![]() ![]() When SW1 is open, the triac acts as an open switch and the lamp passes zero current. Simple AC power switch with resistive (lamp) load. The trigger current sensitivity is greatest when the MT2 and gate currents are both of the same polarity (either both positive or both negative), and is usually about half as great when they are of opposite polarity.įigure 2 shows a triac used as a simple AC power switch, driving a resistive lamp load assume that SW2 is closed.įIGURE 2. ![]() III+ Mode = MT2 current -ve, gate current -ve III+ Mode = MT2 current -ve, gate current +ve I- Mode = MT2 current +ve, gate current -ve I+ Mode = MT2 current +ve, gate current +ve It can be triggered by either positive or negative gate currents, irrespective of the polarity of the MT2 current, and it thus has four possible triggering modes or 'quadrants,' signified as follows: It can conduct current in either direction between its MT1 and MT2 terminals and can thus be used to directly control AC power. Triac BasicsĪ triac is a three-terminal (MT1, gate, and MT2) solid-state thyristor that uses the alternative symbols in Figure 1 and acts like a pair of SCRs wired in inverse parallel and controlled via a single gate terminal. In each design, the user must use a triac with ratings to suit his or her own particular application. Most of the practical circuits show two sets of component values for use with normal domestic/commercial 50Hz or 60Hz AC voltage supplies with nominal values of either 240V (as used in most of Europe) or (in parenthesis) 120V (as used in most of the USA). This two-part article explains its basic operation and shows various ways of using it. ![]() A triac is a controllable medium- to high-power semi-latching solid-state AC power switch. ![]()
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