Схема включения симистора: принцип работы, проверка и включение, схемы

Симистор (триак) — описание, принцип работы, свойства и характеристики

Справочные данные популярных отечественные симисторов и зарубежных
триаков. Простейшие схемы симисторных регуляторов мощности.

Ну что ж! На предыдущей странице мы достаточно плотно обсудили свойства и характеристики полупроводникового прибора под названием тиристор, неуважительно обозвали его «довольно архаичным», пришло время выдвигать внятную альтернативу.
Симистор пришёл на смену рабочей лошадке-тиристору и практически полностью заменил его в электроцепях переменного тока.
История создания симистора также не нова и приходится на 1960-е годы, причём изобретён и запатентован он был в СССР группой товарищей из Мордовского радиотехнического института.

Итак:
Симистор, он же триак, он же симметричный триодный тиристор — это полупроводниковый прибор, являющийся разновидностью тиристора, но, в отличие от него, способный пропускать ток в двух направлениях и используемый для коммутации нагрузки в цепях переменного тока.

Рис.1

На Рис.1 слева направо приведены: топологическая структура симистора, далее расхожая, но весьма условная, эквивалентная схема, выполненная на двух тиристорах и, наконец, изображение симистора на принципиальных схемах.
МТ1 и МТ2 — это силовые выводы, которые могут обозначаться, как Т1&Т2; ТЕ1&ТЕ2; А1&А2; катод&анод. Управляющий электрод, как правило, обозначается латинской G либо русской У.

Глядя на эквивалентную схему, может возникнуть иллюзия, что симистор относительно горизонтальной оси является элементом абсолютно симметричным, что даёт возможность как угодно крутить его вокруг управляющего электрода. Это не верно!!!
Точно так же, как у тиристора, напряжение на управляющий электрод симистора должно подаваться относительно условного катода (МТ1, Т1, ТЕ1, А1).
Иногда производитель может обозначать цифрой 1 «анодный» вывод, цифрой 2 — «катодный», поэтому всегда важно придерживаться обозначений, приведённых в паспортных характеристиках на прибор.

Полярность открывающего напряжения должна быть либо отрицательной для обеих полярностей напряжения на условном аноде, либо совпадать с полярностью «анодного» напряжения (т.е. быть плюсовой в момент прохождения положительной полуволны и минусовой — в момент прохождения отрицательной).

Приведём вольт-амперную характеристику тиристора и схему, реализующую самый простой способ управления симисторами — подачу на управляющий электрод прибора постоянного тока с величиной, необходимой для его включения (Рис.2).

Рис.2

Огромным плюсом симистора перед тиристором является возможность в штатном режиме работать с разнополярными полупериодами сетевого напряжения. Вольт-амперная характеристика является симметричной, надобности в выпрямительном мосте — никакой, схема получается проще, но главное — исключается элемент (выпрямитель), на котором вхолостую рассеивается около 50% мощности.

Давайте рассмотрим работу симистора при подаче на его управляющий вход постоянного тока отрицательной полярности (Рис.2 справа), ведь мы помним, что именно такая полярность открывающего напряжения является универсальной и для положительных, и для отрицательных полупериодов напряжения сети. На самом деле, всё происходит абсолютно аналогично описанной на предыдущей странице работе тиристора.
Повторим пройденный материал.

1. Для начала рассмотрим случай, когда управляющий электрод симистора отключен (S1 на схеме разомкнут, Iу на ВАХ равен 0). Тока через нагрузку нет (участки III на ВАХ), симистор закрыт, и для того, чтобы его открыть, необходимо поднять напряжение на «аноде» симистора настолько, чтобы возник лавинный пробой p-n-переходов полупроводника.
Оговоримся — зафиксировать нам этот процесс не удастся, потому что величина этого напряжения составляет несколько сотен вольт и, как правило, превышает амплитудное значение напряжения сети.
Тем не менее — при достижении этого уровня напряжения (точки II на ВАХ) симистор отпирается, падение напряжения между силовыми выводами падает до единиц вольт, нагрузка подключается к сети — наступает рабочий режим открытого симистора (участки I на ВАХ).
Чтобы закрыть симистор, нужно снизить протекающий через нагрузку ток (или напряжение на «аноде») ниже тока удержания.

2. Для того чтобы снизить величину напряжения включения симистора, следует замкнуть S1 и, тем самым, подать на управляющий электрод ток, задаваемый значением переменного резистора R1. Чем больше ток Iу, тем при меньшем анодном напряжении происходит переключение симистора в проводящее состояние.
А при какой-то величине тока управляющего электрода, называемой током спрямления (на ВАХ не показано), горба на характеристике вообще не будет, и напряжение открывания симистора составит незначительную величину, исчисляемую единицами вольт.
Абсолютно так же, как и в прошлом пункте, чтобы закрыть симистор, необходимо снизить протекающий через нагрузку ток (или напряжение на «аноде») ниже значения тока удержания.

