 mA to stop conducting.

5. Latching Current: Latching current is the minimum Anode current required to maintain the thyristor in the ON-STATE after the gate pulse is removed.”

Difference Between latching and holding current.

Sr. No. Latching Current Holding Current

It is related with turn on process of 1) It is related to turn off process. SCR or thyristor.

Minimum value of anode current Minimum current above which gate 2) below which it must fall to stop losses its control. conducting in forward direction.

Value of latching current is more 3) It is less than latching current. than that of holding current.

Latching current is generally 2 to 3 4) – times of the holding current.

5 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

Two - analogy of SCR

ww.electrical.com The two-transistor analogy of SCR is a method of representing an SCR as a combination of an N-P-N and a P-N-P transistor. SCR is a three terminal device having a P- N-P-N structure. The three terminals are the cathode, anode and the gate terminal.

The two transistor equivalent circuit shows that the collector current of the NPN transistor TR 2 feeds directly into the base of the PNP transistor TR 1, while the collector current of TR 1 feeds into the base of TR 2. These two inter-connected with each other for conduction as each transistor gets its base-emitter current from the other’s collector-emitter current. So until one of the transistors is given some base current nothing can happen even if an Anode-to-Cathode voltage is present.

If the Anode terminal is made positive with respect to the Cathode, the two outer P- N junctions are now forward biased but the centre N-P junction is reverse biased. Therefore forward current is also blocked. If a positive current is injected into the base of the NPN transistor TR 2, the resulting collector current flows in the base of transistor TR 1. This in turn causes a collector current to flow in the PNP transistor, TR 1 which increases the base current of TR 2 and so on

If the thyristors Anode terminal is negative with respect to the Cathode, the centre N- P junction is forward biased, but the two outer P-N junctions are reversed biased and it behaves very much like an ordinary diode. Therefore a thyristor blocks the flow of reverse current until at some high voltage level the breakdown voltage point of the two outer junctions is exceeded and the thyristor conducts without the application of a Gate signal. 6 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

Then we can see that a thyristor blocks current in both directions of an AC supply in its “OFF” state and can be turned “ON” and made to act like a normal rectifying diode by the application of a positive current to the base of transistor, TR 2 which for a silicon controlled rectifier is called the “Gate” terminal.

SCR Triggering: Switching the SCR from forward blocking state (OFF- state) to forward conduction state (ON- state) is known as turning ON process of SCR . It is also called as SCR triggering. SCR Triggering Methods: The SCR can be made to conduct or switching into conduction mode is performed by any one of the following methods. i. Forward voltage triggering.

ii. Temperature triggering.

iii. dv / dt triggering.

iv. Light triggering.

v. Gate triggering.

i. Forward Voltage Triggering : One of the commonly used SCR Turn On methods is by increasing the forward anode to cathode voltage.

By doing this, the depletion layer width is also increasing at junction J 2. This also causes to increase the minority charge carriers accelerating voltage

at junction J2. This further leads to an avalanche breakdown of the junction J 2

at a forward break-over voltage V BO . At this stage SCR turns into conduction mode and hence a large current flow through it with a low voltage drop across it. During the turn ON state the forward voltage drop across the SCR is in the range of 1 to 1.5 volts and this may be increased with the load current. In practice this method is not employed because it needs a very large anode to cathode voltage. And also once the voltage is more than the VBO, it generates very high currents which may cause damage to the SCR. Therefore, most of the cases this type of triggering is avoided. ii. Temperature Triggering : The reverse leakage current depends on the temperature. If the temperature is increased to a certain value, the number of hole-pairs also increases. This causes to increase the leakage current and further it increases the current gains of the SCR. This starts the regenerative 7 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

action inside the SCR since the (α 1 + α 2) value approaches to unity (as the current gains increases).

By increasing the temperature at junction J 2 causes the breakdown of the junction and hence it conducts. This triggering occur in some circumstances particularly when it the device temperature is more (also called false triggering). This type of triggering is practically not employed because it causes the thermal runaway and hence the device or SCR may be damaged.

iii. dv/dt Triggering I = C dv/ dt

Where dv / dt is the rate of change of applied voltage and C is the junction capacitance.

From the above equation, if the rate of change of the applied voltage is large that leads to increase the charging current which is enough to increase the value of alpha. So the SCR becomes turned ON without a gate signal. However, this method is also practically avoided because it is a false turn ON process and also this can produce very high voltage spikes across the SCR so there will be considerable damage to it.

iv. Light Triggering : An SCR turned ON by light radiation is also called as Light Activated SCR (LASCR). This type of triggering is employed for phase controlled converters in HVDC transmission systems. In this method, light rays with appropriate wavelength and intensity are allowed to strike the

junction J 2.These types of SCRs are consisting a niche in the inner p-layer. Therefore, when the light struck on this niche, electron-hole pairs are

generated at the junction J 2 which provides additional charge carriers at the junction leads to turn ON the SCR. v. Gate Triggering: This is most common and efficient method to turn ON the SCR. When the SCR is forward biased, a sufficient voltage at the gate terminal injects some electrons into the junction J2. This result to increase reverse leakage current and hence the breakdown of junction J2 even at the voltage lower than the VBO.In gate triggering method, a positive voltage applied between the gate and the cathode terminals. 8 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

We can use three types of gate signals to turn On the SCR. i. DC signal, ii. AC signal iii. pulse signal. i. DC Gate Triggering : In this triggering, a sufficient DC voltage is applied between the gate and cathode terminals in such a way that the gate is made positive with respect to the cathode. The gate current drives the SCR into conduction mode.

