Triac: the principle of operation, application, device and management

From the article you will learn about what a triac is, how it works, and how it works. But first of all, it's worth mentioning that a triac is the same as a thyristor (symmetrical only). Therefore, one can not do without an article without a description of the principle of functioning of thyristors and their features. Without knowledge of the basics, it will not be possible to design and build even the simplest control scheme.

Thyristors

Thyristor is a switching semiconductor device that is capable of transmitting current in only one direction. It is often called a gate and draws analogies between it and a controlled diode. Thyristors have three terminals, one of which is the control electrode. This, to put it bluntly, is the button by which the element switches to conductive mode. The article will consider a special case of a thyristor - a triac - a device and its operation in various circuits.

The thyristor is still a rectifier, a switch and even a signal amplifier. Often it is used as a regulator (but only when the entire circuit is powered from an AC voltage source). All thyristors have some features, which need to be discussed in more detail.

Properties of thyristors

Among the vast array of characteristics of this semiconductor element, it is possible to distinguish the most significant:

1. Thyristors, like diodes, are capable of conducting electric current in only one direction. In this case, they work in the circuit as a rectifier diode.
2. From the state of the thyristor turned off, the thyristor can be switched by sending a signal with a definite shape to the control electrode. Hence the conclusion - a thyristor as a switch has two states (both are stable). The triac can function in the same way. The principle of operation of an electronic type key based on it is quite simple. But in order to make a return to the initial open state, it is necessary that certain conditions are fulfilled.
3. The current of the control signal, which is necessary for the transition of the thyristor crystal from the locked mode to the open mode, is much less than the working one (literally measured in milliamperes). This means that the thyristor has the properties of a current amplifier.
4. It is possible to fine-tune the average current flowing through the connected load, provided that the load is connected in series with the thyristor. The accuracy of the adjustment directly depends on the length of the signal at the control electrode. In this case, the thyristor acts as a power regulator.

Thyristor and its structure

A thyristor is a semiconductor element that has control functions. The crystal consists of four layers of p and n type, which alternate. The triac is also built exactly. The principle of operation, application, the structure of this element and limitations in use are considered in detail in the article.

The described structure is also called a four-layer structure. The extreme region of the p-structure with the positive polarity of the power supply connected to it is called the anode. Consequently, the second region n (also the extreme one) is the cathode. A negative voltage of the power source is applied to it.

What are the properties of a thyristor

If we perform a full analysis of the structure of the thyristor, we can find in it three transitions (electron-hole). Consequently, an equivalent circuit can be made for semiconductor transistors (polar, bipolar, field) and diodes, which will allow us to understand how the thyristor behaves when the power supply of the control electrode is turned off.

In the case when the anode is positive relative to the cathode, the diode closes, and, consequently, the thyristor also behaves similarly. In case of polarity reversal, both diode are shifted, the thyristor is also locked. The triac also functions in a similar way.

The principle of working on the fingers, of course, is not very easy to explain, but we will try to do this further.

How Thyristor Unlocking Works

To understand the principle of the thyristor, you need to pay attention to an equivalent circuit. It can be composed of two semiconductor triodes (transistors). Here it is convenient to consider the process of unlocking thyristors. A certain current is flowing through the thyristor control electrode. In this case, the current has a direct-directional bias. This current is considered basic for a transistor with the structure of n-p-n.

Therefore, in the collector, the current will be several times greater (it is necessary to multiply the value of the control current by the gain of the transistor). Further it can be seen that this current value is the base value for the second transistor with the conductivity structure pn, and it is unlocked. In this case, the collector current of the second transistor will be equal to the product of the gain factors of both transistors and the initially set control current. Triacs (the principle of operation and management of them are discussed in the article) have similar properties.

Further, this current must be summed up with the previously set current of the control circuit. And it will turn out exactly the value that is necessary to maintain the first transistor in the unlocked state. In the case where the control current is very large, two transistors are saturated simultaneously. The internal OS continues to maintain its conductivity even when the initial current at the control electrode disappears. At the same time, a rather high current value is detected at the thyristor anode.

How to disconnect a thyristor

The transition to the locked state of the thyristor is possible if no signal is applied to the open-circuit control electrode. In this case, the current falls to a certain value, which is called the hypostatic current (or the current of confinement).

