Layer p2, common to the two SCRs, functions as a gate for both sections of the device. The two outer terminals cannot be identified as anode and cathode; instead they are designated main terminal 1 MT1 and main terminal 2 MT2 , as illustrated. In this case current flow is from MT2 to MT1. Now current flow is from MT1 to MT2. Similarly, when MT2 is negative with respect to MT1, a negative gate voltage triggers the device into conduction. From the second line, still with MT2 positive, triggering can be produced by a negative gate voltage;
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The triac can be operated with either positive or negative gate control voltage but in normal operation usually the gate voltage is positive in Ist quadrant and negative in IIIrd quadrant.
The supply voltage at which the triac is turned ON depends upon the gate current. The greater the gate current, the smaller the supply voltage at which the triac is turned on.
This permits to use a triac to control a. Triac Phase Control Circuit A triac can be used to control the average a. This is achieved by changing the conduction angle through the load. Because of this phase shift , the gate voltage lags the line voltage by an angle between 0o and 90o. By adjusting the variable resistance R1, the conduction angle through the load can be changed.
Thus any portion of positive and negative half-cycles of the a. This actin of triac permits it to be used as a controlled bidirectional switch. Circuit Action The operation of triac phase control circuit is as under: i During each positive half-cycle of the a.
As a result, smaller a. We can change the phase angle of gate voltage by adjusting the variable resistance R1. Thus a triac can control the a. This control of a. But as the switch is thrown to position 2, a small gate current a few mA flowing through the gate turns the triac on. Consequently, the lamp is switched on to give full output of watts. However, if it is desired to change the tapping so that input appears across part AB of the primary, then TR2 is turned on and TR1 is turned off.
The gate control signals are so controlled that both triacs are never switched on together. This avoids a dangerous short circuit on the section BC of the primary. I am Sasmita. At ElectronicsPost. And, if you really want to know more about me, please visit my "About" Page. Read More Categories.
DIAC Diode :Basics, Characteristics, Applications
The triac has on and off state characteristics similar to SCR but now the char acteristic is applicable to both positive and negative voltages. This is expected because triac consists of two SCRs connected in parallel but opposite in direc tions. MT2 is positive with respect to MTX in the first quadrant and it is negative in the third quad rant. As already said in previous blog posts, the gate triggering may occur in any of the following four modes. The device, when starts conduction permits a very heavy amount of current to flow through it. This large inrush of current must be restricted by employing external resist ance, otherwise the device may get damaged. The gate is the control terminal of the device.
Understanding Characteristics of DIAC, TRIAC & SCR
Sketch the V-I characteristics of triac . Describe some of its important applications
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