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How to Reduce TRIAC Fault in Switching Circuits?

Author: Apogeeweb
Date: 19 Jan 2021
 1388
triac application

Introduction

What is a triac? TRIAC (Triode for Alternating Current) is an electronic component that is widely used in alternating current power control. It is a three terminal electronic component that conducts current in either direction when triggered. TRIAC is 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. You can find its applications in switching, phase control, chopper designs, brilliance control in lamps, speed control in fans, motors etc.

TRIAC Characteristics, TRIAC Structure and TRIAC Working

Catalog

Introduction

Ⅰ TRIAC vs Silicon Controlled Rectifiers (SCR)

Ⅱ TRIAC Structure and Symbol

Ⅲ How Do You Use a TRIAC

Ⅳ How Does a TRIAC Work?

4.1 TRIAC Leakage Current

4.2 Solutions


Ⅰ TRIAC vs Silicon Controlled Rectifiers (SCR)

Thyristor also called SCR stands for silicon controlled rectifier while TRIAC stands for triode for alternating current. The TRIAC has on and off state characteristics similar to SCR. The main difference between SCR and TRIAC is that thyristor is a unidirectional device while in TRIAC as a bidirectional device. A TRIAC is defined as a three terminal AC switch which is different from the other silicon controlled rectifiers (SCR) in the sense. They can turn and regulate both parts of the AC waveform easily. This makes this component appropriate for a variety of applications where control of AC power is needed. A dimmer circuit will be an example application, and we use it domestically as a ceiling fan regulator circuit. Also it can be used to regulate a motor or electric heater's input power. This is why TRIAC is used for applications of low to medium power, leaving SCR with high-power applications. While this is a very interesting system, a problem known as "leakage current" is present. And we'll talk more about this leakage current, its adverse effects, and some well-known ways to solve these problems in this article. But let's clear out the basics of TRIAC before that.

 

Ⅱ TRIAC Structure and Symbol

As for triac symbol, like any other electronic component, it consists of two SCRs linked in an antiparallel configuration, and if we look very closely at its symbol, it clearly reflects the TRIAC's bidirectional properties. Which you observe from the picture below.

triac symbol

The upgraded variant of the thyristor is the TRIAC. A thyristor can only control current in one direction, as you already know, but a TRIAC can control current in both negative and positive directions. TRIAC turns in each sine wave loop because of the existence of the sine wave, which means we can use the entire cycle, unlike SCRs. Like thyristor, a TRIAC has three terminals, but it becomes a little difficult to assign names to these terminals since they are related simply to the cathode and the anode of two SCRs. Two SCRs are also connected to the gate terminal, which is why it was called Anode 1 and Anode 2 or Main Terminal 1 and Main Terminal 2 (MT1 and MT2).
A multimeter can be used to test the health of a triac. First put the multimeter selector switch in a high resistance mode (100K), then connect the positive lead of multimeter to the MT1 terminal of triac and negative lead to the MT2 terminal of triac (there is no problem if you reverse the connection).

 

Ⅲ How Do You Use a TRIAC

Triacs are semiconductor devices that are widely used for switching medium power AC. Let's acquire a little knowledge of  TRIAC functions before going further. As the following figure shows you.

two-thyristor analogy

Two-thyristor Analogy

We have previously said that as a configuration of two SCRs, a TRIAC can be realized. The above image provides a little bit more clarity on the subject, but it is much more complex to work at the semiconductor level. A TRIAC can be activated in many ways, unlike SCR, regardless of the polarity of the terminals. Regardless of the polarity of the initiating pulse, it may also be activated. When working with TRIAC, one thing to remember is that when the MT2 and gate current are at the same polarity, the sensitivity of the trigger current is much greater. We can now move on to cleaning out our key issue of leakage current with the simple cleared out.

Triac Switching Circuit Example

Ⅳ How Does a TRIAC Work?

4.1 TRIAC Leakage Current

There is structural leakage current in the off state of thyristor, TRIAC, or any other solid-state AC switches, which is why a small amount of current flows through the load, this circuit is sufficient in some cases to charge a load circuit (Inductive) and causes it to flash spontaneously. We need to take careful care of the specifics and design the circuit accordingly to avoid this, and we will talk more about it in this section of this article.
If the voltage of MT2 reaches a certain rated threshold voltage (which can occur due to the transient state of high voltage), the leakage current between the two terminals may enter the point at which the TRIAC breaks into conduction mode. In this state, a sudden localized heat will be produced when a sudden increase in current flows through the TRIAC, so that the TRIAC can be destroyed. Incandescent lamps are most likely the source of strong inrush currents, with capacitive loads.

triac

4.2 Solutions

By applying one or more of the following methods, this condition can be avoided:
1) Maximum Temperature Ratings Tj max. ensure that the temperature is not surpassed. As temperature rises, the current of leakage through the system increases, we can eliminate/reduce this issue by integrating specific TRIAC brands for specific requirements.
2) By placing a broad value resistor from the gate to the cathode, we can reduce the TRIAC's sensitivity. This decreases the gate current, thereby reducing the current of leakage. It, on the other hand, increases the TRIAC turn-on time.
3) If it is not possible to implement the methods described above, we can use a TRIAC with a less sensitive gate during the off time and apply a small degree of reverse bias to the gate. In this process, we have to minimize the dissipation of power through the gate.
4) Depending on the form of load, another strategy for reducing leakage current is to fully eliminate the snubber circuit. The capacitor leakage also becomes the main source of leakage current, so we can decrease the current flow through the snubber and decrease the leakage current by removing the snubber network.
If you want to know more TRIAC info, you can check its Triac I-V Characteristics curves with more examples. Before try these methods to reduce current leakage, please remember safety first!