In this, a continuous gate signal is applied at the gate and hence causes the internal power dissipation (or more power loss). ii. AC Gate Triggering : This is the most commonly used method for AC applications where the SCR is employed for such applications as a switching device. With the proper isolation between the power and control circuit, the SCR is triggered by the phase-shift AC voltage derived from the main supply. The firing angle is controlled by changing the phase angle of the gate signal.

However, only one half of the cycle is available for the gate drive to control the firing angle and next half of the cycle a reverse voltage is applied between the gate and cathode. This is one of the limitation of AC triggering and also separate step down or pulse transformer is needed to supply the voltage to gate drive from the main supply. iii. Pulse Triggering:- The most popular method of triggering the SCR is the pulse triggering. In this method, gate is supplied with single pulse or a train of pulses.The main advantage of this method is that gate drive is discontinuous or doesn’t need continuous pulses to turn the SCR and hence gate losses are reduced in greater amount by applying single or periodically appearing pulses. For isolating the gate drive from the main supply, a pulse transformer is used.

9 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

SCR Turn OFF Methods: Important Note:To turn OFF the conducting SCR the below conditions must be satisfied:

i. The anode or forward current of SCR must be reduced to zero or below the level of holding current and then, ii. A sufficient reverse voltage must be applied across the SCR to regain its forward blocking stage

Commutation of SCR : The turn OFF process of an SCR is called commutation of SCR. There are Two Type of SCR Commutation:. i. Forced commutation and ii. Natural commutation .

i. Natural Commutation :- In natural commutation, the source of commutation voltage is the supply source itself. This method of commutation is also called as source commutation, or line commutation, or class F commutation.

In natural commutation, the source of commutation voltage is the supply source itself. If the SCR is connected to an AC supply, at every end of the positive half cycle the anode current goes through the natural current zero and also immediately a reverse voltage is applied across the SCR. These are the conditions to turn OFF the SCR. 10 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

i. Forced Commutation: In case of DC circuits, there is no natural current zero to turn OFF the SCR. In such circuits, forward current must be forced to zero with an external circuit to commutate the SCR hence named as forced commutation.

Forced commutation is classified into different types such as class A, B, C, D, and E.

i) Class A Commutation : Class A Commutation ( Current Commutation or Self Commutation or Resonance Commutation ) The commutating components L and C are used in order to turn off the SCR. When the load resistance of very small value is used, the inductor L and capacitor C are connected in series with load. The commutating components L and C are connected either parallel or series with the load resistance R as shown below with waveforms of SCR current, voltage and capacitor voltage

The value of load resistance and commutating components are so selected that they forms a under damped resonant circuit to produce natural zero. When the thyristor or SCR is triggered, the forward currents starts flowing through it and during this the capacitor is charged up to the value of

VDC . Once the capacitor is fully charged (more than the supply source voltage) the SCR becomes reverse biased and hence the commutation of the device. This method is simple and reliable. For high frequency operation which is in the range above 1000 Hz, this type of commutation circuits is preferred due to the high values of L and C components.

11 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

ii) Class B Commutation: This is also a self commutation circuit in which commutation of SCR is achieved automatically by L and C components, once the SCR is turned ON. In this, the LC resonant circuit is connected across the SCR but not in series with load as in case of class A commutation and hence the L and C components do not carry the load current. When the DC supply is applied to the circuit, the capacitor charges with

an upper plate positive and lower plate negative up to the supply voltage VDC . When the SCR is triggered, the current flows in two directions, one is through

VDC + – SCR – RL – VDC - and another one is the commutating current through L and C components. Once the SCR is turned ON, the capacitor is starts discharging through C+ – L – SCR – C-. When the capacitor is fully discharged, it starts charging with a reverse polarity. Hence a reverse voltage applied across the SCR which causes the commutating current IC to oppose

load current I L. When the commutating current Ic is higher than the load current, the SCR will automatically turn OFF and the capacitor charges with original polarity.

In the above process, the SCR is turned ON for some time and then automatically turned OFF for some time. This is a continuous process and the desired frequency of ON/OFF depends on the values of L and C. This type of commutation is mostly used in chopper circuits.

12 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

iii) Class C Commutation: In this commutation method, the main SCR is to be commutated is connected in series with the load and an additional or complementary SCR is connected in parallel with main SCR. This method is also called as complementary commutation. In this , SCR turns OFF with a reverse voltage of a charged capacitor. The figure below shows the complementary commutation with appropriate waveforms.

Initially, both SCRs are in OFF state so the capacitor voltage is also

zero. When the SCR 1 or main SCR is triggered, current starts flowing in two

directions, one path is VDC + – RL – SCR 1 – VDC - and another path is the

charging current VDC + – R- C+ – C- SCR 1 – VDC - . Therefore, the capacitor

starts charging up to the value of VDC . When the SCR 2 is triggered, SCR 2 is

turned ON and simultaneously a negative polarity is applied across the SCR 1.

So this reverse voltage across the SCR 1 immediately causes to turn OFF the

SCR 1. Now the capacitor starts charging with a reverse polarity through the

path of VDC + – RL- C+ – C- SCR 2 – VDC -. And again, if the SCR 1 is triggered,

discharging current of the capacitor turns OFF the SCR 2. This commutation is mainly used in single phase inverters with a centre tapped transformers. This is a very reliable method of commutation and it is also useful even at frequencies below 1000Hz.

