Triac characteristics

What are the characteristics of a TRIAC? What is the difference between TRIAC and thyristor? How does a TRIAC conduct current?

Like the SCR, the triac has a number of electrical ratings, the most significant being: 1. Blocking voltage (VDRM)—the maximum voltage that the triac can block in either direction when the gate current is. Maximum on state current (IT (RMS))—the maximum continuous RMS value of current that the.

On-state current – Amperes. Holding current, I H – Milli Amperes. As we know that the SCR as a unidirectional device and has a reverse blocking characteristics that prevents the current flow in reverse biased condition. But for many applications, bidirectional control of current is require particularly in AC circuits. To achieve this with SCRs, two SCRs must be connected in anti-parallel to control over both positive and negative half cycles of the input.

However, this structure can be replaced by special semiconductor device known as a TRIAC to accomplish the bidirectional control. These are often used in motor speed controllers, AC circuits, pressure control systems, light dimmers and other AC control equipments. See full list on electronicshub.

The triac is an important member of the thyristor family of devices. TRI means that the device consisting of three terminals and AC means that it controls the AC power or it can conduct in both directions of alternating current. TRIAC is an abbreviation for a TRIode AC switch. The triac has three terminals namely Main Terminal 1(MT1), Main Terminal (MT2) and Gate (G) as shown in figure.

If MTis forward biased with respect to MT then the current flows from MTto MT2. Similarly, if the MTis forward biased with respect to MT then the current flows from MTto MT1. The above two conditions are achieved whenever the gate is triggered with an appropriate gate pulse.

Similar to the SCR, triac is also turned by injecting appropriate current pu. A triac is a five layer, three terminal semiconductor device. The terminals are marked as MT MTas anode and cathode terminals in case of SCR.

And the gate is represented as G similar to the thyristor. The gate terminal is connected to both Nand Pregions by a metallic contact and it is near to the MTterminal. The terminal MTis connected to both Nand Pregions, while MTis connected to both Nand Pregions.

Hence, the terminals MTand MTconnected to both P and N regions of the device and thus the polarity of applied voltage between these two terminals decides the current flow through the layers of the device. With the gate open, MTis made positive with respect to MTfor a forward biased traic. Similarly for a reverse biased triac , MTis made negative with respect to MTwith gate open.

Until the voltage across the triac is less than the. The four possible electrode potential combinations which make the triac to operate four different operating quadrants or modes are given as.

MTis positive with respect to MTwith a gate polarity positive with respect to MT1. MTis negative with respect to MTwith a gate polarity negative with respect to MT1. In general, latching current is higher in second quadrant or mode whilst gate trigger current is higher in the fourth mode compared with other modes for any triac. Most of the applications, negative triggering current circuit is used that means and quadrants are used for a reliable triggering in bidirectional control and also when the gate sensitiv. The traic function like a two thyristors connected in anti-parallel and hence the VI characteristics of triac in the 1st and 3rd quadrants will be similar to the VI characteristics of a thyristors.

A small leakage current flows through the device provided that voltage across the device is lower than the breakover voltage. Once the breakover voltage of the device is reache then the triac turns ON as shown in below figure. However, it is also possible to turn ON the triac below the VBO by applying a gate pulse in such that the current through the device should be more than the latching current of the triac. Similarly, when the terminal MTis made negative with respect to MT the traic is in reverse blocking mode.

Hence the positive or negative pulse to. As compared with the anti-parallel thyristor configuration which requires two heat sinks of slightly smaller size, a triac needs a single heat sink of slightly larger size. In DC applications, SCRs are required to be connected with a parallel diode to protect against reverse voltage. But the triac may work without a diode, a safe breakdown is possible in either direction.

Due to the bidirectional control of AC, triacs are used as AC power controllers, fan controllers, heater controllers, triggering devices for SCRs, three position static switch, light dimmers, etc. The TRIAC Operation and Characteristics is defined as operating in one of the four quadrants : I, II , III , or IV. We may say that a triac is a diac with a gate. Its performance in each direction is similar to a controllable diac, and it is used for AC electricity. Its terminals are called MTand MT, or anode and anode , similar to a diac.

The schematics of its structure and its symbol are shown in Figure 4. Operating characteristics of triac in the 1st and 3rd quadrants are similar but for the direction of flow of current and applied voltage. The V-I characteristics of triac in the first and third quadrants are basically equal to those of an SCR in the first quadrant. The triac can be operated with either positive or negative gate control voltage but in normal operation usually the gate voltage is positive in Ist quadrant and negative in IIIrd quadrant. We now know that a “triac” is a 4-layer, PNPN in the positive direction and a NPNP in the negative direction, three-terminal bidirectional device that blocks current in its “OFF” state acting like an open-circuit switch, but unlike a conventional thyristor, the triac can conduct current in either direction when triggered by a single gate pulse. Then a triac has four possible triggering modes of operation as follows.

