scr,traic,diac report

Introduction to SCR, DIAC, TRIAC

Introduction to Thyristor:

In Our study, We learn the diode, transistor,capacitor etc Electronics devices which will help in the field of electronics. Now, to understand the Scr ,diac,Triac first we have to know the term Thyristor. In general, Thyristor is a semiconductor device having three or more junctions. A semiconductor component that is unidirectional like a diode, but is switched like a transistor. It is widely used to control high currents and voltages in motors, lighting, heating and power applications. Once turned on at the gate.

These is the physical difference between the diode,thyristor,and transistor. Diode will made by diffusing the n-type and p-type material. While transistor will made by diffusing two back to back diode. And thyristor will made by diffusing like one transistor and one diode. So, it serves the purpose of diode and transistor also.


SCR(Silicon controlled Rectifier):

As its name suggests its main function is to convert ac voltage to dc voltage when gate pulse is applied. The Silicon Controlled Rectifier (one type of thyristor) is a four layer device, PNPN from anode (A) to cathode (here labeled "C", sometimes also labeled "K"). An SCR is like a junction transistor with a fourth layer and therefore a total of three P-N junctions. Meanwhile, a third terminal, the gate (G), makes an SCR function like an odd hybrid of transistor and diode.

Symbol:

History of SCR:

SCR will be invented in 1957,an SCR can be used as a controlled switch to perform various functions such as rectification, inversion, and regulation of power flow.

SCR was first applied in Japan in the mid-1970s and later introduced in Europe during the 1980s. At an early stage, ALSTOM followed the development of the SCR technology in Japan and entered into cooperation with Hitachi Zosen and Electric Power Development Company (EPDC). Later agreements with Babcock


Hitachi KK, one of the largest SCR suppliers in the world, enabled us to gain access to the SCR technology at an early stage.

From the 1980s, ALSTOM Power built and operated several pilot plants to develop the SCR technology for European conditions and gain knowledge from different processes to develop full competence as an SCR process supplier. Another major contribution from ALSTOM to the further development of the SCR process is expert knowledge in fluid dynamics, creating optimum conditions by new proprietary mixing systems for completeness of the chemical reactions, as well as minimum ducting and attractive plant layouts for different applications.

To benefit from the development of catalysts with improved performance possibilities, ALSTOM has continued to test different catalysts in pilot and full scale applications for both old and new processes, such as the combustion of biofuel and waste. This makes it possible to combine our process experience in a multitude of gas cleaning processes with the requirements for the best SCR performance.

ALSTOM's broad experience from commercial reference plants in different fields of SCR applications ensures that we can meet our customers’ high expectations for performance, reliability and operating economy.

Features of SCR :

It is latching type device It can handle a very large power. It is a current control device, because the gate current controls the SCR. It acts as a open or closed switch. The on state voltage drop is very low. It can handle thousand of ampere of current.


Construction :

The basic structure of a SCR is as shown in fig. It is a four layer p-n-p-n device, with three terminal brought out for the users, they are Anode, cathode, and Gate. The gate terminal is the control terminal that can turn on the device whenever required.

Due to the four layer structure there are three junctions j1, j2, j3. It is essential terminal is added to it. The direction of forward anode current and voltage across the SCR are shown in the figure.

The direction of the anode and gate current in the fig. are the conventional current directions. It is clearly indicates that SCR is a unidirectional device and also that gate current can only be positive.

Now, It is clear that it is essentially that an ordinary rectifier and a junction transistor combined in one unit to form pnpn device.


V-I characteristic :

Forward characteristic:

When anode is positive w.r.t cathode , the curve between V and I is called the forward characteristic. If the supply voltage is increased from zero, a point is reached when the SCR starts conducting. Under this condition the voltage across SCR suddenly drops as shown by dotted lines and most of the supply voltage appears across the load resistance RL.

The two outer junctions are forward biased by the voltage , but the inner junction is reverse biased. As a result, a small current in the gate electrode can turn on a current between anode and cathode. SCRs are latching devices; once an SCR is turned on, or "fired," it remains on (i.e., the current will continue to flow) until the driving voltage between anode and cathode is removed. The minimum anode - cathode current required to keep the SCR on (once triggered) is called the holding current, IH.