13 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

iv) Class D Commutation : This is also called as auxiliary commutation because

it uses an auxiliary SCR to switch the charged capacitor. In this, the main

SCR 1 is commutated by the auxiliary SCR 2. The main SCR 1 with load

resistance RL forms the power circuit while the diode D, inductor L and SCR 2 forms the commutation circuit.

When the supply voltage VDC is applied, both SCRs are in OFF state and hence the capacitor voltage is zero. In order to charge the capacitor,

SCR 2 must be triggered first. So the capacitor charges through the path VDC +

– C+ – C- – SCR 2- RL- VDC -. When the capacitor is fully charged the SCR 2

becomes turned OFF because no current flow through the SCR 2 when

capacitor C is charged fully. If the SCR 1 is triggered, the current flows in two

directions; one is the load current path VDC + – SCR 1- RL- VDC - and another

one is commutation current path C+ – SCR 1- L- D- C-. As soon as the capacitor completely discharges, its polarities will be reversed but due to the

presence of diode the reverse discharge is not possible. When the SCR 2 is

triggered capacitor starts discharging through C+ – SCR 2- SCR 1- C-. When

this discharging current is more than the load current the SCR 1 becomes

turned OFF. Again, the capacitor starts charging through the SCR 2 to a supply

voltage VDC and then the SCR 2 is turned OFF. Therefore, both SCRs are turned OFF and the above cyclic process is repeated. This commutation method is mainly used in inverters and also used in chopper circuit. 14 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

v) Class E Commutation: This is also known as external pulse commutation. In this, an external pulse source is used to produce the reverse voltage across the SCR..

If the SCR need to be commutated, pulse duration equal to the turn OFF time of the SCR is applied. When the SCR is triggered, load current flows through the pulse transformer. If the pulse is applied to the primary of the pulse transformer, an e. m. f or voltage is induced in the secondary of the pulse transformer. This induced voltage is applied across the SCR as a reverse polarity and hence the SCR is turned OFF. The capacitor offers a very low or zero impedance to the high frequency pulse.

Series and Parallel connections of an SCR

In many power control applications the required voltage and current ratings exceed the voltage and current that can be provided by a single SCR. Under such situations the SCRs are required to be connected in series or in parallel to meet the requirements.

i. Series Connection of an SCRs: When the required voltage rating exceeds the SCR voltage rating, a number of SCRs are required to be connected in series to share the forward and reverse voltage. 15 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

As it is not possible to have SCRs of completely identical characteristics, deviation in characteristics lead to the following two major problems during series connections of the SCRs: i) Unequal distribution of voltage across SCRs . ii) Difference in recovery characteristics. As the voltage drop across the thyristors would be unequal & to solve this issue following parameters are set by default. i) Resistors are connected across each thyristor to avoid the unequal voltage sharing. ii) The values of the resistors should be selected, so the equivalent resistance of thyristor and resistor would be same. i. Parallel Connection of SCRs: When the load current exceeds the SCR current rating, SCRs are connected in parallel to share the load current.

But when SCRs are operated in parallel, the current sharing between them may not be proper. The device having lower dynamic resistance will tend to share more current. This will raise the temperature of that particular device in comparison to other, thereby reducing further its dynamic resistance and increasing current through it and continues till the device gets damaged . This process is known as Thermal Runway. 16 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

Thermal runaway can be avoided by following these steps: i) There should be a common heat sink, making sure that the thyristors operate at similar temperature. ii) Current sharing should be equal. For this resistors or inductors may be used in series with the thyristors. TRIAC : TRIAC: It is a Bidirectional Five Layers device that can pass the current in

both forward and reverse biased conditions and have three terminals MT 1 , MT 2 &

Gate The MT 1 and MT 2 terminals are used to connect the Phase lines while the Gate is used to feed the triggering pulse. The Gate can be triggered either by a positive voltage or negative voltage. TRIAC (Triode for AC) is the widely used in power control and switching applications like switching, phase control, chopper designs, brilliance control in lamps, speed control in fans, motors etc. Hence it is an AC control device. TRIAC is an abbreviation for a TRIode for Alternating Current switch . The TRIAC is equivalent to two back to back SCRs connected with one gate terminal as shown in figure.

17 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

V-I Characteristics of TRIAC

Figure shows the V -I characteristics of TRIAC Because the TRIAC essentially consists of two SCRs of opposite orientation fabricated in the same crystal, its operating characteristics in the first and third quadrants are the same except for the direction of applied voltage and current flow. The following points may be noted from the TRIAC characteristics: 1. The V-I characteristics for TRIAC in the 1st and 3 rd quadrants are essentially identical to those of an SCR in the 1st quadrant. 2. The TRIAC can be operated with either positive or negative gate control voltage but i n normal operation usually the gate voltage is positive in 1st quadrant and negative in 3rd quadrant. 3. The supply voltage at which the TRIAC is turned ON depends upon the Gate current. The greater the gate current, the smaller the supply voltage at which t he TRIAC is turned ON . This permits to use a TRIAC to control AC Power in a load from zero to full Power in a smooth and continuous manner with no loss in the controlling device. 18 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

DIAC: DIAC: A DIAC is a bi-directional semiconductor device which has three layers, two junctions and Two Terminals MT 1 & MT 2. The DIAC is a combination of two diodes connected in anti parallel, one in forward bias and the other one is in reverse bias condition with respect to both sides. DIAC is a diode that conducts electrical current only after its break-over voltage

(V BO ). DIAC stands for “Diode for Alternating Current”. DIACs are used in the triggering of SCR and TRIAC.