То бишь — всё полностью аналогично тиристору. Для открывания симистора следует подать на управляющий электрод прибора постоянный ток с величиной, необходимой для его включения, для закрывания — снизить протекающий через нагрузку ток (или напряжение на «аноде») ниже значения тока удержания.
Т.е. в нашем случае, представленном на Рис.2 — симистор будет открываться при замыкании S1 в каждый момент превышения «анодным» напряжением некоторого значения, зависящего от номинала R1, а закрываться с каждым полупериодом сетевого напряжения в момент приближения его уровня к нулевому значению.

Описанный выше способ управления симистором посредством подачи на управляющий электрод постоянного напряжения обладает существенным недостатком — требуется довольно большой ток (а соответственно и мощность) управляющего сигнала (по паспорту — до 250мА для КУ208). Поэтому в большинстве случаев для управления симисторами используется импульсный метод, либо метод, при котором открытый симистор шунтирует цепь управления, не допуская бесполезного рассеивания мощности на её элементах.

В качестве примера рассмотрим простейшую, но вполне себе работоспособную схему симисторного регулятора мощности, позволяющего работать с нагрузками вплоть до 2000 Вт.

Рис.3

Как можно увидеть, на схеме помимо симистора VS2 присутствует малопонятный элемент VS1 — динистор. Для интересующихся отмечу — на странице ссылка на страницу мы подробно обсудили принцип работы, свойства и характеристики приборов данного типа.

А теперь — как это всё работает?
В начале действия положительного полупериода симистор закрыт. По мере увеличения сетевого напряжения конденсатор С1 заряжается через последовательно соединённые резисторы R1 и R2. Причём увеличение напряжения на конденсаторе С1 отстаёт (сдвигается по фазе) от сетевого на величину, зависящую от суммарного сопротивления резисторов и номинала ёмкости С1. Чем выше значения резисторов и конденсатора — тем больше сдвиг по фазе.
Заряд конденсатора продолжается до тех пор, пока напряжение на нём не достигнет порога пробоя динистора (около 35 В). Как только динистор откроется (следовательно, откроется и симистор), через нагрузку потечёт ток, определяемый суммарным сопротивлением открытого симистора и нагрузки.
При этом симистор остаётся открытым до конца полупериода, т.е. момента, когда полуволна сетевого напряжения приблизится к нулевому уровню.
Переменным резистором R2 устанавливают момент открывания динистора и симистора, производя тем самым регулировку мощности, подводимой к нагрузке.

При действии отрицательной полуволны принцип работы устройства аналогичен.

Диаграммы напряжения на нагрузке при различных значениях переменного резистора приведены на Рис.3 справа.

Для предотвращения ложных срабатываний триаков, вызванных переходными процессами в индуктивных нагрузках (например, в электродвигателях), симисторы должны иметь дополнительные компоненты защиты. Это, как правило, демпферная RC-цепочка (снабберная цепь) между силовыми электродами триака, которая используется для ограничения скорости изменения напряжения (на схеме Рис.3 показана синим цветом).
В некоторых случаях, когда нагрузка имеет ярко выраженный ёмкостной характер, между силовыми электродами необходима индуктивность для ограничения скорости изменения тока при коммутации.

А под занавес приведём основные характеристики отечественных симисторов и зарубежных триаков.

Тип	U макс, В	I max, А	Iу отп, мА
КУ208Г	400	5
BT 131-600	600	1
BT 134-500	500	4
BT 134-600	600	4
BT 134-600D	600	4
BT 136-500Е	500	4
BT 136-600Е	600	4
BT 137-600Е	600	8
BT 138-600	600	12
BT 138-800	800	12
BT 139-500	500	16
BT 139-600	600	16
BT 139-800	800	16
BTA 140-600	600	25
BTF 140-800	800	25
BT 151-650R	650	12
BT 151-800R	800	12
BT 169D	400	12
BTA/BTB 04-600S	600	4
BTA/BTB 06-600C	600	6
BTA/BTB 08-600B	600	8
BTA/BTB 08-600C	600	8
BTA/BTB 10-600B	600	10
BTA/BTB 12-600B	600	12
BTA/BTB 12-600C	600	12
BTA/BTB 12-800B	800	12
BTA/BTB 12-800C	800	12
BTA/BTB 16-600B	600	16
BTA/BTB 16-600C	600	16
BTA/BTB 16-600S	600	16
BTA/BTB 16-800B	800	16
BTA/BTB 16-800S	800	16
BTA/BTB 24-600B	600	25
BTA/BTB 24-600C	600	25
BTA/BTB 24-800B	800	25
BTA/BTB 25-600В	600	25
BTA/BTB 26-600A	600	25
BTA/BTB 26-600B	600	25
BTA/BTB 26-700B	700	25
BTA/BTB 26-800B	800	25
BTA/BTB 40-600B	600	40
BTA/BTB 40-800B	800	40
BTA/BTB 41-600B	600	41
BTA/BTB 41-800B	800	41
MAC8M	600	8
MAC8N	800	8
MAC9M	600	9
MAC9N	800	9
MAC12M	600	12
MAC12N	800	12
MAC15M	600	15
MAC12N	800	15

Симисторы с обозначение BTA отличаются от других наличием изолированного корпуса.
Падение напряжения на открытом симисторе составляет примерно 1-2 В и мало зависит от протекающего тока.

