Triac in stud-mounted package
You can of a triac as two back-to-back SCRs with the cathode of one SCR connected to the anode of the other. The gates are connected together. Because we have a two SCRs configuration you can control the switching of both half cycles.
A triac is a 4-layer PNPN device in the positive direction and a 4-layer NPNP device in the negative direction. It is a three terminal device that blocks current in its OFF state. Unlike an SCR, a triac can conduct current in either direction. A triac has four possible triggering modes.
Trigger mode 1
1. Terminal MT2 and the gate are positive relative to terminal MT1. Current flow will be in the direction of P1-N1-P2-N2. The junction among the layers like P1-N1 and P2-N2 are connected in forward biased while the Junction N1-P2 is connected in reverse bias.
Trigger mode 2
2. Terminal MT2 and the gate are negative relative to terminal MT1. Current flow will be in the direction of P1-N1-P2-N2. The junction among the layers P2-N2 are connected in forward biased and current carriers are added into layer P2.
Trigger mode 3
3. Terminal MT2 is positive relative to terminal MT1, but the gate is negative relative to terminal MT1. Current flow will be in the direction of P2-N1-P2-N2. The junction among the two layers P2-N1 and P1-N4 is forward biased while the junction among layers N1-P1 are reverse biased.
Trigger mode 4
4. Terminal MT2 is negative relative to terminal MT1, but the gate is positive relative to terminal MT1. The junction among P2-N2 is forward biased and current carriers are added into layer N2.
Phase Control Circuit
The schematic above shows the use of a triac to produce the variable power to the load by controlling the phase at which the triac's gate is triggered.
During the positive half cycle of the power source diode D1 is forward biased and a positive trigger current is applied to the gate. During the negative half cycle of the power source diode D2 is forward biased and a negative trigger is applied to the gate.
The amount of AC power delivered to the load is controlled in either direction by applying the proper mode of gate signal. The angle at which the triac conducts is controlled by varying the resistance of R2.
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