DIAC V-I Characteristics For the positive half cycle, the characteristic obtains in the first quadrant. For the negative half cycle, the characteristic obtains in the third quadrant. The V-I characteristic is as shown in the below figure.

19 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

i) When the terminal MT 1 is made positive with respect to MT 2 and Applied Voltage less than the break over voltage, only leakage current flows through the device. ii) This leakage current is due to the drift of electrons and holes at the depletion region which are not sufficient to drive the device into conduction state. iii) The region OA represents blocking or non conducting state as shown in the Figure. iv) When the voltage across its terminal reaches to break over voltage, the device starts to conduct. As the current through the device increases, the voltage drop across device decreases. v) The device exhibits negative resistance characteristic and region AB is known as conducting state.

vi) When the MT 2 is made Positive with respect to MT 1, the device has similar characteristics as that of in the first quadrant. This is due to fact that the doping level is same at the two junctions of the device. vii) Once the device starts conducting, the current increases sharply which is limited by external characteristics. Applications i. DIACs are uses as a triggering device for TRIAC / SCR. It is also use in below control schemes. ii. It is used Lamp dimmer. iii. It is used Fan speed regulator. iv. It is used Temperature controller. Disadvantages i. It is Low power device. ii. It has no control terminal. Uni-junction Transistor (UJT)

Uni-junction Transistor: A Unijunction Transistor (UJT) is a three-terminals ( an emitter

(E) and two bases (B 1 and B 2) ) with only one junction electronic semiconductor device. It is a three terminal semiconductor switching device which is consists of a bar of n- type silicon material with a non-rectifying contact at either end (base 1 and base 2), and with a rectifying contact (emitter) alloyed into the bar part way along its length, to form the only junction within the device (hence the name ‘Unijunction’). The Unijunction Transistor is also known as Double Base Diode. 20 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

The UJT is a three-terminal semiconductor device having 2 layers which exhibits negative resistance and switching characteristics for use as a relaxation oscillator in phase control applications. It is called so because of the presence of only one junction. It has the ability to limit large power with a small input signal and is also known as a double base diode .. Characteristics of Uni-junction transistor ( UJT )

The figure below shows the characteristic of UJT, it is a graph between VE and I E.

The emitter voltage that lies in the left direction towards the peak point. The region is known as the cut-off region . When emitter potential becomes equal to peak voltage. After this emitter potential reduces on, any further increase in I E, that simply shows a reduction in R B1 . This is the reason why the device is said to possess negative resistance characteristic and the region is called negative resistance region . The negative resistance characteristic of a Unijunction transistor is the basis for its operation and due to this, the device can be used as an oscillator. After this, a valley point is reached, where the device comes to saturation region with any additional increase in the emitter current of the device. 21 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

Advantages of Uni-junction Transistor (UJT) 1. It is a low-cost device. 2. UJT is a device with a high pulse current capability. 3. It possesses negative resistance characteristic 4. It is a device that absorbs less power during operation. Applications of Uni-junction Transistor (UJT) 1. The unijunction transistor (UJT) used as relaxation oscillator. 2. It is used as voltage detector. 3. The UJT is used for switching. 4. It is widely used as triggering device for silicon control rectifier (SCR) and TRIAC. 5. The UJT is used in phase control circuit. 6. The UJT is used in timing circuit. UJT as Relaxation Oscillator . An oscillator is a device that produces a Non-Sinusoidal waveform by its own, without any input, when DC voltage is applied to it. The circuit diagram of a UJT relaxation oscillator is as shown below, in which R1 & R2 are current limiting resistors and Resistor R & capacitor C determines the frequency of the oscillator .

When the Power Supply is Switched ON, the capacitor usually starts charging and

continues to charge until the maximum voltage V cc . But in this circuit, when the voltage across capacitor reaches a value, which enables the UJT to turn ON (the peak voltage) then the capacitor stops to charge and starts discharging through UJT. Now, this discharging continues until the minimum voltage which turns the UJT OFF (the valley voltage). This process continues and the voltage across the capacitor, when indicated on a graph, the following waveform is observed. 22 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

So, the charge and discharge of capacitor produces the sweep waveform as shown above. The charging time produces increasing sweep and the discharging time produces decreasing sweep. The repetition of this cycle, forms a continuous sweep output waveform. As the output is a non-sinusoidal waveform, this circuit is said to be working as a relaxation oscillator. Programmable UJT (PUT): Programmable UJT (PUT): Programmable Uni-Junction Transistor ( PUT ) is a close relative of the thyristor family. Its has a four layered construction just like the thyristors and have three terminals named anode (A), cathode (K) and gate (G) again like the thyristors. and parameters have much similarity to that of the unijunction transistor. It is called programmable because the parameters like intrinsic standoff ratio (η), peak voltage (Vp) etc can be programmed with the help of two external resistors. In a UJT, the parameters like Vp, η etc are fixed and we cannot change it.

The Schematic diagram and symbol of PUT are shown below.

Programmable Unijunction Transistor (PUT), offering many advantages over conventional Unijunction transistors. The designer can select R 1 and R 2 to program Unijunction characteristics such as intrinsic standoff ratio, interbase resistance, and valley point current to meet particular needs .

