What is The TRIAC

Chapter 7.6 The TRIAC

Lessons In Electric Circuits Volume III – Semiconductors Book
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Lessons In Electric Circuits Volume III – Semiconductors Book

  • 7.6. THE TRIAC343cathode. SCRs may be turned off by anode current falling below the holding current value(low-current dropout), or by ”reverse-firing” the gate (applying a negative voltage to thegate). Reverse-firing is only sometimes effective, and always involves high gate current.• A variant of the SCR, called a Gate-Turn-Off thyristor (GTO), is specifically designed tobe turned off by means of reverse triggering. Even then, reverse triggering requires fairlyhigh current: typically 20% of the anode current.• SCR terminals may be identified by a continuity meter: the only two terminals showingany continuity between them at all should be the gate and cathode. Gate and cathodeterminals connect to a PN junction inside the SCR, so a continuity meter should obtaina diode-like reading between these two terminals with the red (+) lead on the gate andthe black (-) lead on the cathode. Beware, though, that some large SCRs have an internalresistor connected between gate and cathode, which will affect any continuity readingstaken by a meter.• SCRs are true rectifiers: they only allow current through them in one direction. Thismeans they cannot be used alone for full-wave AC power control.• If the diodes in a rectifier circuit are replaced by SCRs, you have the makings of a con-trolledrectifier circuit, whereby DC power to a load may be time-proportioned by trigger-ing the SCRs at different points along the AC power waveform.7.6The TRIACSCRs are unidirectional (one-way) current devices, making them useful for controlling DConly. If two SCRs are joined in back-to-back parallel fashion just like two Shockley diodes werejoined together to form a DIAC, we have a new device known as the TRIAC: (Figure 352,7.39)TRIAC equivalent circuitTRIAC schematic symbolGateMain Terminal 1Main Terminal 2(MT2)(MT1)Main Terminal 2(MT2)Main Terminal 1(MT1)GateFigure 7.39:The TRIAC SCR equivalent and, TRIAC schematic symbolBecause individual SCRs are more flexible to use in advanced control systems, these aremore commonly seen in circuits like motor drives; TRIACs are usually seen in simple, low-power applications like household dimmer switches. A simple lamp dimmer circuit is shown in