 

90000 What is TRIAC: Switching Circuit and Applications 90001 90002 Power electronic switches like BJT, SCR, IGBT, MOSFET, and TRIAC are very important components when it comes to switching circuits like 90003 DC-DC converters 90004, 90003 Motor Speed ​​Controllers 90004, 90003 Motor Drivers 90004, and 90003 Frequency Controllers 90004 etc. Each device has its own unique property and thus they have their own specific applications. In this tutorial we will learn about the 90003 TRIAC 90004, which is a bidirectional device meaning it can conduct in both the direction.Due to this property TRIAC is exclusively used where sinusoidal AC supply is involved. 90013 90002 90013 90016 90003 Introduction to TRIAC 90004 90019 90002 The term 90003 TRIAC 90004 stands for 90003 TRI 90004 ode for 90003 A 90004 lternating 90003 C 90004 urrent. It is a three terminal switching device similar to SCR (Thyristor) but it can conduct in both the directional since it construct by combining two SCR in anti-parallel state. The symbol and pin out of TRIAC is shown below.90013 90002 90031 90013 90002 90013 90002 Since the TRIAC is a bi-directional device the current can either flow from MT1 to MT2 or from MT2 to MT1 when the gate terminal is triggered. For a TRIAC this trigger voltage that is to be applied to the gate terminal can either be positive or negative with respect to terminal MT2. Thus this puts the TRIAC into four 90003 operating modes 90004 as listed below 90013 90039 90040 Positive Voltage at MT2 and positive pulse to gate (Quadrant 1) 90041 90040 Positive Voltage at MT2 and negative pulse to gate (Quadrant 2) 90041 90040 Negative Voltage at MT2 and positive pulse to gate (Quadrant 3) 90041 90040 Negative Voltage at MT2 and negative pulse to gate (Quadrant 4) 90041 90048 90002 90013 90016 90003 V-I Characteristics of a TRIAC 90004 90019 90002 The below picture illustrates the status of TRIAC in each quadrant.90013 90002 90058 90013 90002 90013 90002 The turn on and turn off characterises of the TRIAC can be understood by looking at the VI characterises graph of the TRIAC which is also shown in the above picture. Since the TRIAC is just a combination of two SCR in anti-parallel direction the V-I characteristics graph looks similar to that of an SCR. As you can see the TRIAC mostly operates in the 1 90063 st 90064 Quadrant and the 3 90063 rd 90064 Quadrant. 90013 90002 90013 90002 90003 Turn-On Characteristics 90004 90013 90002 To turn on a TRIAC a positive or negative gate voltage / pulse has to be supplied to the gate pin of the TRIAC.When triggered one of the two SCR inside, the TRIAC begins to conduct based on the polarity of the MT1 and MT2 terminals. If MT2 is positive and MT1 is negative the first SCR conducts and if the MT2 terminal is negative and MT1 is positive then second SCR conducts. This way either one of the SCR always stays on thus making the TRIAC ideal for AC applications. 90013 90002 The minimum voltage that has to be applied to gate pin to turn ON a TRIAC is called as 90003 the threshold gate voltage (V 90078 GT 90079) 90004 and the resulting current through the gate pin is called as the 90003 threshold gate current (I 90078 GT 90079).90004 Once this voltage is applied the gate pin the TRIAC gets forward biased and starts to conduct, the time taken for the TRIAC to change from off state to on state is called as 90003 turn-on time (t 90078 on 90079). 90004 90013 90002 Just like an SCR the TRIAC once turned on will remain turned on unless it is commutated. But for this condition the load current through the TRIAC should be greater than or equal to 90003 the latching current (I 90078 L 90079) 90004 of the TRIAC. So to conclude a TRIAC will remain turned on even after removing the gate pulse as long as the load current is greater than the value of latching current.90013 90002 Similar to latching current, there is another important value of current called holding current. The minimum value of current to keep the TRIAC in forward conduction mode is called as the 90003 holding current (I 90078 H 90079). 90004 A TRIAC will enter into continuous conduction mode only after passing though the holding current and the latching current as shown in the graph above. Also the value of Latching current of any TRIAC will always be greater than the value of the holding current.90013 90002 90013 90002 90003 Turn-off characteristics 90004 90013 90002 The process of turning off an TRIAC or any other power device is called as 90003 commutation 90004, and the circuit associated with it to perform the task is called as a commutational circuit. The most common method used to turn off a TRIAC is by reducing the load current though the TRIAC until it reaches below the value of holding current (I 90078 H 90079). This type of commutation is called as forced commutation in DC circuits.We will learn more about how a TRIAC is turned On and turned Off through it application circuits. 90013 90002 90013 90016 90003 TRIAC Applications 90004 90019 90002 TRIAC is very commonly used in places where AC power has to be controlled for example, it is used in the speed regulators of ceiling fans, AC bulb dimmer circuits etc. Let us look into a simple TRIAC switching circuit to understand how it works practically. 90013 90002 90123 90013 90002 90013 90002 Here 90003 we have used the TRIAC to turn On and off an AC load through a push button 90004.The mains power source is then wired to a small bulb through the TRIAC as shown above. When the switch is closed the phase voltage is applied to the gate pin of the TRIAC through the resistor R1. If this gate voltage is above the gate threshold voltage then a current flows through the gate pin, which will be greater than the gate threshold current. 