Page 70 - Electrician - TT (Volume 2)
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ELECTRICIAN - CITS


            Power control circuit using SCR,DIAC,TRIAC & IGBT


           Objectives: At the end of this lesson you shall be able to
           •  explain the construction and working of SCR,DIAC,TRIAC & IGBT
           •  explain power control circuits using SCR
           •  explain power control circuit using DIAC & TRIAC
           •  explain  the construction and using of IGBT.


           Introduction to power electronics devices

           Industrial electronics is concerned primarily with electronics  applied to industries such as industrial equipments,
           controls and processes. An important application of electronics in industries is in controlling of machinery.
           In communication electronics, domestic & entertainment electronics, generally, the electronic devices operate
           with currents in the order of Microampere to Milliampere. For industrial applications, most frequently, devices
           are required to handle currents in the range of ampere to several thousands of ampere. This, therefore calls for
           high power electronic devices. One such high power electronic device  frequently used in industrial electronic
           application is the SCR,TRIAC,IGBT and  DIAC for  associate triggering circuits.

           This devices can be used  to run, dc motors from an ac power source, control power tool speed,also to control
           motor speeds of small appliances like, mixers and food blenders, illumination control, temperature control and
           so on.
           Silicon Controlled Rectifier (SCR)
           Before Silicon controlled rectifiers were invented(1956), a glass tube device called Thyratron was used for high
           power applications. Silicon Controlled Rectifier (SCR) is the first device of the thyristor family. The term thyristor
           is coined from the expression Thyratron-transistor. SCR is a semiconductor device. SCR does the function of
           controlled rectification. Unlike a rectifier diode, SCR has an additional terminal called the gate which controls the
           rectification(gated silicon rectifier).
           The basic principle application of SCRs is to control the amount of power delivered to a load(motor, lamp, etc.,).A
           rectifier diode will have one PN junction. SCRs on the other hand will have two PN junctions (P-N-P-N layers).
           Fig 1 shows the electrical symbol, basic construction and a typical SCR packages.


               Fig 1












           Basic operation of SCR
           When  a  gate  direct current is applied to the gate terminal, forward current conduction commences in the
           SCR(latched into conduction). When the gate current is removed, the forward current through the SCR does not
           cut-off. This means, once the SCR is latched into conduction, the gate loses control over the conduction. The
           current through the SCR can be turned off only by reducing the current through it(load current) below a critical
           value called the Holding current.

           Fig 2 shows how an SCR can be gated into conduction or turned off.
           In Fig 2a, with switched S1 open the SCR is in OFF state and no current is flowing through the load.
           In Fig 2b, when S1 is closed, a small gate current(around 1/1000 or less compared to load current) turns-ON
           (fires) the SCR. A heavy load current starts flowing through the SCR and load RL.






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                                    CITS : Power - Electrician & Wireman - Lesson 60-69
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