This makes the triac ideal to control a lamp or AC motor load with a very basic triac switching circuit given below. Another common type of triac switching circuit uses phase control to vary the amount of voltage, and therefore power applied to a loa in this case a motor, for both the positive and negative halves of the input waveform. This type of AC motor speed control gives a fully variable and linear control because the voltage can be adjusted from zero to the full applied voltage as shown. Triacs are widely used in AC power control applications. This makes triac circuits ideal for use in a variety of applications where power switching is needed.

One particular use of triac circuits is in light dimmers for domestic lighting, and they are also used in many other power control situations including motor control. As a result of their performance, trials tend to be used for low to medium power applications, leaving thyristors to be used for the very heat duty AC power switching applications. They are often used in low to medium power AC switching requirements. The basic triac symbol used on circuit diagram indicates its bi-directional properties. Like a thyristor, a triac has three terminals.

However the names of these are a little more difficult to assign, because the main current carrying terminals are connected to what is effectively a cathode of one thyristor, and the anode of another within the overall device. There is a gate which acts as a trigger to turn the device on. In addition to this the other terminals are both called Anodes, or Main Terminals These are usually designated Anode and Anode or Main Terminal and Main Terminal (MTand MT2). When using triacs it is both MTand MThave very similar properties. Before looking at how a triac works, it helps to have an understanding of how a thyristor works.

In this way the basic concepts can be grasped for the simpler device and then applied to a triac which is more complicated. For the operation of the triac , it can be imagined from the circuit symbol that the triac consists of two thyristors in parallel but around different ways. The operation of the triac can be looked on in this fashion, although the actual operation at the semiconductor level is rather more complicated. The triac structure is shown below and it can be seen that there are several areas of n-type and p-type material that form what is effectively a pair of back to back thyristors. It can conduct current irrespective of the voltage polarity of terminals MTand MT2.

It can also be triggered by either positive or negative gate currents, irrespective of the polarity of the MTcurrent. The typical IV characteristic of a triac can be seen in the diagram below with the four different quadrants labelled. Although these devices operate very well, to get the best performance out of them it is necessary to understand a few hints on tips on using triacs.

It is found that because of their internal construction and the slight differences between the two halves, triacs do not fire symmetrically. This in harmonics being generated: the less symmetrical the triac fires, the greater the level of harmonics that are produced. It is not normally desirable to have high levels of harmonics in a power system and as a result triacs are not favoured for high power systems. To help in overcoming the problem of the triac non-symmetrical firing, and the resulting harmonics, a device known as a diac (diode AC switch) is often placed in series with the gate of the triac.

The inclusion of this device helps make the switching more even for both halves of the cycle. This from the fact that the diac switching characteristic is far more even than that of the triac. Since the diac prevents any gate current flowing until the trigger voltage has reached a certain voltage in either direction, this makes the firing point of the triac more even in both directions. Triacs have many specifications that are very similar to those of thyristors, although obviously they are intended for triac operation on both halves of a cycle and need to be interpreted as such.

However as their operation is very similar, so too are the basic specification types. Parameters like the gate triggering current, repetitive peak off-state voltage and the like are all required when designing a triac circuit, ensuring there is sufficient margin for the circuit to operate reliably. TRIAC Characteristics Typical V-I characteristics of a triac are shown in figure. The triac has on and off state characteristics similar to SCR but now the char acteristic is applicable to both positive and negative voltages.

This is expected because triac consists of two SCRs connected in parallel but opposite in direc tions. Being a solid state device, thyristors can be used to control lamps, motors, or heaters etc. However, one of the problems of using a thyristor for controlling such circuits is that like a diode, the “thyristor” is a unidirectional device, meaning that it passes current in one direction only, from Anode to Cathode.

If you are completely news to Thyristors like SCR you can check the Introduction to SCR article. Therefore the triac can conduct and be controlled during both positive and negative half cycles of the mains waveform. Tri’ – indicated that the device has three terminalsand ‘ac’ means that the device controls alternating current or can conduct current in either direction.

Experiment Name: Study of V- I Characteristics of TRIAC. AiTo study the V-I characteristics of TRIAC. Apparatus Required: 1. Inductive loads have a tendency to deliver a “back current” when they turn on.

Triac is an abbreviation for triode a. This back current is like a voltage spike coming through the system. This could be dangerous to output relays.