Reverse characteristic :

When anode is negative w.r.t cathode, the curve between V and I is known as reverse characteristic. The reverse voltage does come across SCR when it is operated with a.c. supply. If the reverse voltage is gradually increased , at first the anode current remains small and at some reverse voltage avalanche breakdown occurs and the SCR starts conducting heavily in the reverse directions. This maximum reverse voltage at which SCR starts conducting heavily is known as reverse breakdown voltage.

An SCR is called a rectifier because (much like a "vanilla" diode) it conducts current in only direction. It is a unidirectional thyristor, in contrast to a triac. You can think of an SCR as being a conventional (diode) rectifier, with a gate controlling forward resistance. Additionally, an SCR can be switched on (in the


absense of a gate voltage) by applying enough forward voltage to overcome its internal resistance. This is normally considered a design limitation, though, and switching is normally controlled via gate voltage.

Knowing all this, an SCR's characteristic V - I curve then looks like this:

.


Testing of SCR :

A rudimentary test of SCR function, or at least terminal identification, may be performed with an ohmmeter. Because the internal connection between gate and cathode is a single PN junction, a meter should indicate continuity between these terminals with the red test lead on the gate and the black test lead on the cathode like this

All other continuity measurements performed on an SCR will show "open" ("OL" on some digital multimeter displays). It must be understood that this test is very crude and does not constitute a comprehensive assessment of the SCR. It is possible for an SCR to give good ohmmeter indications and still be defective. Ultimately, the only way to test an SCR is to subject it to a load current.

:


If you are using a multimeter with a "diode check" function, the gate-to-cathode junction voltage indication you get may or may not correspond to what's expected of a silicon PN junction (approximately 0.7 volts). In some cases, you will read a much lower junction voltage: mere hundredths of a volt. This is due to an internal resistor connected between the gate and cathode incorporated within some SCRs. This resistor is added to make the SCR less susceptible to false triggering by spurious voltage spikes, from circuit "noise" or from static electric discharge. In other words, having a resistor connected across the gate-cathode junction requires that a strong triggering signal (substantial current) be applied to latch the SCR. This feature is often found in larger SCRs, not on small SCRs. Bear in mind that an SCR with an internal resistor connected between gate and cathode will indicate continuity in both directions between those two terminals.

Application:

DC motor speed control AC motor speed control Inverter Battery charger Battery operated vehicles High voltage dc transmission Uninterrupted power supplies Switched mode power supplies Relay controls Static switches


DIAC :

Diac is a device used for triggering. It is popularly used as the triggering of the triac. The DIAC, or diode for alternating current, is a bidirectional trigger diode that conducts current only after its breakdown voltage has been exceeded momentarily. When this occurs, the resistance of the diode abruptly decreases, leading to a sharp decrease in the voltage drop across the diode and, usually, a sharp increase in current flow through the diode. The diode remains "in conduction" until the current flow through it drops below a value characteristic for the device, called the holding current. Below this value, the diode switches back to its high-resistance (non-conducting) state. When used in AC applications this automatically happens when the current reverses polarity.

DIACs are also called symmetrical trigger diodes due to the symmetry of their characteristic curve. Because DIACs are bidirectional devices, their terminals are not labeled as anode or cathode but as A1 and A2 or MT1 ("Main Terminal") and MT2.

DIACs are a form of thyristor but without a gate electrode. They are typically used for triggering both thyristors and TRIACs - a bidirectional member of the thyristor family. Because of this common usage, many TRIACs contain a built-in DIAC in series with the TRIAC's "gate" terminal.

Symbol :


Construction :

.

The DIAC is basically a two terminal device. It is a parallel inverse combination of the semiconductor layers that permits triggering in either direction. The diac is used extensively triggering device for the triac.

The basic structure of the diac are shown in the figure. The two terminals are MT1 & MT2 are called the main terminal.

When MT1 is made positive with respect to the MT2, the conduction will takes place to the structure p-n-p-n Which is a four layer diode. When MT2 is made positive with respect to the MT1, the conduction will take place to the structure p-n-p-n. which is another four layer diode.

The diac can be conduct for both the direction. And the another symbol for the diac will be the looks like as the transistor which is shown below:

.



The characteristic of the diac are shown in the figure.