Applications of programmable UJT (i) It is used as relaxation oscillators. (ii) It is used for thyristor firing. (iii) It is used as pulse circuits. (iv) It is used as timing circuits. 23 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

OTHER THYRISTORS

GTO Thyristors : A Gate Turn off Thyristor or GTO is a three terminal, bipolar (current controlled minority carrier) semiconductor switching device. Similar to SCR / Thyristor, It has also the same terminals are anode, cathode and gate as shown in figure below. As the name indicates, it has gate turn off capability by their third lead, the Gate lead.

Metal Oxide Semiconductor Field Effect Transistor : The MOSFET is a four terminal device with source(S), gate (G), drain (D) and body (B) terminals. The body of the MOSFET is frequently connected to the source terminal so making it a three terminal device like field effect transistor .

The aim of the MOSFET is to be able to control the voltage and current flow between the source and drain. It works almost as a switch.

Silicon Unilateral Switch : Silicon Unilateral Switch ( SUS ) is a device of the thyristor family and is used as a trigger element in circuits using SCRs. Symbol

24 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

Silicon controlled switch : Silicon controlled switch (SCS), like the SCR, is a unilateral, four layer three junction P-N-P-N silicon device with four electrodes namely Cathode K,

Cathode Gate G 1, Anode Gate G 2 and the Anode A, as shown in figure. In-fact, the SCS is a low power device compared with the SCR.

SCS has one extra Gate Terminal which allows more control to be exerted over the device, particularly in the mode of forced commutation , where the device has not yet fallen below the holding current value.

LASCR or light activated SCR : LASCR or light activated SCR is a semiconductor device which turns ON when it is exposed to light. The LASCR is a type of thyristor which is triggered by photons present in the light rays.

It is a three terminal device, consists of Cathode , Anode and Gate terminals. The gate terminal is used when the electrical triggering is supplied to the LASCR . The advantage of using triggering of the Thyristor by light is prevention from electrical noise disturbances. Thus, LASCR is considered to be one of the best devices. 25 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

Heat Sink Heat Sink : A heat sink is a thermal conductive metal device that absorb and disperse heat away from a high temperature object such as a computer processor . A heat sink is a heat exchanger that transfers and are made up of metal s, such as a copper or aluminum alloy, and are attached to a device releasing energy in the form of heat, with the aim of dissipating that heat to a surrounding fluid in order to prevent the device overheating .

This tool is designed to calcula te junction temperature of an electronic device (power devices) given four parameters: the maximum ambient temperature, the device's junction - to-package thermal resistance, the thermal resistance of the heat sink, and the power applied. It can also calcul ate the maximum power that the device can handle given its junction temperature, the maximum ambient temperature, the junction -to-package thermal resistance, and the heat sink's thermal resistance. Equations

TJ = P (R case + R 1 + R 2 ) + Ta

TJ = junction temperature P = Power dissipated

Rcase = thermal resistance of device junction to case

R1 = thermal resistance of device junction to air (if no heat sink) or thermal resistance of heat sink

R2 = thermal resistance of device junction to air

Ta = Ambient Temperature 26 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

Heat Sink Types: The heat sinks are classified into two types, v. Active heat sinks vi. Passive heat sinks. Active Heat Sinks : These are generally fan type and utilize power for cooling purpose. They can also be termed as Heat sink or fans. Sometimes these types of heat sinks are referred to as an HSF , which is short for heat sink and fan . Passive heat sinks: Passive heat sinks are those that have no mechanical components. Passive heat sinks are made of an aluminum finned radiator that dissipates heat through convection Heat Sink Types The most common types of air-cooled heat sinks include: 1. Stampings : Stamping (also known as pressing) is the process of placing flat sheet metal in either blank or coil form into a stamping press where a tool and die surface forms the metal into a net shape. Stamping includes a variety of sheet-metal forming manufacturing processes, such as punching using a machine press or stamping press, blanking, embossing, bending, flanging, and coining. Copper or aluminum sheet metals are stamped into desired shapes. they are used in traditional air cooling of electronic components and offer a low cost solution to low density thermal problems. They are suitable for high volume production, because advanced tooling with high speed stamping would lower costs.

27 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

2. Extrusion Heat Sink : Extruded aluminum is one of the most popular and cost- efficient heat sink fabrication methods An Extruded Heat sink uses the Extrusion manufacturing process where material is pushed through a die of the desired cross-section. Metal extrusion is a metal forming process in which a object / work piece of a certain length and cross section, is forced to flow through a die of a smaller cross sectional area, thus forming the work to the new cross section. Extrusion is a process used to create objects of a fixed cross-sectional profile. A material is pushed through a die of the desired cross-section. Extrusion may be continuous (theoretically producing indefinitely long material) or semi- continuous (producing many pieces).

3. Bonded/Fabricated Fins Heat Sink : Bonded fin heat sinks are an assembly of a grooved base with individual fins bonded into the grooves . This type of heat sink fabrication allows for high fin densities and fin aspect ratios Most air cooled heat sinks are convection limited, and the overall thermal performance of an air cooled heat sink can often be improved significantly if more surface area can be exposed to the air stream. These high performance heat sinks utilize thermally conductive aluminum-filled epoxy to bond planar fins onto a grooved extrusion base plate.

28 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

4. Castings Heat Sink : A die cast heat sink uses the cast process by forcing molten metal under high pressure into a molded cavity. The molded cavity of the die cast heat sink is created using a hardened tool steel die which is carefully machined into a pre- designated shape.

