INTRODUCTION TO OTHERELECTRONIC DEVICESCHAPTER 6BYPN. RUHIYAH NAZIHAH ZAHKAIELECTRICAL ENGINEERING DEPARTMENTPOLYTECHNIC SULTAN IDRIS SHAH
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
UJT (UNIJUNCTION TRANSISTOR) A unijunction transistor (UJT) isan electronic semiconductor device that has onlyone junction.
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.
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.
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.
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