Triac Switching and gate trigger circuit
Triac Switching and gate trigger circuit:
- Unlike a thyristor a triac gets limited time to turn off due to bidirectional conduction. As a result the triacs are operated only at power frequency. Switching characteristics of a triac is similar to that of a thyristor.
- However, turn off of a triac is extremely sensitive to temperature variation and may not turn off at all if the junction temperature exceeds certain limit. Problem may arise when a triac is used to control a lagging power factor load.
- At the current zero instant (when the triac turns off) a reverse voltage will appear across the triac since the supply voltage is negative at that instant. The rate of rise of this voltage is restricted by the triac junction capacitance only. The resulting dv/dt may turn on the triac again.
- Similar problem occurs when a triac is used to control the power to a resistive element which has a very low resistance before normal working condition is reached. If such a load (e.g. incandescent filament lamp) is switch on at full supply voltage very large junction capacitance charging current will turn ON the device.
- To prevent such condition an R-C snubber is generally used across a triac. The triac should be triggered carefully to ensure safe operation.
- For phase control application, the triac is switched on and off in synchronism with the mains supply so that only a part of each half cycle is applied across the load. To ensure ‘clean turn ON’ the trigger signal must rise rapidly to provide the necessary charge. A rise time of about 1 μs will be desirable.
- As Vi increases voltage across C1 increases due to current flowing through load, R1, R2 and C1. The voltage drop across diac D1 increases until it reaches its break over point. As D1 conducts a large current pulse is injected into the gate of the triac. By varying R2 the firing can be controlled from zero to virtually 100%.