Thyristors: THE TRIAC AND DIAC

Introduction

Triac is one of the most interesting components of the thyristors family; being able to control the passage of the current in both directions, it represents one of the most efficient and economical solutions for the control of the power absorbed by the users working with alternating voltages.

THE TRIAC

The triac can be considered as two SCR diodes connected in antiparallel, or side by side, but with opposite directions (scheme a) of figure 1). The anodes of the two SCRs become the main terminals of the triac, and take the name of MT2 and MT1 (Main Terminal 1 and Main Terminal 2). The gates of the two SCRs are connected together, and become the gate of the triac

In b) the block construction of a triac is seen, while in c) its schematic symbol is shown.

As mentioned above, the TRIAC can be crossed by the current in both directions; it should also be noted that its transition to the "on" state, ie conduction, can occur by applying a positive or negative voltage to the gate. These multiple possibilities of operation can better be illustrated by referring to a graph like that of figure 2, called "four quadrant". Each quadrant represents a different condition of operation of the triac; the polarities and therefore the voltages are always referred to the terminal MT1.

1°: The MT2 terminal is postive with respect to the MT1 terminal; the current flowing through the triac actually flows from top to bottom. The gate, in turn, is positive compared to MT1, and in fact the gate current is "entering"

2°: MT2 is always positive compared to MT1, while the gate is negative; the gate current is a current that "goes out"

3°: MT2 is negative compared to MT1, and in fact the current crosses the triac from the bottom to the top; the voltage applied   to the gate is negative with respect to MT1

4°: MT2 is negative compared to MT1, while a positive voltage is applied to the gate.

The choice of making Triac work in one quadrant rather than another, or to choose a positive or negative gating voltage, modifies the performance of the device in a more or less important way. Following the physical arrangement of the semiconductor layers that make up the triac, the values of the "latching current" (IL), of the "holding current" (IH) and of the "gate trigger current" (IGT), vary from one quadrant to the other.

The most common operation is that corresponding to the 1st and 3rd quadrants, ie when the voltage applied to the gate has the same polarity as that applied to the terminal MT2; in these quadrants an excellent gate sensitivity is obtained. When it is not possible to work in said quadrants, the best alternative is to use the pair of 2nd and 3rd quadrants.

Below the table with all characteristic:

For convenience and clarity, a table follows that summarizes the main characteristics of thyristors, with the English name and the corresponding meaning in Italian:

The table below gives an example of the values assumed by the current characteristics in the various quadrants, for a 4 A triac.

As can be seen, the gate current is only 10 mA when the triac is made to work in the conditions corresponding to the first quadrant, confirming with this value the best sensitivity; the same current goes to 27 mA for the 4th quadrant, the one with the lowest sensitivity.
The high value (48 mA) of the "latching current" in the 2nd quadrant coincides with a certain difficulty in triggering the triac.

 THE DIAC

The DIAC is obtained by diffusing type N impurities on both sides of a P-type wafer, so as to obtain a two-terminal device with symmetrical electrical characteristics. The structure of a DIAC is similar to that of an open-base NPN transistor. This is a bidirectional structure, which has a high impedance (and therefore does not let current flow) until the voltage applied to the two terminals does not exceed a certain value, called "breakover voltage". Above this value, the Diac enters a negative resistance zone, where the avalanche conduction effect manifests itself.

Being a bi-directional device, the diac is a valid and inexpensive triggering system for triacs in phase control circuits such as light regulators, engine speed control systems, etc. Indeed, this is the only important application of the diacs.

As has been mentioned, the passage in conduction of the diac can only take place by overcoming the breakover voltage; in fact, the diac has only two terminals, called anode 1 and anode 2, and therefore does not have a gate.
The primer obtained by applying to its terminals a voltage higher than that of breakover is usefully practicable only with the diacs; even SCRs and Triacs could be conducted in a similar way, but for the latter the method is inadvisable, since the repeated exceeding of the breakover voltage could cause damage to the devices themselves. The Diacs used in phase control circuits are sufficiently protected against excessive breakover current, and therefore can work safely when the capacitor they discharge is not of excessive capacity.
Figure 5 shows the static characteristic of the Diac, which appears symmetrical with respect to the two polarities of the voltage applied to the terminals. In one sense or another, the current that attracts the diac is minimal up to a certain voltage value, VB0; exceeded this value, the voltage at the ends of the diac drops abruptly to a lower value, V0, called "breakback voltage", while the current assumes the maximum value allowed by the circuit. The current I_B0,corresponding to the breakover, it is called precisely "breakover current".

For a diac like the one shown in figure 6 (it's the Philips BR100), the characteristic quantities have the following values:

• Breakover voltage VB0: from 27 to 36 V
• Output voltage V0: 7 V
• Repetitive direct current peak: 2 A

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