5. Folded Fins Heat Sink : Folded fin heat sinks are manufactured by bending aluminum or copper sheets into a variety of fin shapes which are then bonded to a separate base. This type of folded fin heat sink allows the base and the folded fins to be manufactured from different metals.

Most material that can be formed into thin sheets and easily folded can be used as a folded fin material. In thermal management, the most popular materials are copper and aluminum, since those have the highest thermal conductivities.

29 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

Applications Of Thysristors

Circuit Diagram of Battery Charger Using SCR :

The battery charger consists of a full -wave rectifier (diodes D1 and D 2). The output voltage of the rectifier is pulsating and is applied directly to the battery to be charged through the thyristor (SCR 1).

When the battery starts charging, the thyristor (SCR 2) is in the cut -off state (the thyristor is like an open circuit). This means that the SCR 1 thyristor gate has enough voltage to trigger the thyristor and to deliver current to the battery. The current necessary to trigger the SCR comes through the resistor R 1. When the battery charge starts (the battery is discharged) the voltage at the potentiometer slider is l ow. This voltage is too small to “reverse bias” the 11 volt zener diode. Thus the Zener diode is like an open circuit and SCR 2 is kept in the cut state. As the battery charges, the battery voltage increases, as the voltage at the potentiometer slider. Eventually this voltage is enough to “reverse bias” the zener diode and it triggers the SCR 2 thyristor which now is like a short circuit.

When the SCR 2 thyristor conducts a voltage division is created with R 1 and

R3 resistors. This voltage division causes the voltage at the anode of diode D 3 to be very small and SCR 1 thyristor does not conducts, stopping the current flow into the battery. When this occurs the battery is fully charged. If the battery is discharged again, the process starts automatically.

Capacitor C is used, to prevent the thyristor SCR 2 from triggering f or undesired reasons . 30 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

Fan Regulator Circuit The fan regulator circuit is used to control the speed of the fan according to our desire. The objective of the circuit is to vary the Fan Speed by using a fixed source. A simple TRIAC can do the work for you. are used in this circuit as a Fan Regulator as it is easy to design and control, and are very much economic due to its high efficiency and low buying costs.

Circuit Detail : This is the circuit diagram of the simplest fan regulator. The circuit is based on the principle of power control using a TRIAC. The circuit works by varying the firing angle of the

TRIAC . Resistors R 1, R 2 ( Variable ) and capacitor C 1 are associated with this. The firing angle can be varied by varying the value of any of these components. Here R 2 is selected as the variable element. By varying the value of R 2 the firing angle of TRIAC changes (in simple words, how much time should TRIAC conduct) changes. This directly varies the load power, since load is driven by TRIAC. The firing pulses are given to the gate of TRIAC using DIAC.

31 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

Operation of Regulator Circuit

1. Before giving the power supply to this simple fan regulator circuit, keep the variable

resistor R 2 or potentiometer in maximum resistance position so that no triggering is applied to TRIAC and hence the TRIAC will be in cutoff mode. 2. Turn ON the power supply of the circuit and observe whether the fan is in standstill

condition or not. Vary the potentiometer position R2 slowly so that the capacitor starts

charging at the time constant determined by the values of R 1 and R 2. 3. Once the voltage across the capacitor is more than the break over voltage of the DIAC, DIAC starts conducting. Thus, the capacitor starts discharging towards the gate terminal of TRIAC through DIAC. 4. Therefore, TRIAC starts conducting and hence the main current starts flowing into the fan through the closed path formed by TRIAC.

5. By varying the potentiometer R 2, the rate at which capacitor is going to be charged get varied this means that if the resistance is less, the capacitor will charge at a faster rate so the earlier will be the conduction of TRIAC.

6. As the potentiometer resistance gradually increases, the conduction angle of TRIAC will be reduced. Hence the average power across the load will be varied. 7. Due to the bidirectional control capability of both TRIAC and DIAC, it is possible to control the firing angle of the TRIAC in both positive and negative peaks of the input. Advantages of Fan Regulator Circuit 1. Continuous and step less control of the fan speed is possible 2. Power saving is achieved at all the speeds by minimizing the energy losses 3. Simple circuit which requires less number of components 4. Efficient as compared with resistive type due to lower power consumption

5. Low Cost.

32 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

Light Intensity Control Circuit The Light Intensity control circuit is used to control the light brightness of bulb according to our desire. The objective of the circuit is to vary in intensity the brightness of the light bulb by using a fixed source. There is no need to replace the bulb with a higher wattage one to do so. A simple TRIAC can do the work for you. TRIACs are used in this circuit as a dimmer as they are easy to design and control, and are very much economic due to their high efficiency and low buying costs. Circuit Detail: This is the circuit diagram of the simplest lamp dimmer. The circuit is based on the principle of power control using a TRIAC. The circuit works by varying the firing angle of the

TRIAC . Resistors R 1, R 2 and capacitor C 2 are associated with this. The firing angle can be varied by varying the value of any of these components. Here R 2 is selected as the variable element. By varying the value of R 2 the firing angle of TRIAC changes (in simple words, how much time should TRIAC conduct) changes. This directly varies the load power, since load is driven by TRIAC. The firing pulses are given to the gate of TRIAC using DIAC.

