Triac

A TRIAC or alternating current triode is a semiconductor device, of the thyristor family. The difference with a conventional thyristor is that it is unidirectional and the TRIAC is bidirectional. In a colloquial way, it could be said that the TRIAC is a switch capable of switching alternating current.

Its internal structure somewhat resembles the arrangement that two SCRs would form in opposite directions. It has three electrodes: MT1, MT2 (in this case they lose the name of anode and cathode) and gate (G). The TRIAC is triggered by applying a current to the gate electrode.

Most common applications

• Its versatility makes it ideal for the control of alternating current (C.A.).
• One of them is its use as a static switch offering many advantages over conventional mechanical switches and relays.
• It works as an electronic switch and also a battery.
• Low-power TRIACs are used in many applications such as light attenuators, speed controls for electric motors, and in computerized control systems of many home elements. However, when used with inductive loads such as electric motors, the necessary precautions should be taken to ensure that the TRIAC is turned off properly at the end of each half-cycle of the alternating current wave.

Operation

Activation modes. Quadrant, 1 (up on the right), 2 (up on the left), 3 (down on the left), 4 (down on the right)

To explain the operation of a TRIAC, its regime is usually divided into quadrants according to the polarity of the gate (G) and the secondary terminal (MT2), both with respect to the primary terminal (MT1).

In quadrants 1 and 2, MT2 is positive, and current flows from MT2 to MT1 through P, N, P, and N layers. The N region attached to MT2 is not significantly involved. In quadrants 3 and 4, MT2 is negative, and current flows from MT1 to MT2, also through P, N, P, and N layers. The MT2-bound N region is active, but the MT1-bound N region is only active. participates in the initial trip, does not contribute to the initial flow of current.

The relative sensitivity depends on the physical structure of a particular triac, but as a general rule, quadrant 1 is the most sensitive (lowest gate current required), and quadrant 4 is the least sensitive (highest gate current required).).

In most applications, the gate current is drawn from the same connection of MT2, so quadrants 1 and 3 are the only modes of operation (G and MT2 positive or negative with respect to MT1). Other applications with a single polarity trigger from a drive circuit can operate in quadrants 2 and 3.

Quadrant 1

Triac in Quadrant 1

The Quadrant 1 trade occurs when the gate and MT2 are positive with respect to MT1.

The gate current turns on an equivalent NPN transistor switch, which in turn draws current from the base of an equivalent PNP transistor, turning itself on. Part of the gate current (dotted line) is lost via the ohmic path through the p-doped silicon, flowing directly into MT1 without passing through the base of the NPN transistor. In this case, injection of holes into the p-silicon causes the n, p, and n stacked layers below MT1 to behave like an NPN transistor, which is turned on by the presence of a current at its base. This, in turn, causes the p, n, and p layers on MT2 to behave like a PNP transistor, which turns on because its n-type base is forward-biased with respect to its emitter (MT2). Therefore, the activation scheme is the same as an SCR.

However, the structure is different from SCRs. In particular, TRIAC always has a small current flowing directly from the gate to MT1 through the p-type doping silicon bypassing the p-n junction between the base and emitter of the equivalent NPN transistor. This current is indicated by a red dotted line and is the reason why a TRIAC needs more gate current to turn on than a comparably rated SCR.

Quadrant 2

Triac in Quadrant 2

The Quadrant 2 trade occurs when the gate is negative and MT2 is positive with respect to MT1.

The device's turn-on is triple and starts when current from MT1 flows to the gate through the p-n junction under the gate. This switches a structure made up of an NPN transistor and a PNP transistor, which has the gate as the cathode.

As the current in the gate increases, the potential on the left side of the silicon p under the gate rises towards MT1, since the potential difference between the gate and MT2 tends to go down: this establishes a current between the gate and MT2. left and right side of the p-silicon, which in turn activates the NPN transistor under the MT1 terminal and as a consequence also the PNP transistor between MT2 and the right side of the upper p-silicon. Thus, in the end, the structure that is traversed by most of the current is the same as the I-quadrant operation

Quadrant 3

Triac in quadrant 3

The Quadrant 3 trade occurs when the gate and MT2 are negative with respect to MT1.

The process also occurs in different stages. In the first phase, the pn junction between the MT1 terminal and the gate is forward biased (step 1). Since forward bias involves the injection of minority carriers into the two layers that bind the junction, electrons are injected into the p-layer under the gate. Some of these electrons do not recombine and escape into the underlying n region (step 2). This in turn lowers the potential of the n region, acting like the base of a pnp transistor being turned on (turning the transistor on without directly lowering the base potential is called remote gate control). The bottom p layer acts as the collector of this PNP transistor and has its voltage increased: actually this p layer also acts as the base of an NPN transistor made up of the last three layers just above the MT2 terminal which, in turn, is activated.

Quadrant 4

Triac in quadrant 4

The Quadrant 4 trade occurs when the gate is positive and MT2 is negative with respect to MT1.

Activation in this quadrant is similar to activation in quadrant 3. The process uses a remote door control. As current flows from the p-layer under the gate into the n-layer under MT1, minority carriers are injected in the form of free electrons into the p-region some of them are collected by the underlying np junction and pass into the neighboring n junction -region not recombined. As in the case of a quadrant 3 trigger, this reduces the potential of the n layer and turns on the PNP transistor made up of the n layer and the two p layers next to it. The bottom p layer acts as the collector of this PNP transistor and has its voltage increased: actually this p layer also acts as the base of an NPN transistor made up of the last three layers just above the MT2 terminal, which in turn turns on.

Application example: Phase control (power)

Construction of the TRIAC.

Figure 1. Dimmer circuit (light dimmer)

In figure 1 a fundamental application of the triac is presented. In this condition, it is controlling the AC power to the load by switching on and off during the positive and negative regions of the sinusoidal input signal. The action of this circuit during the positive part of the input signal is very similar to that found for the Shockley diode. The advantage of this configuration is that during the negative part of the input signal, the same type of response will be obtained since both the diac and triac can trigger in the reverse direction. The resulting waveform for the current through the load is given in the "phase control" figure. By varying the resistance R2, it is possible to control the conduction angle. There are units currently available that can handle loads in excess of 10kW.

Typical values