Sunday, November 18, 2018

How Triacs are used in Industrial Control Applications


The Triac is a three-terminal device that is similar to the SCR except that the Triacs can conduct current in both directions. Its primary use is to control power to AC loads such as turning AC motors on and off or varying the power for lighting and heating systems. The Triac is a solid state device that acts like two SCRs that have been connected in parallel with each other (inversely) so that one SCR will conduct the positive half-cycle and the other will conduct the negative half-cycle. Before the triac was designed as a single component, two SCRs were actually used for this purpose.
The figure 1 below shows the symbol for the triac, and its pn Structure. The terminals of the triac are identified as main terminal 1 (MT1), main terminal 2 (MT2), and gate. The multiple pn structure is actually a combination of two four-layer (pnpn) junctions.


Triacs
Figure 1

As the name suggests, the load current passes through the main terminals, and the gate controls the flow. Figure 2 below shows the equivalent circuit of the triac, which consists of two back-to-back SCRs with a common gate.
Triacs
Figure 2
Operation of the Triac
The operation of the triac can be explained by the two-SCR model in Figure 2.  From the figure, you can see the SCRs are connected in an inverse parallel configuration. One of the SCRs will conduct positive voltage and the other will conduct negative voltage.
When MT2 is more positive, the current flows through first SCR; when MT1 is more positive the current flows through Second SCR.
Unlike the two SCRs, the Triac is triggered by a single gate. This prevents problems of one SCR not firing at the correct time and overloading the other.
The operating characteristics of the Triac are best explained using the characteristic curve shown in Figure 3:


Triacs
Figure 3
In the figure above, you can see that the triac can conduct both positive and negative current. The Voltage is shown along the horizontal x-axis, and current is shown along the vertical y-axis. This diagram also shows a second graph with four quadrants. These quadrants are used to explain the operation of the triac as polarity to its MT1 and MT2 and gate changes.
Notice that the right half of the graph (in quadrant 1) looks just like the SCR curve; no current flows until either the break over voltage is reached or the gate is triggered (indicated by dashed line).
This same pattern is repeated in quadrant 3 (for voltage and current of the opposite direction). Also, like the SCR, once the triac is triggered on, it will remain on by itself until the load current drops below the holding current value (IH)
A Single cycle of AC has a positive and a negative half-cycle. The triac requires a trigger pulse at the gate for each half-cycle and works best if the trigger is positive for the positive half-cycle and negative for the negative half-circle (Although in most cases the triac will also trigger if the gate goes negative in the positive half-cycle and if it goes positive in the negative half-cycle.
Applications of Triacs
The Triac is required in circuits where AC Voltage and Current need to be controlled like the SCR controls dc current. Another difference between the triac and SCR is that the Triac can be turned on either by a positive or negative gate pulse. The gate pulse need only be momentary and the Triac will remain in conduction until the conditions for commutation are satisfied.
A triac can be used as an-off solid-state switch for AC loads or to regulate power to an AC load, such as dimmer switch.
Triacs are available in various packages, some of which can handle currents up to 50 A (which is considerably less than the SCR).
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