Operation of Light Intensity Control Circuit

1. Before giving the power supply to this simple Light Intensity Control circuit, keep the

variable resistor R 2 or potentiometer in maximum resistance position so that no triggering is applied to TRIAC and hence the TRIAC will be in cutoff mode. 2. Turn ON the power supply of the circuit and observe whether the fan is in standstill

condition or not. Vary the potentiometer position R2 slowly so that the capacitor starts

charging at the time constant determined by the values of R 1 and R 2. 33 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

3. Once the voltage across the capacitor is more than the break over voltage of the DIAC, DIAC starts conducting. Thus, the capacitor starts discharging towards the gate terminal of TRIAC through DIAC. 4. Therefore, TRIAC starts conducting and hence the main current starts flowing into the Load / Bulb through the closed path formed by TRIAC.

5. By varying the potentiometer R 2, the rate at which capacitor is going to be charged get varied this means that if the resistance is less, the capacitor will charge at a faster rate so the earlier will be the conduction of TRIAC. 6. As the potentiometer resistance gradually increases, the conduction angle of TRIAC will be reduced. Hence the average power across the load / Bulb will be varied. 7. Due to the bidirectional control capability of both TRIAC and DIAC, it is possible to control the firing angle of the TRIAC in both positive and negative peaks of the input.

Advantages of Light Intensity Control Circuit

1. Continuous and step less control of the fan speed is possible 2. Power saving is achieved at all the speeds by minimizing the energy losses 3. Fan Regulator is Simple circuit which requires less number of components 4. Efficient as compared with resistive type due to lower power consumption 5. Fan Regulator is of Low Cost.

Universal Motor :

The universal motor is a type of electric motor that can operate on either AC or DC power and uses an electromagnet as its stator to create its magnetic field. It is a commutated series-wound motor where the stator's field coils are connected in series with the rotor windings through a commutator .

34 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

Speed Control of Universal Motor

The speed control of universal motor is shown in the figure A. The single phase bridge rectifier converts alternating voltage into direct voltage. The zener diode provides constant voltage to RC circuit. The charging of capacitor is done through variable resistor R. When the voltage across capacitor becomes equal to DIAC break over voltage, the DIAC turns on. The gate terminal of SCR receives gate pulse and therefore it turns on. As soon as the SCR turns on, current passes through universal motor. The discharging of capacitor is done through DIAC and gate cathode circuit. The speed control of DC motor is done through variable resistor R. If the variable resistor R sets at higher value, the charging rate of capacitor decreases and firing angle increases. This will result in speed of universal motor decreases. There are others several method to control speed of universal motor.

35 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

FILL IN THE BLANKS: 1. SCR Combines the features of transistor and ………………… both. 2. Number of PN junctions in a thyristor are ………….. 3. SCR, DIAC & TRIAC are the devices that belongs to……….….….family. 4. Process of turning ON of SCR is called its …………or……….. 5. Process of Turning OFF of a SCR is called………….. 6. UJT stands for…………… 7. UJT is used to generate……………..for triggering of SCRs 8. The device which is used to generate pulses for triggering SCRs but is not a member of Thristor family is………… 9. TRIAC is …………..device. 10. Controlling the instant of firing an SCR is called ……………………. 11. DIAC is a ………………….device. 12. DIAC is used for triggering of………………….. 13. Forward voltage at which a device is turned ON is called………….. 14. PUT is a……………….. device like SCR. 15. GTO is a Gate Turn off……………….. 16. IGBT stands for …………………….. 17. SCS is a silicon …………….switch. 18. SUS is a silicon…………..switch. 19. PUT stands for …………………… 20. Snubber circuit is used to overcome the effects of ………………….

Answers 1) Rectifier. 2) Three. 3) Thyristor. 4) Triggering, Firing 5) Commutation 6) Uni-junction Transistor 7) Pulses 8) UJT 9) Bidirectional 10) Phase control 11) Bidirectional 12) TRIACs/ SCRs

13) Forward break over voltage 14) PNPN 15) Thyristor

16) Insulated Gate Bipolar Transistor 17) Controlled 18) Unilateral

19) Programmable Uni-Junction Transistor 20) Over Current

36 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

FILL IN THE BLANKS: 21. While SCR conducting ,the resistance of SCR is ………………………….. 22. LASCR stands for ……………….…….. . 23. ……………… technique is very convenient method of ac motor speed control. 24. MOSFET stand for…………………… 25. The response of transistor is ………………… in comparison to thyristor. 26. An SCR is a ……………………..layer …………………. junction device. 27. Size and shape of a thyristor as well as its heat sink is designed as per its ………….. 28. The method of switching ON a thyristor is known as its ………………….. 29. Speed of a motor ……………………. With increase in firing angle. 30. Holding current is …………………… latching current. 31. Thyristor is turned ON by increasing the …………………… current. 32. ……………… is the minimum anode current to maintain thyristor in ON state. 33. An SCR can be considered to be prepared by sandwiching………………….. 34. The most commonly used method for firing an SCR is …………………….. 35. SCRs are connected in series to enhance their …………………………rating. 36. …………………… is the process of turning off a thyristor. 37. GTO can be turned OFF by a ……………………. Gate singal. 38. 18. An SCR has …………………… terminals. 39. 19. A DIAC is used to …………………. TRIAC. 40. SCRs are connected in parallel to enhance……………….capability.

