新型电子课件第9章triac.ppt

3. the operation of Triac (1) Notice that the schematic symbol for the triac is similar to the connection of the two SCRs. The gate must be connected to the same polarity as MT2 to turn the triac on. (2) When the voltage applied to MT2 increases in the positive direction, the gate fires the half of the triac which is forward biased during that half of the cycle. (3) Since the other SCR half of the triac is reverse biased during that half cycle, it cannot be fired. (4) When the applied AC voltage becomes negative at MT2, the gate fires the other half of the triac which is now forward biased. 4. Adding a diode If a diode is connected in series with the gate, it permits only half of the AC cycle to provide gate current: assume the positive half cycle. If an oscilloscope is connected across the load resistor, only the positive half of the AC waveform is seen. The diode is reverse biased during the negative half cycle and no gate current flows to turn the negative half of the triac on. If a diode is removed from the circuit and replaced facing in the opposite direction, it blocks the positive half cycles and passes the negative half. The oscilloscope connected across the load resistor shows the negative half of the AC waveform. This is a good example of the two SCR arrangement of the triac. 5. Adding a variable resistor (1) If a variable resistor is used to control the gate of the triac, the same control problems exist as did with the SCR. (2) A simple resistor control, as shown below, permits the triac to control only half of the waveform just like an SCR which has not been phase shifted. ◆ both halves of the AC cycle provide gate current ◆ both halves of the AC waveform can be seen across the load. ◆ the gate current is locked in phase with the voltage applied to the MT1 and MT2 ◆ the triac can not be fired after the AC voltage reached 90°and 270° 6. Testing the triac the triac can be tested with an ohmmeter; since it is basically two SC