The thyristor will shut off also in the event that a disconnection occurs in the load circuit. Or when the voltage that is applied to the circuit (external) changes its polarity. This occurs at the end of each half-cycle in the case when the circuit is powered by an AC source.

When the thyristor operates in a DC circuit, the locking can be done with a simple switch or a mechanical type button. It is connected to the load in series and used to de-energize the circuit. Similarly, the principle of the power regulator on a triac, however, there are some features in the circuit.

Methods for switching off thyristors

But it is possible to connect the switch in parallel, then it shunts the current of the anode, and the thyristor is put into a locked state. Some types of thyristors can be switched on again if the switch contacts are opened. This can be explained by the fact that during the opening of the contacts, the parasitic capacitors of thyristor transitions accumulate a charge, thereby creating interference.

Therefore, it is desirable to position the switch so that it is located between the cathode and the control electrode. This will ensure that the thyristor turns off normally, and the holding current is cut off. Sometimes, for convenience and speed and reliability, an auxiliary thyristor is used instead of a mechanical key. It should be noted that the work of the triac is in many respects similar to the functioning of thyristors.

Triacs

And now closer to the topic of the article - you need to consider a special case of a thyristor - a triac. The principle of its operation is similar to that which was considered earlier. But there are some differences and characteristics. Therefore, we need to talk about it in more detail. The triac is an instrument based on a semiconductor crystal. It is often used in systems that operate on alternating current.

The simplest definition of this device is a switch, but controllable. In the locked state, it works just like a switch with open contacts. When a signal is applied to the control electrode of the triac, the device switches to the open state (conductivity mode). When operating in this mode, it is possible to draw a parallel with a switch whose contacts are closed.

When there is no signal in the control circuit, in any of the half-cycles (when operating in alternating current circuits), the triac switches from open to closed. Triacs are widely used in relay mode (for example, in the construction of light-sensitive switches or thermostats). But they are also often used in control systems that operate according to the principles of phase control of voltage on the load (they are smooth regulators).

Structure and principle of operation of the triac

A triac is nothing more than a symmetric thyristor. Therefore, based on the name, we can conclude - it is easy to replace it with two thyristors, which are turned on and off in parallel. In any direction, it is able to pass current. The triac has three main outputs - the control, for signaling, and the main (anode, cathode) so that it can pass working currents.

The triac (the principle of operation for the "dummies" of this semiconductor element is given to your attention) is opened when the minimum required current value is supplied to the control terminal. Or in the case when the difference of potentials between the two other electrodes is higher than the maximum permissible value.

In most cases, exceeding the voltage causes the triac to operate spontaneously at the maximum amplitude of the supply voltage. The transition to the locked state occurs when the polarity changes or when the operating current decreases to a level lower than the holding current.

How to unlock the triac

When feeding from an alternating current network, the operating modes change due to a change in the polarity of the voltage across the working electrodes. For this reason, depending on the polarity of the control current, four types of this procedure can be distinguished.

Suppose voltage is applied between working electrodes. And on the control electrode, the voltage across the sign is opposite to that which is applied to the anode circuit. In this case, the triac will move along the quadrant - the principle of operation, as you can see, is quite simple.

There are 4 quadrants, and for each of them a current of unlocking, holding, inclusions is defined. The opening current must be retained until that time, until it exceeds by several times (in 2-3) the value of the holding current. This is precisely the current of the triac switching-the minimum required current of unlocking. If you get rid of the current in the control circuit, the triac will be in a conducting state. And it will work in this mode until then, while the current in the anode circuit will be greater than the retention current.

What are the limitations of using triacs

It is difficult to use when the load is of inductive type. The rate of change in voltage and current is limited. When a triac goes from a locked mode to an open one, a significant current appears in the external circuit. The voltage does not fall instantly on the power terminals of the triac. And the power will instantly develop and reach quite large values. The energy that dissipates, due to small space, sharply increases the temperature of the semiconductor.

If the critical value is exceeded, the crystal is destroyed, due to an excessively rapid increase in the current strength. If the triac, which is in the locked state, apply some voltage and increase it dramatically, the channel will open (if there is no signal in the control circuit). This phenomenon can be observed because the charge accumulates in the internal parasitic capacitance of the semiconductor. And the charge current has enough value to unlock the triac.