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Thyristors and triacs

Thyristors are controllable multi-layer diodes. They are semiconductors and have a non-linear volt-ampere characteristic with two stable states – of low and high conductivity. Thyristors are key elements in heavy-current engineering. They are used as a substitute for various contactors and relays in trigger circuits, regulators, rectifiers, inverters, automatic devices, etc., because they lack switching parts and that is why they have higher speed and longer life. Their name is derived from the combination of the names of two elements: thyratron and transistor. Thyristors can be low power, medium power and high power.

High-power thyristors are used as electronic switches for switching voltages of the order of 500 to 1000 volts and currents in the range of 50 to 500 amps. Here it is used the property of the thyristor to switch very high currents in the anode circuit by very low power of the control signal. Thus thyristors can switch quickly and reliably circuits with high voltages and currents.
The main parameters of thyristors are: direct current, control current, maximum reverse voltage and control voltage. A thyristor can be triggered by positive input current only if its anode voltage is positive. When the anode voltage is negative, the thyristor remains blocked. The triggered thyristor has unidirectional, not bidirectional conductivity, just like a diode.


Structure of a thyristor

A thyristor has four semiconductor regions P1-N1-P2-N2 and three junctions. The terminal junctions are called emitters and the central junction is collector. The electrode connected to the P1 region is called anode and the one, which is connected to the N2 region – cathode. The control electrode is connected to the P2 region. Regions N1 and P2 are called bases and the terminal junctions P1 and N2 are called emitters. Thyristors are usually made of silicon Si, and the technology is alloy, diffuse or planar.

tiristor tablet
When voltage is supplied between the anode and the cathode and junctions P1 – N1 and P2 – N2 are directly connected, there is leakage current from the reversely connected junction N1 – P2. If we increase the voltage, the leakage current also increases and triggers a transistor effect through layer P2.

Due to the great variety of thyristors, different types of housings are used. Thus, for example, high-power thyristors are sealed in metal housings with a screw and metal-ceramic disk housings. The semiconductor structure is not sealed to the base but is pressed through springs, thus realizing the mechanically-electrical contact with the external outlets. This leads to high reliability, as the linear expansions in the various materials are compensated.

Thyristors are cooled through artificial and natural convection.  Aluminum or copper cooling-air jackets, which dissipate the heat, are often mounted to thyristors. The most powerful thyristors have water-cooling.
There are thyristors with a control electrode. They are called three-electrode thyristors, trinistors or just thyristors. There are also thyristors which have no control electrode and they are called diode thyristors or dinistors, but they are not widely used.

A symistor (symmetrical triode thyristor) or triac (triode for alternating current) is a semiconductor component, a variety of a thyristor and is used for switching in AC circuits. In electronics it is often referred to as a controllable switch. Unlike a thyristor, which has an anode and a cathode, the main (power) terminals of a triac can’t be called anode and cathode, because in the process of work they swap their positions (the anode becomes a cathode and vice versa).
Typical of a triac is that when triggered, it has conductivity in both directions. Moreover, unlike thyristors, it is not necessary to continuously supply signal to the control electrode in order to maintain the triac turned on.


The triac remains turned on until the main current drops below a certain level, called holding current. The symistor was invented by Valentin Vasilenko on 22 June 1963. It has five-layer semiconductor structure, i.e. 5 PN-junctions.

simistor shema
In simple words, a symistor can be represented as two triode thyristors (trinistors) connected in opposite parallel, but controlling a symistor differs from controlling two trinistors connected in opposite parallel. The volt-ampere characteristic of a triacs is symmetrical to the origin of the coordinate system.
To trigger a triac, there has to be voltage supply to its control electrode in terms of its conditional cathode. The polarity of the supplied voltage must be either negative or match the polarity of the conditional anode. Therefore the method of controlling a triac is such that the signal of the control electrode is fed to the conditional anode through a current-limiting resistor or a circuit-breaker. It is often convenient to control a triac by setting the control electrode to certain current magnitude, sufficient to trigger the triac.


The usаge of triacs has certain limitations. One of them is associated with inductive loads. The restriction concerns the speed of voltage alteration dU/dt between the main electrodes of the triacs and the speed of alteration of the operating current di/dt. The acceleration of the voltage alteration and the magnitude of this voltage can lead to undesired triggering of the triac. The acceleration of the increase of the current between the main electrodes, as well as the magnitude of this current can cause burning-out of the triac. There are other parameters that restrict the use of triacs in accordance with the limits of the permitted modes of operation. These parameters include the current and voltage of the control electrode, the temperature of the housing, the dissipated power per unit time, etc. The danger of accelerating the increase of the current is the following. Thanks to the profound positive feedback, the triggering of the triac goes off like a snowball. Nevertheless, this process still lasts a few microseconds during which the triac is fed with both large current and large voltage. Therefore the instantaneous power of the triac when triggered can be extremely high. This is accompanied by the release of heat that cannot dissipate for such a short time and can lead to overheating and damage of the crystal.


One way to protect the triac from the voltage wave, when working with inductive loads, is to switch a varistor in parallel with the main outlets of the triacs. An analogically switched RC circuit serves to prevent the acceleration of voltage alteration. The protection of the triac from exceeding the speed of increasing the current depends on the internal resistance and the inductance of the power source and the load.

The bi-directionality of symistors makes them widely used in contactless regulators for AC current, for controlling low-power inductive loads, in dimmable lighting fixtures, etc.

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