90013 90002 At this condition the TRIAC enters forward bias and the load current will flow though the Bulb. If the loads consumes enough current the TRIAC enter into latching state.But since this is an AC power source the voltage will reach zero for every half cycle and thus the current will also reach zero momentarily. Hence latching is not possible in this circuit and the TRIAC will turn off as soon as the switch is opened and no commutation circuit is required here. This type of commutation of TRIAC is called as 90003 natural commutation 90004. Now let us build this circuit on a breadboard using the 90003 BT136 TRIAC 90004 and check how it works. 90013 90002 High caution is needed while working with AC power supplies the operating voltage is stepped down for safety purpose The standard AC power of 230V 50Hz (In India) is stepped down to 12V 50Hz using a transformer.A small bulb is connected as a load. The experimental set-up looks like this below when completed. 90013 90002 90140 90013 90002 90013 90002 When the button is pressed the gate pin receives the gate voltage and thus the TRIAC is turned ON. The bulb will glow as long as the button is held pressed. Once the button is released, the TRIAC will be in latched state, but since the input voltage is AC the current though the TRIAC will go below the holding current and thus the TRIAC will turn off, the complete working can also be found in the 90003 video given 90004 at the end of this tutorial.90013 90002 90013 90016 90003 TRIAC control using Microcontrollers 90004 90019 90002 When TRIACs are used as light dimmers or for Phase control application, the gate pulse that is supplied to the gate pin has to be controlled using a microcontroller. In that case the gate pin will also be isolated using an opto-coupler. The circuit diagram for the same is shown below. 90013 90002 90157 90013 90002 90013 90002 To control the TRIAC using a 5V / 3.3V signal we will use an 90003 opto-coupler like the MOC3021 90004 which has a TRIAC inside it.This TRIAC can be triggered by 5V / 3.3V through the Light Emitting Diode. Normally a PWM signal will be applied to the 1 90063 st 90064 pin of MOC3021 and the frequency and duty cycle of the PWM signal will be varied to get the desired output. This type of circuit is normally used for Lamp brightness control or motor speed control. 90013 90002 90013 90016 90003 Rate Effect — Snubber Circuits 90004 90019 90002 All TRIACs suffer from a problem called Rate Effect. That is when the MT1 terminal is subjected to sharp increase in voltage due to switching noise or transients or surges the TRIAC miss-interrupts it as a switching signal and turns ON automatically.This is because of the internal capacitance of present between the terminals MT1 and MT2. 90013 90002 The easiest way to overcome this problem is by using a Snubber circuit. In the above circuit, the Resistor R2 (50R) and the Capacitor C1 (10nF) together forms an RC network which acts as a Snubber circuit. Any peak voltages supplied to MT1 will be observed by this RC network. 90013 90002 90013 90016 90003 Backlash Effect 90004 90019 90002 Another common problem that will be faced by designers while using TRIAC is the Backlash effect.This problem occurs when a potentiometer is used for controlling the gate voltage of the TRIAC. When the POT is turned to minimum value, no voltage will be applied to gate pin and thus the Load will be turned off. But when the POT is turned to maximum value the TRIAC will not switch on because of the capacitance effect between the pins MT1 and MT2, this capacitor should find a path to discharge else it will not allow the TRIAC o turn ON. This effect is called as the Backlash effect. This problem can be rectified by simply introducing a resistor in series with switching circuit to provide a path for the capacitor to discharge.90013 90002 90013 90016 90003 Radio Frequency Interference (RFI) and TRIACs 90004 90019 90002 TRIAC switching circuits are more prone to Radio Frequency interference (EFI) because when the load is turned on, the current raises form 0A to maximum value all of a sudden thus creating a burst of electric pulses which causes Radio Frequency Interface. The larger the load current is the worse will be the interference. Using Suppressor circuits like an LC suppressor will solve this problem. 90013 90002 90013 90016 90003 TRIAC — Limitations 90004 90019 90002 When required to switch AC waveforms in both the directions obviously TRIAC will be the first choice since it is the only bi-directional power electronic switch.It acts just like two SCRs connected in back to back fashion and also share the same properties. Although while designing circuits using TRIAC the following limitations must be considered 90013 90039 90040 The TRIAC has two SCR structures inside it, one conducts during positive half and the other during negative half. But, they do not trigger symmetrically causing difference in the positive and negative half cycle of the output 90041 90040 Also since the switching is not symmetrical, it leads to high level harmonics which will induce noise in the circuit.90041 90040 This harmonics problem will also lead to Electro Magnetic Interference (EMI) 90041 90040 While using inductive loads, there is a huge risk of inrush current flowing towards the source, hence it should be ensured that TRIAC is turned off completely and the inductive load is discharged safely through an alternate path 90041 90048 90002 90212 90213 90013 .90000 What is a Triac — Triac Switch »Electronics Notes 90001 90002 Triacs are semiconductor devices that are widely used for switching medium power AC — their advantage is that they can switch both halves of alternating cycle. 