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
Triac terminology I GT – The magnitude of current that must flow into the gate terminal of the triac in order to trigger it into the ON conducting state. I HO – The minimum amount of main terminal current that must continue flowing to prevent a conducting triac from reverting to the OFF blocking mode.
Triac terminology V DROM – The maximum permissible main terminal voltage that can be applied to the triac repetitively, with certainty that the triac will not break over into the ON conducting state. V TM – The voltage existing across the main terminals of the triac when it is in the ON conducting state.
Triac terminology I Trms – The main terminal current being carried from the source through the triac to the load under a given set of circuit operating conditions. Note: For a medium-power triac(3A): I GT = 5mA and V TM = 1.2 V.
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
Unijunction Transistors The unijunction transistor (UJT) is a three- terminal semiconductor device that has only one p-n junction. The UJT is a breakover-type switching device whose characteristics make it useful in many industrial circuits, including timers, oscillators, waveform generators, and gate control circuits for SCRs and TRIACS. The unijunction transistor (UJT) has two base leads, B 1 and B 2 and an emitter (E) lead.
Unijunction Transistors The interbase resistance, R BB of a UJT is the resistance of its n-type silicon bar. The ratio R B1 /(R B1 + R B2 ) is called the intrinsic standoff ratio, designated η . The reasonable value for a UJTs standoff ratio, η = 0.7 UJTs are used in conjunction with SCRs and Triacs to control their conduction angle.
Unijunction Transistors Figure 27 Construction of a UJT is shown in Figure 27 A bar of n-type silicon (Si) is placed on two separate pieces of ceramic.
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