TRIAC

1. TRIAC Triac was developed by the need to control the current in both directions which a single SCR was not able to do. Before triacs, two SCRs in paralle to accomplish this bidirectional task. The triac is three-terminal device used to control the average current flow to a load. The triac, like SCR, is often used in an AC circuit to control the power on load. Triac can carry current in either direction (positive and negative directions),so theoratically it is possible for a triac to conduct for a 360 per cycle with appropriate triggering. SCRs carry current in positive direction only. A triac can operate in full-wave phase control circuits while an SCR can operate in half-wave phase control circuits.

2. Figure 6–1(a) Schematic symbol and terminal names of a triac. (b) Triac circuit showing how the supply voltage, the load, and the triac are connected. Equivalent Circuit of the triac

3. Operation Of the Triac • The symbol of the triac is shown in Figure 6-1(a) with the names of its terminals. • When the triac is turned OFF, no current can flow between the main terminals whatever the polarity of the applied voltage. It acts as open switch. • When the triac is turned ON, there is a very low-resistance current flow path between the main terminals. • The direction of the current flow will depend on the polarity of applied voltage.

4. When the voltage is more positive on MT1 with respect to MT2 the current will flow from MT1 to MT2. If the voltage is more positive on MT2 with respect to MT1 the current will flow from MT2 to MT1. • In Figure 6-1(b), the average current delivered to the load can be varied by varying the amount of time per cycle that the triac spends in its ON state. • With proper triggering arrangement, triac can conduct for a full 360o per cycle. • Triacs (SCRs, Transistors) have no contact bounce, no arcing across partially opened contacts and they operate much faster (more precise control) than mechanical switches.

5. Triac Waveforms (b) (a) Figure 6–2Waveforms of triac main-terminal voltage and load voltage for three different conditions. (a) Firing delay equals 30 for both the positive half cycle and the negative half cycle. (b) Firing delay equals 120 for both half cycles. (c) Unequal firing delay angles for the positive and negative half cycles. This is usually undesirable. (c)

6. Triac waveforms are very much like SCR waveforms except that they can fire on the negative half cycle. • In figure 6-2 (a), the triac is OFF during the first 30o(firing delay angle) of each half cycle. During this time the entire line voltage is dropped across the main terminals of the triac, with no voltage applied to the load. • After 30o has elapsed, the triac fires (turn ON) and it becomes like a closed switch and start conducting current to the load thru its main terminals for the remainder of the half cycle (for the conduction angle). • for the negative half cycle the triac will be turned ON after 210o=180o+30o and it will conduct current in the opposite direction (from load to supply) for the remainder of the negative half cycle.

7. Electrical Characteristics of Triac • Refer to Figure 6-2 (c), there is mismatch in the delay angles for the positive and negative half cycles. This is a common problem for the triacs; some have tendency to easily fire on the positive half cycle and some have the reverse tendency. • This because IGT for a triac for the forward main terminal polarity (See figure -3) may be quite a bit different from IGT for the reverse main terminal bias. • For most medium sized triacs, VGT=0.6V—2V and IGT=0.1mA—20mA. • Like SCR, Triac does not require continuous gate current once it has been fired. It will remain ON until the bias is changed or its terminal current drops below IHO (medium sized triac IHO=100mA). Definition of some terms of a triac:See page# 243 of your text book

8. Figure 6–3(a) The situation when a triac has forward main terminal bias. Normally the gate current and gate voltage would have the polarities indicated. (b) The situation at a different point in time when the triac is reverse biased. Normally the gate current and voltage are also reversed.

9. Triggering Methods For Triacs Figure 6–4(a) Simple gate control circuit (triggering circuit) for a triac. Firing delay is adjusted by potentiometer R2. (b) Improved gate control circuit, which allows a wider range of adjustment of firing delay.

10. Triggering Methods For Triacs • RC Gate Control Circuits • Fig.6-4(a) shows the simplest triggering circuit; C charges thru R1, R2 during the delay angle of each half cycle. • When C is charged to a value enough to deliver sufficient gate current (IGT) thru R3 the Triac will fire. • The charging rate of C is set by R2; big R2 will give charging rate slow causing long firing delay (big delay angle) and vice versa. • To establish wider range of delay angle, the double RC network is used (Fig.6-4(b)).