 

Frequently Asked Questions about TRIAC Basic and Its Applications

1. What is a triac used for?
The Triac is most commonly used semiconductor device for switching and power control of AC systems as the triac can be switched “ON” by either a positive or negative Gate pulse, regardless of the polarity of the AC supply at that time.

 

2. Which is an example of Triac?
TRIAC Applications
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.

 

3. What is triac and its characteristics?
A Triac is defined as a three terminal AC switch which is different from the other silicon controlled rectifiers in the sense that it can conduct in both the directions that is whether the applied gate signal is positive or negative, it will conduct. Thus, this device can be used for AC systems as a switch.

 

4. What is a triac switch?
A Triac is a high-speed solid-state device that can switch and control AC power in both directions of a sinusoidal waveform. Being a solid state device, thyristors can be used to control lamps, motors, or heaters etc.

 

5. What is the working principle of Triac?
The triac is another three-terminal ac switch that is triggered into conduction when a low-energy signal is applied to its gate terminal. Unlike the SCR, the triac conducts in either direction when turned on.

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Related Articles

pinglun 5 comments

    • pingluntus
    • RachelYe on 2021/6/7 14:37:13

    Hi, Can a triac be used in place of an SCR for a DC rectifying circuit? if so, should the AC side go MT2 or MT1, how should the gate drive be organized? Is there a danger of conduction the wrong way?

      • pingluntu
      • author on 2021/6/8 10:18:50
        author

      Re:

      Yes. In opposite to an SCR the triac may be conductive in both directions. Besides this problem you may use a triac instead of an SCR.
      Also.....whereas the SCR triggers in a single quadrant, and with the Triac you can trigger it in several quadrants.
      The trigger voltage has to be applied between G and MT1, MT1 is acting as cathode and MT2 as anode when replacing the SCR.

      There are basically two ways to trigger the triac with unwanted polarity
      - applying trigger voltage while MT2 is negative against MT1
      - self triggering due to high dv/dt

      On-state voltage of triac should be similar to SCR with same current rating.

    • pingluntus
    • Harper on 2021/6/11 10:18:48

    I need an SCR to repair a 250V 300W power tool speed controller. The one that died was a C106B1 (200V, obviously should have been 400V). Would it work if I substituted a triac or thyristor of adequate (or greater) voltage and current rating?

      • pingluntu
      • author on 2021/6/15 9:39:53
        author

      Re:

      No, you cannot use a triac or thyristor for your circuit because the voltage of your circuit is very high. Just find a replacement SCR for the one that died or its equivalent.

    • pingluntus
    • kauffmanan on 2021/6/17 10:06:35

    Hey, I'm a student.. While looking for resources to read, most people seem to interchange or use both "triac" and "thyristor" like they are both the same component! And to add to confusion "SCR" well a sillicon controlled rectifier can be a triac or thyristor right? In short thyristors allow ac to flow in 1 direction, triacs allow ac to flow in both directions, search for thyristor circuits and you'll indeed find triacs not thyristors but a lot of people seem to confuse the two which confused me lol

      • pingluntu
      • author on 2021/6/18 10:29:43
        author

      Re:

      Basically, SCRs and thyristors are for switching dc and triacs for switching ac.
      A believe "thyrisor" is just a fancy name for a high current SCR.
      So a triac and a thyristor are not the same components, but a scr and a thyristor are essentially the same. 🙂

    • pingluntus
    • Mia on 2021/7/2 17:28:30

    I have a dimmer for a lamp that has a bad triac. I was wondering if there was something else that could be used in its place. The part in question is a MAC97A6. It's a logic level triac. Could a MOSFET be used instead?

      • pingluntu
      • author on 2021/7/3 14:48:37
        author

      Re:

      Actually the body diode in the MOSFET will make it conduct in the reverse direction.
      The impedance of the gate of a MOSFET is much higher than the gate of a TRIAC, so it's possible that you'd exceed Vgs(max) and destroy the gate. It would be hard to predict what happens next.
      When all you have is a hammer, the superficial similarity between nails, screws, golf tees, and sewing needles should not lead you think the tool you have is the right one for all of them. 😶

    • pingluntus
    • Filley on 2021/7/28 14:19:16

    Hi, budy can you advised a good TRIAC/SCR book?

      • pingluntu
      • author on 2021/7/29 13:50:46
        author

      Re:

      I was actually looking for a good reference a couple days ago to understand an SCR flip-flop circuit I saw on YouTube but I didn't find a good reference. But today, my Google powers are better.
      SCR Manual Including Triacs and Other Thyristors – Fifth Edition, by General Electric from 1972 looks pretty comprehensive in over 700 pages.

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