90003 90004 90005 90006 Triac, Diac, SCR Tutorial Includes: 90007 90008 Thyristor basics Thyristor device structure Thyristor operation Gate turn off thyristor, GTO Thyristor specifications What is a triac Triac specifications Diac overview 90009 90004 90005 Triacs are electronic components that are widely used in AC power control applications.They are able to switch high voltages and high levels of current, and over both parts of an AC waveform. This makes triac circuits ideal for use in a variety of applications where power switching is needed. 90009 90005 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 and electronic switches. 90009 90005 As a result of their performance, triacs tend to be used for low to medium power electronic switching applications, leaving thyristors to be used for the very heat duty AC power switching applications.90009 90017 A medium current triac 90018 Triac basics 90019 90005 The triac is a development of the thyristor. While the thyristor can only control current over one half of the cycle, the triac controls it over two halves of an AC waveform. 90009 90005 As such the triac can be considered as a pair of parallel but opposite thyristors with the two gates connected together and the anode of one device connected to the cathode of the other, etc .. 90009 90024 Triac switching waveform 90005 The fact that the triac switching action occurs on both halves of an AC waveform means that for AC electronic switching applications, the complete cycle can be used.For basic thyristor circuits, only half the waveform is used and this means that basic circuits using thyristors will not utilise both halves of the cycle. Two devices are required to utilise both halves. However the triac only requires one device to control both halves of the AC waveform and in many respects it is an ideal solution for an electronic switch for AC. 90009 90018 Triac symbol 90019 90005 Like other electronic components, the triac has its own circuit symbol for use on circuit diagrams and this indicates its bi-directional properties.The triac symbol can be seen to be a couple of thyristor symbols in opposite senses merged together. 90009 90031 Triac circuit symbol 90005 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. 90009 90005 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 1 and Anode 2 or Main Terminal 1 and Main Terminal 2 (MT1 and MT2). When using triacs it is both MT1 and MT2 have very similar properties. 90009 90018 How does a triac work? 90019 90005 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 semiconductor device and then applied to a triac which is more complicated.90009 90005 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. 90009 90042 Triac equivalent circuit 90005 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.90009 90045 Triac basic structure 90005 The triac is able to conduct in a number of ways — more than the thyristor. It can conduct current irrespective of the voltage polarity of terminals MT1 and MT2. It can also be triggered by either positive or negative gate currents, irrespective of the polarity of the MT2 current. This means that there are four triggering modes or quadrants: 90009 90048 90049 90006 90051 I + Mode 90052 90007 MT2 current is + ve, gate current is + ve 90054 90049 90006 90051 I- Mode 90052 90007 MT2 current is + ve, gate current is -ve 90054 90049 90006 90051 III + Mode: 90052 90007 MT2 current is -ve, gate current is + ve 90054 90049 90006 90051 III- Mode: 90052 90007 MT2 current is -ve, gate current is -ve 90054 90073 90005 It is found that the triac trigger current sensitivity is greatest when the MT2 and gate currents are both of the same polarity, i.e. both positive or both negative. If the gate and MT2 currents are of the opposite polarity then the sensitivity is typically about half the value of when they are the same. 90009 90005 The typical IV characteristic of a triac can be seen in the diagram below with the four different quadrants labelled. 90009 90078 Triac IV characteristic 90018 Triac applications 90019 90005 Triacs are used in many applications. These electronic components are often used in low to medium power AC switching requirements.Where large levels of power need to be switched, two thyristors / SCRs tend to be used as they can be controlled more easily. 90009 90005 Nevertheless triacs are widely used in many applications: 90009 90048 90049 Lighting control — especially domestic dimmmers. 90054 90049 Control of fans and small motors. 90054 90049 Electronic switches for general AC switching and control 90054 90073 90005 There are naturally many other triac applications, but these are some of the most common.90009 90005 In one specific application, triacs can be included in modules called solid state relays. Here an optical version of this semiconductor device is activated by an LED light source turning the solid state relay on according to the input signal. 90009 90005 Typically within solid state relays, the LED light or infrared source and the optical triac are contained within the same package, sufficient isolation being provided to withstand high voltages which may extend to hundreds of volts or possibly even more.90009 90005 Solid state relays come in many forms, but those used for AC switching may use a triac. 