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How to Reduce TRIAC Fault in Switching Circuits?

Author: Apogeeweb Date: 19 Jan 2021  173

triac application

Introduction

TRIAC 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 to Minimize Leakage Current of TRIAC?

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 thyristor 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

TRIAC has its own circuit symbols, 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

Let's acquire a little knowledge of how a 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.

 

Ⅳ How to Minimize Leakage Current of TRIAC?

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 the difference between DIAC and Triac?
A Triac device comprises of two thyristors that are connected in opposite direction but in parallel but, it is controlled by the same gate. ... The 'G' current to switch 100A from triac is not more than 50mA or so. TRIAC. The DIAC is a bi-directional semiconductor switch that can be switched on in both polarities.

 

2.What is triac output?
The TRIAC Output Module (TM) is a single normally open contact for 24 VAC loads only, 0.5 A. Ideal for switching 24 VAC control circuit loads found in packaged HVAC systems, and 24 VAC two-position or floating point actuators. Sold in packages of 10.

 

3.What is the difference between a triac and a transistor?
Thyristor is a four layer semiconductor device which is used for rectification and switching. Transistor is a three layer semiconductor device which is used mainly for amplification and switching.

 

4.What happens when a triac fails?
Fail-closed is the usual failure mode for power semiconductors, triacs, diodes etc. An overcurrent event will tend to overheat the semiconductor, which melts everything into a highly doped and therefore conductive mess. An overvoltage event will tend to punch through insulation layers, with much the same result.

 

5.Why triac is known as bidirectional device?
It can switch both halves of an AC waveform, that's why it is called a bidirectional device. ... The TRIAC is an ideal device to use for AC switching applications because it can control the current flow over both halves of an alternating cycle. A thyristor is only able to control them over one half of a cycle.

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