TRIAC as PAC
TRIAC as PAC:
Fig: TRIAC – Back to back SCR Fig: Phase angle controller circuit for a single-phase ac regulator using TRIAC
- A Triac is equivalent to two thyristors connected back to back. Thus, it is a bidirectional switching device, in contrast to the thyristor, which is a unidirectional device, having reverse blocking characteristic, preventing the flow of current from Cathode to Anode.
- TRIAC in conduction mode, current flows in both directions (forward and reverse). This switching device is called as TRIAC (TRIode AC switch). The three terminals of the triac are designated asMT1 , MT2 and gate. These are similar to the terminals – A (Anode), K (Cathode) and G (Gate), of the thyristor.
- The terminal,MT1 is taken as the reference point for the measurement of the voltages and currents at other two terminals, G (gate) and MT2 . The gate (G) is near to the terminal MT1. The thyristor conducts with the current direction from Anode to Cathode (positive), when a positive pulse is fed at the Gate terminal with respect to Cathode, and at that time, with positive voltage applied between Anode and Cathode terminals, being connected in series with the load.
- The triac conducts in the positive direction from MT2 to MT1 , when a positive pulse is applied at the gate (G) terminal with respect to MT1 and at the same time, the positive voltage is applied between two terminals, MT2 ( ) and MT1 (−).
- Similarly, the triac conducts in negative direction from MT1 to MT2 , when a negative pulse is applied at the gate (G) terminal with respect to MT1 and at the same time, the positive voltage is applied between two terminals,MT1 ( ) and MT2 (−).
- Note that the voltage between two terminals, MT1 and MT2 , is negative, in this case. So, the triac can conduct in both directions (positive and negative) as given here, whereas the thyristor conducts in one (positive) direction only. Only one triac is needed, whereas it is to be replaced by two thyristors, with consequent change in the control circuit.
- A thyristor turns off (non-conducting mode), if the current through it, falls below holding current. Similarly, a triac turns off (non-conducting mode), if the magnitude of the current, irrespective of its direction, falls below holding current.
- As a triac is connected in an ac circuit, and if the load in the circuit is resistive, the triac turns off at the zero crossing points of the voltage in each half (the supply (input) voltage reaches zero at the end of each half cycle).
- This will be nearly valid, if the load inductance is small, though the triac in that case turns off, as the current though it goes to zero, after the zero crossing point is reached in each half. The triac is a low power device, used in voltage control circuits, used as light dimmers, speed control for fan motors (single-phase), etc. Some of the advantages and disadvantages of the triac vis-a-vis thyristor are given.
1. Triacs are triggered by positive or negative polarity voltages applied at the gate terminal.
2. A triac needs a single heat sink of slightly larger size, whereas anti-parallel thyristor pair needs two heat sinks of slightly smaller sizes, but due to the clearance total space required is more for thyristors
1. Triacs have low rating as compared to thyristors. dv/dt
2. Triacs are available in lower rating as compared to thyristors.
3. Since a triac can be triggered in either direction, a trigger circuit for triac needs careful consideration.
4. The reliability of triacs is lower than that of thyristors.