90009 90018 Using triacs 90019 90005 There are a number of points to note when using triacs. Although these semiconductor 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. 90009 90005 It is found that because of their internal construction and the slight differences between the two halves, these electronic components do not fire symmetrically.This results 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. Instead for these systems two thyristors may be used as it is easier to control their firing. 90009 90005 To help in overcoming the problem of the triac non-symmetrical firing, and the resulting harmonics, another semiconductor device known as a diac (diode AC switch) is often placed in series with the gate of the triac.The inclusion of this semiconductor device helps make the switching more even for both halves of the cycle and thereby creating a more effective electronic switch. 90009 90005 This results 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. 90009 90111 Internal circuitry of triac light dimmer 90018 Triac circuit examples 90019 90005 There are many ways in which triacs can be used.The two examples below give a taste of what can be done with these semiconductor devices. 90009 90048 90049 90006 90051 Simple triac electronic switch circuit: 90052 90007 The triac can function as an electronic switch — it could enable a trigger pulse of a low power switch to turn the triac on to control a much higher power levels that might be possible with a simple switch. 90122 Simple triac switch circuit 90054 90049 90006 90051 Triac variable power or dimmer circuit: 90052 90007 One of the most popular triac circuits varies the phase on the input of the triac to control the power that can be dissipated into load.90008 90130 A basic triac circuit using phase of input waveform to control dissipated power in the load 90054 90073 90005 There are many more triac circuits that can be used. The device is very versatile and can be used in a variety of circuits, typically to provide various forms of AC switching. 90009 90135 Note on Triac Circuits & Design: 90136 90005 Triac circuits are able to switch both halves on an alternating waveform using single device and this makes them very attractive for use in many small to medium power AC switching circuits.90009 90005 Read more about 90006 90051 Triac Circuits & Design 90052 90007 90009 90018 Triac specifications 90019 90005 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. 90009 90005 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.90009 90005 Triacs are ideal devices for use in many AC small power applications. Triac circuits for use as dimmers and small electronic switches are widespread and they are simple and easy to implement. When using triacs, diacs are often included in the circuit as mentioned above to help reduce the level of harmonics produced. 90009 90005 90006 More Electronic Components: 90007 90008 Resistors Capacitors Inductors Quartz crystals Diodes Transistor Phototransistor FET Memory types Thyristor Connectors RF connectors Valves / Tubes Batteries Switches Relays 90008 90006 90051 Return to Components menu.. . 90052 90007 90009.90000 SPDT Relay Switch Circuit using Triac 90001 90002 An efficient solid state single pole double throw or SPDT switch can be built using triacs for replacing a mechanical SPDT. 90003 90002 The post details a simple solid state triac SPDT relay circuit, using an optocoupler and a couple of triacs, which can be used as an effective replacement for mechanical relays. The idea was requested by «Cypherbuster». 90003 90006 Introduction 90007 90002 In one of the other posts we learned how to make an DPDT SSR using mosfets, however this design could be used only for high current DC loads, and not with AC loads at the mains level.90003 90002 In this article we will see how a simple mains operated solid-state relay can be made using triacs and an optocoupler. 90003 90002 The working of any relay is specifically intended to operate two different high power loads individually and alternately with the help of an external isolated low power trigger. 90003 90002 In a conventional mechanical type of rely this is done by toggling the loads across its N / O and N / C contacts in response to the activation applied across its coil. 90003 90002 However mechanical relays have their own drawbacks such as higher degree of wear and tear, lower life, generation of RF disturbance due to sparks across the contacts, and the most vital being the delayed switching response which could be crucial in systems like UPS.90003 90006 Circuit Operation 90007 90002 In our triac SPDT relay circuit the same function is executed through the switching of two triacs via two BJT stages and an isolating optocoupler which ensures that the changeover operation for this relay has no drawbacks as mentioned above. 90003 90002 Referring to the diagram, the left side triac represents the N / O contact while the right side triac operates like the N / C contact. 90003 90024 Circuit Diagram 90025 90002 While the optocoupler is in the non-triggered mode, the BC547 directly associated with the opto goes into the triggered mode, which keeps the second BC547 switched OFF.This situation enables the right side triac to remain switched ON, and the other triac is held switched OFF. 90003 90002 In this condition any load connected with the right triac becomes operational and stays switched ON. 90003 90002 Now as soon as a trigger is applied to the opto coupler, it switches ON, and in turn switches OFF the connected BC547. 