The diac resembles a bipolar transistor with no base connection. The device can be turn on by either positive or negative half cycle of the ac voltage. For positive half cycle of the ac voltage , if the applied voltage is less than the forward break over a very small current called the leakage current flows through the device. This current is produced due to the drift of the electrons and the holes at the depletion region and is not sufficient to cause condition. Hence the diac remains in the practically in the non conducting mode . It is called the blocking state. As soon as the applied voltage reaches the break over voltage, the device starts the conducting. The current through the device starts increasing and the voltage across it starts decreasing. This region is called the “conduction State”. The break over voltage for the diac remains unchanged as the gate terminal is absent here.

The forward characteristic of the diac is very similar to the four layer diode. The forward characteristic appears in the first quadrant.

The reverse characteristic appears in the third quadrant. It is similar to the forward characteristic.

The break over voltage of the diac ranges between 28 volts to 42 volts. The typical turn on time ranges from 50 to 500nsec. The turn off is much longer upto 100nsec. Diacs have a power handling capability between 300mW to 1W.

Typical Diac voltage and current relationships. Once the voltage exceeds the turn-on threshold, the device turns on and the voltage rapidly falls while the current increases.

In this way, their behavior is somewhat similar to (but much more precisely controlled and taking place at lower voltages than) a neon lamp.


Application :

Triggering device for triac phase control circuit Light dimmer Motor speed control Heater control


Triac :

In the thyristor family , after SCR triac is the most widely used device for power control. Triac with the large current and voltage ratings are available in the market.

Symbol :

Construction :


Triac is the three terminal device, with the terminal named as the MT1, MT2 and gate. Here the gate terminal is the control terminal.

Triac is the bi directional device it means that current can flow in both the direction through it. The basic structure of triac are shown in the figure. Its structure is like that we can say that two scrs are connected in the anti-parallel. So traic will acts as the switch for both the directions.

Here the gate terminal is the near the MT1 when the gate is open,the triac will block both the polarities of the voltage across the MT1 and MT2 if the magnitude of the voltage is less than the break over voltage of the device. It says that triac will remaining in the off state at that time.

The another structure of the triac will two scr will connected in anti parallel. And it is shown in the figure below:



The only difference between scr and triac is that SCR conducts only in the forward direction where as the triac conduct in both the directions. The other difference in the operation in the triggering mechnisum. The triac can be turned on by applying either a positive or negative voltage the gate with respect to terminal MT1. Whereas the SCR can be triggered only by a positive gate signed.

The characteristic of the triac is same as that of two back to back connected SCRs. The effect of the gate current is also same it means with increase in the gate current the breakdown voltage decrease. In the gate current can be positive or negative whereas in SCR the gate current can be only positive.

As can be seen from the fig it is verymuvh similar to the characteristic of the SCR. It has been three region of the operation.


Blocking or off state Transition or unstable state Conduction or on state

Application :

Illumination control Temperature control Liquid level control Motor speed control Static switch to turn ac power on and off

Comparison :

SCR TRIAC


Scr is the uni directional device Triac is the bi directional deviceThe Gate current can be only positive The Gate current can be positive or

negativeScr can be operated in only one quadrant of V-I characteristic

Triac can be operated in two quadrant of V-I charactiristics.

Anti parallel SCRs are preferred over triac for power control of inductive loads. They can be successfully used for the resistive loads as well.

Triac is not suitable for power control of inductive loads but suitable for the resistive loads.

UJT is used for triggering the SCR DIAC is used for triggering the Triac

Symbol : Symbol :

DIAC TRIACIt is two terminal device It is three terminal deviceBreak over voltage can not be controlled

Break over voltage can be controlled by adjusting the gate current

Diac is a low power device Triac is the high power deviceIt is used as the triggering device fro the triac

It is used in the application like fan control light dimmer etc.

Symbol : Symbol :


Conclusion :

After the all the discussion we will conclude that SCR ,TRIAC, DIAC are useful for the power electronics application for controlling purpose.

References :

Electronics Principle by sanjay Sharma Industrial electronics by N.G.Palan Principle of electronics V.K.Mehta Industrial Electronics by S.k.bhattacharya www.allaboutcircuits.com www.answers.com

http://www.answers.com/

http://www.allaboutcircuit.com/

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