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EE201 -Chapter 6


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Physical structure, schematic symbol and characteristic - SCR, Diac, Triac and UJT. Application of these devices in electronics.

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EE201 -Chapter 6

  2. 2. SILICON CONTROLLED RECTIFIER(SCR) An SCR consist of four layers of alternating Pand N type semiconductor materials. As the terminology indicates, the SCR is arectifier constructed of silicon material with athird terminal for control purposes. Silicon was chosen because of its hightemperature and power capabilities.
  4. 4. SCR MODES OF OPERATION In the normal "off" state, the device restrictscurrent to the leakage current. When the gate-to-cathode voltage exceeds acertain threshold, the device turns "on" andconducts current. The device will remain in the "on" state evenafter gate current is removed so long as currentthrough the device remains above the holdingcurrent.
  5. 5. CONT’D Once current falls below the holding current foran appropriate period of time, the device willswitch "off". If the gate is pulsed and the current through thedevice is below the holding current, the devicewill remain in the "off" state.
  6. 6. SCR APPLICATION Mainly used in devices where the control of highpower, possibly coupled with high voltage, isdemanded. Suitable for use in medium to high-voltage ACpower control applications, such as lampdimming, regulators and motor control. SCRs and similar devices are used forrectification of high power AC in high-voltagedirect current power transmission.
  7. 7. DIAC The DIAC, or diode for alternating current, is adiode that conducts current only after its break-over voltage has been reached momentarily. The diac is basically a two-terminal parallel-inverse combination of semiconductor layers thatpermits triggering in either direction.
  9. 9. CONT’D DIACs are also called symmetrical triggerdiodes due to the symmetry of theircharacteristic curve. Because DIACs are bidirectional devices, theirterminals are not labeled as anode and cathodebut as A1 and A2 or MT1 ("Main Terminal") andMT2.
  10. 10. DIAC APPLICATION The diacs, because of their symmetrical bidirectionalswitching characteristics, are widely used astriggering devices in triac phase control circuitsemployed for lamp dimmer, heat control, universalmotor speed control etc.
  11. 11. TRIAC TRIAC, from Triode for Alternating Current,is a genericized tradename for an electroniccomponent which can conduct current in eitherdirection when it is triggered (turned on). The triac is fundamentally a diac with a gateterminal for controlling the turn-on conditions ofthe bilateral device in either direction. For either direction the gate current can controlthe action of the device in a manner very similarto that demonstrated for an SCR.
  13. 13. CONT’D It can be triggered by either a positive or anegative voltage being applied to its gate electrode. Once triggered, the device continues to conduct untilthe current through it drops below a certain thresholdvalue, the holding current, such as at the end of ahalf-cycle of alternating current(AC) mains power. This makes the TRIAC a very convenient switch forAC circuits, allowing the control of very large powerflows with milliampere-scale control currents.
  14. 14. TRIAC APPLICATION Low power TRIACs are used in manyapplications such as light dimmers, speedcontrols for electric fans and other electricmotors. In the modern computerized control circuits ofmany household small and major appliances.
  15. 15. UJT (UNIJUNCTION TRANSISTOR) A unijunction transistor (UJT) isan electronic semiconductor device that has onlyone junction.
  16. 16.  A slab of lightly doped (increased resistancecharacteristic) n-type silicon material has twobase contacts attached to both ends of onesurface and an aluminum rod alloyed to theopposite surface.
  17. 17.  The p-n junction of the device is formed at theboundary of the aluminum rod and the n-typesilicon slab. The single p-n junction accounts for theterminology unijunction.
  18. 18. UJT APPLICATION The most important applications of UJTs orPUTs is to trigger thyristors (SCR, TRIAC, etc.). DC voltage can be used to control a UJT circuitsuch that the "on-period" increases with anincrease in the DC control voltage. Thisapplication is important for large AC currentcontrol. UJTs can also be used to measure magnetic flux.