90003 90002 This situation switches ON the second BC547 and consequently the right side triac is switched OFF, ensuring that the left side triac is now switched ON.90003 90002 The above condition immediately toggles the second load ON and switches OFF the earlier load, effectively fulfilling the required alternate switching of the load with the help of an isolated external DC trigger. 90003 90002 The two LED connected with the bases of the two BJTs indicate which load is in the activated state at any moment while the triac SPDT relay circuit is being operated. 90003 90024 Adding an attached power supply and Delay Effect 90025 90002 The above design could be further enhanced and made fully independent of an external DC power source by upgrading it with its own transformerless power supply, as shown below: 90003 90002 You will find the following changes in this upgraded diagram: 90003 90002 Addition of a 1K at the base of the right BC547 to ensure correct triggering of the left side triac 90003 90002 Addition of R / C network across the gates of the triacs to ensure that the two triacs are never ON together at any given instance or during the changeover periods.The diodes can be 1N4148, resistors can be 22K or 33K, and the capacitors can be around 100uF / 25V. 90003 90002 There’s one more thing that seems to be missing in the diagram, and it is a limiting resistor (approximately 22 ohms) between the 12V zener diodes and the 0.33uF capacitor, this may be important to safeguard the zener diode from sudden in rush surge through the capacitor during power switch ON. 90003 90002 90051 Warning: The circuit shown above is not isolated from the mains AC input supply and therefore is extremely dangerous to touch in the switched ON condition.90052 90003 90054 About Swagatam 90055 90002 I am an electronic engineer (dipIETE), hobbyist, inventor, schematic / PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. 90057 If you have any circuit related query, you may interact through comments, I’ll be most happy to help! 90003.90000 Triac Circuits & Circuit Design »Electronics Notes 90001 90002 Unlike thyristors or SCRs, triac circuits are able to switch both halves of an alternating waveform, making them ideal for many AC control and switching applications. 90003 90004 90005 90006 Thyristor Circuit Design Includes: 90007 90008 Thyristor circuit design primer Circuit operation Triggering / firing circuit design Overvoltage crowbar Triac circuits 90009 90004 90005 There are many circuits in which triacs can be used — often they are used for relatively low power switching and control applications in situations like domestic light dimmers, small heating controls and the like.90009 90005 In these types of circuit the triac is a very useful device, enabling circuits to be designed using a minimum of components. 90009 90005 Triac circuits can be very straightforward requiring only a few components, and they are able to provide a good degree of control and switching, although they tend not to be used for high power levels where two discrete back to back thyristors provide better performance. 90009 90017 90018 Triac technology 90019 90005 The triac can be considered as being two thyristors or SCRs back to back to accommodate both halves of an AC waveform cycle.Being a single device, this has considerable advantages, especially for domestic products where costs are of paramount importance. 90009 90005 The triac has the property that when a trigger is applied to the gate, the device turns on and remains conducting until the voltage across the anodes or main terminals of the device falls below a certain value — nominally when the supply voltage falls to nearly zero . This condition occurs when an alternating waveform crosses the zero voltage line, an din this way the triac is able to control each half waveform.90009 90024 Triac switching waveform 90025 Note on Triac Component Technology: 90026 90005 Triacs can be considered as back to back thyristors, but being contained within one device, their technology and operation is a little more complicated. 90009 90005 Read more about 90006 90031 Triac Component Technology 90032 90007 90009 90018 Simple triac switch circuit 90019 90005 The triac can function as a switch — it could enable a trigger pulse of a low power switch to turn the triac on to control a much higher power levels that might be possible with a simple switch.90009 90039 Simple triac switch circuit 90005 In this circuit the resistor R1 may be 100R or more dependent upon the triac in question. 90009 90018 Triac variable power or dimmer circuit 90019 90005 One of the most popular triac circuits varies the phase on the input of the triac to control the power that can be dissipated into load. This is the form of circuit that is widely used in circuits for incandescent light dimmers in domestic applications. Sadly this simple circuit is not suitable for LEDs as it clips the leading edge of the waveform, and LEDs typically require the trailing edge to be cut.90009 90005 This circuit operates because the capacitor and resistor network requires time for the capacitor to charge — the waveform at the junction of the capacitor and resistor is effectively delayed and this delays the turn on for the triac in the circuit. As the triac turns on part way through the half of each cycle, this means that the overall power in the circuit is reduced. 90009 90048 A basic triac circuit using phase of input waveform to control dissipated power in the load 90005 Notice the diac placed in circuit next to the gate of the triac.This is required because the switching characteristics of triacs are not particularly symmetrical from one half waveform to the following half as detailed below. This results from the structure of the triac. 