About diac traic scr & mosfet

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  • #1082
    Profile photo of Vicky
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    Sir please tell me difference in principle of scr , diac , traic , mosfet

    And their application

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    Profile photo of Kiran Daware
    Kiran Daware
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    Thyristor or SCR – Silicon Controlled Rectifier

    An SCR is basically a Shockley diode with just an added Gate terminal. Hence, an SCR is a three terminal device with 4 layers – PNPN. An SCR has three PN junctions J1, J2 and J3. The three terminals of an SCR are – Anode which is connected to P type layer, Cathode connected to N type layer and the Gate connected to P type layer nearest to the cathode as shown in the image below. Operation of an SCR can be understood by visualizing it’s analogous  BJT equivalent circuit as shown below.

    thyristor or scr

    Image source: Wikimedia commons

    Applications of SCR

    SCRs are generally used as switching devices for the control of high power at high voltages. They are also used in rectifying high power AC for the purpose of HVDC (High Voltage DC) transmission.

    DIAC (Diode on AC)

    A DIAC is basically two zener diodes connected back to back. It is a two terminal device with three layers – PNP or NPN. Though the structure of a DIAC looks similar to the structure of BJT, there are significant differences between them – (i) no terminal is attached to the base layer of a DIAC, (ii) unlike BJT, the three regions of a DIAC are of nearly identical size also with identical doping concentrations. DIAC is a bidirectional device wich conducts after a break-over voltage. 5-layer structure of diac is also available which produces a very similar I-V curve as of the 3-layer structure.

    diac 3 and 5 layer structure and symbol

    Applications of DIAC

    Because of their symmetrical bidirectional switching characteristics, DIACs are popularly used as triggering devices for TRIAC based phase control systems.

    TRIAC (Triode on AC)

    A TRIAC is a three terminal, five layer device whose forward and reverse characteristics are identical to the forward characteristics of SCR.  The three terminals are – main terminal MT1, main terminal MT2 and Gate. Basic structure of a TRIAC is similar to the two SCRs connected in inverse parallel.

    triac structure and symbol

    Applications of TRIAC

    TRIAC is generally used in light dimmers, speed control of electric fans and motors etc. They can be used in both AC as well as DC circuits.

    MOSFET (Metal Oxide Semiconductor Field Effect Transistor)

    MOSFET is a type of field effect transistor with an insulated gate whose voltage determines the conductivity of the device. It is a four terminal device named as Source (S), Gate (G), Drain (D) and Body or Substrate (B). But, usually, substrate is connected to the source terminal making it essentially a three terminal device.

    It is not possible to explain all the devices descriptively in a single answer. Please refer popular books or website to study about each more.

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