A triac is a bi-directional thyristor used to control the power in ac circuits. It is like a diac with a gate control or equivalent to two SCRs in parallel. A triac has two leads designated M T1 , and M T2 or A 1 and A 2 . It can conduct current in either direction when it is trigger ON (depending on polarity of voltage across it terminal A 1 and A 2 ). A triac has a gate lead which is used to control its conduction,which can be turned on by a pulse of gate current and does not require the breakover voltage to initiate conduction.
Triac Figure 24 The schematic symbol for a triac is shown in Figure 24 (a). There are two anode terminals, A 2 and A 1 , and a gate lead. The triac is the equivalent of two SCRs connected in parallel, as shown in Figure 24 (b).
Triac Figure 25 A 2 Gate Mode + + 1 + − 2 − + 3 − − 4 Figure 25 shows the four operating modes for a triac. When the anode 2 (A 2 ) terminal is positive, the triac can be turned on with either a positive or negative gate voltage. When the anode 2 (A 2 ) terminal is negative, the triac can be turned on with either a positive or negative gate voltage.
Triac Figure 26 Figure 26 shows a very effective way to provide a wide range of control over load current. R 1 -C 1 and R 2 -C 2 provide the required phase shift necessary for full control of the load current.
Triac Construction Neither terminal is referred to as cathode Contain 2 anodes, anode 1(electrode 1) and anode 2(electrode 2) & 1 gate terminal.
Triac Characteristic Curve
Current in direction depending on the polarity across the terminal. It turn OFF when the current drop sufficient low level. Breakover potential decrease as the gate current increase. Triac Characteristic Curve
Triac Basic Operation Terminal A1 is biased positive with respect to A2
Triac Basic Operation Terminal A2 is biased positive with respect to A1.
Triac Typical Packages and Pin Identification
1. (a) In the given figure, the switch is closed. If the triac has fired, what is the approximate current through the 22 Ω ? (b) When the switch is closed. The MPT32 is a diac with a breakover voltage of 32 V. If the triac has a trigger voltage of 1 V and a trigger current of 10 mA, what is the capacitor voltage that trigger the triac? Example
Diac and Triac Comparison Diac Triac 2 terminal 3 terminal Contain 2 anodes Contain 2 anodes & 1 gate OFF when the current drop below the holding current(I H ) OFF when the current drop sufficient low level
Both conduct current in either direction when properly activate. Both cease to conduct when the anode drops below specified value of holding current(I H ). Both have same characteristic curve. Diac and Triac Similarities