90009 90018 Issues with triac circuits 90019 90005 Triacs are not the complete solution to all AC switching requirements. Triacs have some issues when they are used in various circuits, and these must be accommodated when designing the circuits. 90009 90005 Some of get effects to be accommodated within the circuit design are mentioned below: 90009 90057 90058 90006 90031 dV / dt effect: 90032 90007 Triacs suffer from an issue sometimes called the rate effect or dV / dt effect.If either main terminal is subjected to a sharp change in voltage, exceeding the rated dV / dt rating, then can cause sufficient breakthrough to the gate to cause the triac to turn on. These transients can occur as a result of switching spikes or electrical discharges that are carried along the power lines. Another cause of transients can arise when driving inductive loads like motors. Here the line currents and voltages can be out of phase and under these circumstances large voltages can suddenly appear which are sufficient to exceed the triac dV / dt rating.It arises because the triac unlatches as its main terminal current falls to near-zero during each operating half-cycle. 90005 90064 A basic triac circuit with a transient snubber This issue can resolved to a large degree by adding a transient suppressor across the line — a resistor, R1, possibly around 100R and series capacitor, C2, possibly around 10 nF or 100nF dependent upon the installation. Beware that the capacitor must be able to handle the voltage (and current) and the resistor must be large enough to dissipate the required energy, especially that of the voltage spike.For normal 240 volt power lines, the capacitor should have a working voltage of at least 400 volts and preferably more. 90009 90066 90058 90006 90031 Backlash effect: 90032 90007 This effect encountered in some triac circuits results when a potentiometer and capacitor is used to control the gate voltage. 90005 It is found that if the potentiometer it turned to provide the minimum value, there is no leakage path for the triac MT1 to MT2 capacitance discharge, preventing the triac from turning on.The solution is to introduce a high value resistor to allow this capacitance to discharge. 90009 90066 90058 90006 90031 Non-symetrical firing: 90032 90007 Owing to the internal construction of trials, there are slight differences between the sections for covering the different half cycles. This leads to non-symmetrical firing of the triac and in turn this results in high levels of harmonics being generated which can be bad for EMC performance, etc. Although the action of the triac, even switching symmetrically will generate harmonics, the asymmetry will cause much higher levels to be generated giving rise to greater levels of interference.To help overcome this issue and provide a much more defined gate trigger signal for the triac circuit, a diac is normally placed in series with the gate. 90005 A diac is able to improve the performance of the triac circuit because its switching characteristic is far more even than that of the triac. Since the diac prevents any gate current flowing until the diac trigger voltage of about 35 volts has reached, this makes the firing point of the triac more equal for both polarities. 90009 90005 Some years ago, trials that incorporated discs within the package were developed and sold.However, for some reason, these were not a commercial success and have been discontinued. 90009 90066 90058 90006 90031 Harmonic filtering: 90032 90007 Any switching circuit that switches during the course of the waveform, like a triac will generate harmonics. This is even worse if the triggering in non-symmetrical. These harmonics can give rise to interference that may affect the sorption of other electronic equipment into e vicinity, especially if wireless communications are used.Although it is best to remove any harmonics at source for EMC, even when a diac is fitted there is likely to be the need for some filtering to remove the harmonics. 90005 90091 A basic triac circuit with a harmonic / interference filter For most triac circuits a simple LC filter will provide sufficient good filtering. A small series inductor, RFC1, and a capacitor, C2 across the triac will generally provide sufficient attenuation for many applications. A choke of around 100μH together with a 0.1μF capacitor will generally work adequately well. The choke should be able to withstand the current, and the capacitor must be able to withstand the voltage. If mains / power line voltage is being switched, as in the case of a light dimmer, the capacitor must be able to withstand the peak line voltage which is √2 times the RMS voltage plus a good margin to accommodate any transients that may come along the line. 400 volt rated capacitors are often used for 240 volt line systems. 90009 90066 90094 90005 There are many triac circuits that can be used.The basic circuits are very simple and provide good performance where this level of functionality is needed. The switching function can also be controlled via a processor enabling very intelligent forms of strike witching circuit to be developed. 90009 90005 90006 More Circuits & Circuit Design: 90007 90008 Op Amp basics Op Amp circuits Power supply circuits Transistor design Transistor Darlington Transistor circuits FET circuits Circuit symbols 90008 90006 90031 Return to Circuit Design menu.. . 90032 90007 90009.