What is a triac, how does it work and what is it for

Triac is a semiconductor device. Its full name is a symmetric triode thyristor. Its feature is that it is possible to conduct current in both directions. This circuit element has three outputs: one is the control, and the other two are power. In this article we will consider the principle of operation, the device and the purpose of the triac in various circuits of electrical appliances.

Design and principle of operation

A feature of the triac is the bi-directional conductivity of the electric current flowing through the device. The design of the device is based on the use of two counter-parallel thyristors with common control. This principle of operation gave the name from the abbreviation "symmetric thyristors". Since electric current can flow in both directions, it makes no sense to designate power outputs as anode and cathode. The control electrode complements the overall picture.

The symbol on the diagram according to GOST:

The appearance is as follows:

There are five transitions in the triac that allow you to organize two structures. Which one will be used depends on the place of formation (specific power output) of negative polarity.

How does a triac work? Initially, the semiconductor device is in a locked state and current does not pass through it. When a current is applied to the control electrode, the latter goes into the open state and the triac starts passing current through itself. When using AC power, the polarity of the contacts is constantly changing. The scheme where the element in question is used will work without problems. After all, current is passed in both directions. In order for the triac to perform its functions, a current pulse is applied to the control electrode, after taking the pulse, the current through the conditional anode and cathode continues to flow until the circuit is broken or they are under reverse polarity voltage.

When used in an alternating current circuit, the triac closes on the inverse half-wave of a sinusoid, then you need to apply a pulse of opposite polarity (the same one under which the "power" electrodes of the element are located).

The operating principle of the control system can be adjusted depending on the specific case and application. After opening and the beginning of the flow, it is not necessary to supply current to the control electrode. The power circuit will not break. If it is necessary to turn off the power, lower the current in the circuit below the level of the holding value or short-circuit the power supply circuit.

Control signals

To achieve the desired result with the triac use not voltage, but current. To open the device, it must be at a certain small level. For each triac, the strength of the control current can be different, it can be found from the datasheet for a specific element.For example, for the KU208 triac this current should be more than 160 mA, and for the KU201 — at least 70 mA.

The polarity of the control signal must match the polarity of the conditional anode. To control the triac, they often use a switch and a current limiting resistorif it is controlled by a microcontroller, it may be necessary to install an additional transistor so as not to burn the MK output, or use a triac optical driver, such as MOC3041 and the like.

Four-quadrant triacs can be unlocked by a signal with any polarity. There is a drawback to this advantage - an increased control current may be required.

In the absence of the device is replaced by two thyristors. In this case, you should correctly select their parameters and redo the control circuit. After all, the signal will be fed to two control pins.

Advantages and disadvantages

What is the semiconductor device in question for? The most popular use case is switching in AC circuits. In this regard, the triac is very convenient - using a small element, you can provide control of high-voltage power.

Solutions are popular when they replace the usual electromechanical relay. The advantage of this solution is that there is no physical contact, due to which the power supply becomes more reliable, the switching is silent, the resource is orders of magnitude greater, and the speed is higher. Another advantage of the triac is its relatively low price, which, together with the high reliability of the circuit and the mean time between failures, looks attractive.

The developers failed to completely avoid the minuses. So, the devices heat up very much under load. It is necessary to provide heat dissipation. Powerful (or "power") triacs are installed on radiators. Another drawback that affects the use is the creation of harmonic electrical interference some circuits of triac regulators (for example, a household dimmer for adjusting the illumination).

Note that the voltage on the load will differ from the sine wave, which is associated with the minimum voltage and current at which inclusion is possible. Because of this, only a load that does not impose high power requirements should be connected. When setting the task to achieve a sine wave, this switching method will not work. Triacs are highly susceptible to noise, transients and interference. High switching frequencies are also not supported.

Application area

Characteristics, low cost and simplicity of the device allows the successful use of triacs in industry and everyday life. They can be found:

1. In the washing machine.
2. In the oven.
3. In the oven.
4. In an electric motor.
5. In rotary hammers and drills.
6. In the dishwasher.
7. In dimmers.
8. In a vacuum cleaner.

On this list, where this semiconductor device is used, is not limited. The use of the conductor device in question is carried out in almost all electrical appliances that only exists in the house. It is entrusted with the function of controlling the rotation of the drive motor in washing machines, they are used on the control board to start the operation of various devices - it is easier to say where they are not.

Main characteristics

Consider a semiconductor device designed to control circuits. Regardless of where it is used in the circuit, the following characteristics of triacs are important:

1. The maximum voltage. The indicator, which, being achieved on power electrodes, will not cause, in theory, failure. In fact, it is the maximum allowable value subject to the temperature range. Be careful - even a short-term excess can result in the destruction of this element of the chain.
2. The maximum short-term pulse current in the open state. The peak value and the valid period for it, indicated in milliseconds.
3. Working temperature range.
4. Triggering control voltage (corresponds to the minimum constant triggering current).
5. On time.
6. The minimum direct current control required to turn on the device.
7. Maximum repetitive surge voltage when closed. This parameter is always indicated in the accompanying documentation. Indicates the critical voltage limit for this instrument.
8. The maximum voltage drop across the triac in the open state. Indicates the voltage limit that can be set between the power electrodes when open.
9. The critical slew rate of current in open state and voltage in closed state. Indicated respectively in amperes and volts per second. Exceeding the recommended values ​​can lead to a breakdown or erroneous opening out of place. It is necessary to ensure working conditions to comply with the recommended standards and to exclude interference in which the dynamics exceeds a given parameter.
10. Triac body. It is important for thermal calculations and affects the power dissipation.

So we examined what a triac is, what it is responsible for, where it is used and what characteristics it has. The theoretical basics considered in simple language will help lay the foundation for future productive activities. We hope the information provided was useful and interesting for you!