Answers

21) Very Low 22) Light Activated Silicon Controlled Rectifier

23) Thyristorised Control 24) Metal Oxide Semiconductor Field Effect Transistor 25) Fast 26) Four, Three 27) Rating 28) Triggering 29) Decreases 30) Less than 31) Gate Current 32) Latching Current 33) PNPN layer 34) Gate Triggering 35) Voltage 36) Commutation 37) Negative 38) Three 39) Trigger 40) Current

37 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

FILL IN THE BLANKS: 41. A SCR is an ………………….carrier semi controlled device. 42. A SCR can conduct current in ………. Direction and block voltage in ………direction. 43. A SCR can be turned ON by applying a forward voltage greater than forward ………………….. voltage or by injection a positive ………………… current pulse under forward bias condition. 44. To turn OFF a SCR, the anode current must be brought below ……………….. current and a reverse voltage must be applied for a time larger than ……………………. Time of the device. 45. An SCR may turn ON due to large forward ……………….. 46. Forward breakover voltage of an SCR decrease with increase in the ………………….. current. 47. Reverse ……………… Voltage of a SCR is ……………… of the gate current. 48. Reverse saturation current of an SCR …………………. With gate current. 49. In the pulsed gate current of an SCR, the gate current pulse width should be larger than the ……………..time of device. 50. To prevent unwanted turn ON of an SCR all spurious noise signals between the gate and the cathode must be less than the gate ……………….. voltage. 51. The gate non-trigger voltage specification of an SCR useful for avoiding unwanted turn ON of the SCR due to …………….. voltage signals at the gate. 52. An SCR is turned ON by applying a ……………….. gate current pulse when it is ………………………. Biased. 53. A SCR can be turned OFF by bringing its anode current below …………………. Current and applying a reverse voltage across the device for duration larger than the ………………….. time of the device. 54. TRIAC is a …………………. Minority carrier device. 55. A TRIAC behaves like two ………………….. connected SCRs.

Answers: 41) Minority 42) Forward , Reverse 43) Break Over, Gate 44) Holding, Turn-Off 45) dv/dt 46) Gate

47) Break Down, Independent 48) Increase 49) Turn ON 50) Non-Trigger 51) Noise 52) Positive, Forward 53) Holding, Turn-off 54) Bi-directional 55) Anti-Parallel

38 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

FILL IN THE BLANKS: 56. A TRIAC operates either in the ……………………. Or the ……………….. quadrant of the VI characteristics. 57. The maximum possible voltage and current rating of a TRIAC is considerably ………………. as Compared to SCR . 58. A MOSFET is a …………………. Controlled ………………….. carrier device.

59. A MOSFET operates in the …………………….. mode when VGS < V GS(th). 60. A GTO is a ……………….. controlled …………………… carrier device. 61. A GTO has ………………… layers and …………………… terminals. 62. A GTO can be turned ON by injecting a ………………… gate current and turned OFF by injecting a …………………. Gate current. 63. After a GTO turns ON, the gate current can be ……………………… 64. A conducting GTO reverts back to the blocking mode when the anode current falls below ………………… current. 65. To turn OFF a conducting GTO, the gate terminal is biased ………………. With respect to the …………………… 66. A SCR operates either in the ……………………. Or the ……………….. quadrant of the VI characteristics. 67. A DIAC behaves like two ………………….. connected Diodes. 68. SCRs are connected in parallel to enhance their …………………………rating. 69. Commutation is the process of …………….. a thyristor. 70. Forward breakover voltage of an SCR increase with decrease in the ………………….. current. 71. DIAC is …………..device. 72. Speed of a motor increase with ……………. in firing angle. 73. Latching current is …………………… Holding current. 74. TRIAC is a ……………………..layer …………………. terminals device. 75. …………………. is a Bi-directional Minority carrier device.

Answers: 56) First, Third 57) Lower 58) Voltage, Majority 59) Cut-off 60) Current Minority 61) Four, Three 62) Positive, Negative 63) Removed 64) Holding 65) Negative, Cathode 66) First, Third 67) Anti-Parallel 68) Current 69) Turn-off 70) Gate 71) Bi-directional 72) Decrease 73) Greater than 74) Four, Three 75) TRIAC

39 POWER ELECTRONICS UNIT - I INTRODUCTION To THYRISTORS AND OTHER POWER ELECTRONICS

State TRUE / FALSE of the Followings: 1. UJT has a negative resistance region. 2. An SCR is a three layer device. 3. An SCR is a three terminal, four layer device. 4. When Triggering pulse is removed , an SCR is turned off. 5. Holding current is more than the latching current. 6. SCR is a bidirectional device. 7. SUS triggers one fixed Anode to cathode voltage. 8. MOSFET is a current controlled device. 9. PUT is a PNPN device with gate near to the Anode. 10. Latching current is more than holding current. 11. UJT is not member of Thyristor family. 12. A UTT after reaching the valley point goes to its saturation state. 13. An SCR is a bi-directional device. 14. UJT relaxation oscillator is used for triggering an SCR. 15. A GTO has advantage of reduction in noise. 16. A DIAC is a 3 terminals device. 17. In an emergency light, the lamp glows only when AC power is not available. 18. A TRIAC is a bi-directional device. 19. The thermal resistance is the resistance that the heat energy meets in its flow from cold to hot. 20. DRIAC is a bidirectional device.

Answers: 1) TRUE 2) FALSE 3) TRUE 4) FALSE 5) FALSE 6) FALSE 7) TRUE 8) FALSE 9) TRUE 10) TRUE 11) TRUE 12) TRUE 13) FALSE 14) TRUE 15) TRUE 16) FALSE 17) TRUE 18) TRUE 19) FALSE 20) TRUE