TRIAC stands for Triode for Alternating Current. It is a genericized trademark for an electrical device that has 3 terminals and when triggered conducts electrical current in both directions. Its conventional name is bilateral triode thyristor or bidirectional triode thyristor. A thyristor is comparable to a relay where a small voltage induced current can control a lot bigger current and voltage.

Thyristors can be said to be the superset of TRIACs. TRIACs are also relatable to SCRs (Silicon Controlled Rectifiers). TRIACs and SCRs can be distinguished in one characteristic feature i.e. the first one is bidirectional, the flow of current is in both the direction but the later one allows the flow of current in only one direction. Generally, the TRIACs get initiated by the consequence of a positive or a negative potential to the gate but the SCR needs only a positive potential. Once activated, SCRs and TRIACs keep on conducting, regardless of whether the current in the gate ceases, until the principle current dips under a specific level called the holding current.

The GTOs or Gate Turn-Off Thyristors are the same as TRIACs but when the signal in the gate ceases, it provides much more control by turning OFF.

The bidirectional feature of this equipment makes it a good switch for AC or Alternating Current. A trigger, when applied in the main circuit at AC's controlled phase angle, permits control of the average current streaming into a load. This feature is popularly used for managing the speed of dimming lamps, electric heaters, and universal motor.

Factors that affect the working of TRIAC

TRIACS, unlike SCRs, needs appropriate enhancement for its legitimate working. There are some inherent drawbacks like TRIACs Backlash effect, Rate effect, etc. Thus designing it requires proper case.

• Backlash Effect

This effect is one of the most acute Control Hysteresis that grows in the speed control or the lamp control circuits with the help of a Potentiometer for controlling the Gate current. The glowing brightness of the lamp gets a minimum when the potentiometer's resistance increases to the maximum. The lamp never gets ON when the pot is turned back until and unless the resistance of the pot drops to its lowest value. The main reason behind this is the discharging of the TRIAC's capacitor. The triggering pulse is provided to the gate by a Diac. Triac releases through the Diac, generating the Backlash effect.

• Rate Effect

The Rate effect generally takes place due to high inrush current when overwhelming inductive loads switch on and also because of a momentary variation in frequency, voltage or current in the mains.

• Effect of RFI

RFI or the Radio Frequency Interference seriously influences the working of Triacs. Depending on the resistance and the supply voltage of the load, the load current is seen to sharply increase from 0 to a high value whenever the TRIAC switches on the load. This forms pulses of RFI.

This was all regarding TRIACs. Brands that manufacture it are Littelfuse, STMicroelectronics, and NXP. We hope that you liked the article. If you want to refer to the brands then just follow the links as given.

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A Semiconductor Controlled Rectifier or an SCR or a Silicon Controlled Rectifier is a 4 layered solid-state current controlling electrical equipment. This technique of 4 P-N-P-N switchings was established in 1956 by Tanenbaum, Holonyak, and Goldey. In January 1958, Dr. Ian M. Mackintosh of Bell Laboratories presented the practical explanation of SCR. The name Silicon Controlled Rectifier is the trade name for General Electric for a kind of thyristor. The Silicon Controlled Rectifier was made by a group of power engineers which was led by Gordon Hall, furthermore, marketed by Frank W. "Bill" Gutzwiller in 1957.

A few sources characterize thyristors and Silicon Controlled Rectifiers as almost the same thing but some say that they are an appropriate subset of the arrangement of thyristors and it comes with at least 4 layers of alternating P and N-type materials. Bill Gutzwiller says that the word “Controlled Rectifier” and “SCR” were used earlier and the word “thyristor” came into existence when the equipment started to get spread internationally.

SCRs are devices that can make the current flow in only one direction (i.e. unidirectional). It is completely different from TRIACs (Triode for Alternating Current) where current can flow in both the directions (i.e. bidirectional). SCRs can be activated regularly just by the flow of current into the gate rather than TRIACs, which can be activated ordinarily by either a negative or a positive current applied to its gate electrode.

Modes of Operation

Depending on the biasing given to an SCR, it can be classified into 3 modes.

Forward Blocking Mode

In this category, the cathode is provided with a negative potential and the anode gets a positive potential and the gate is kept disconnected i.e. at Zero potential.

As you can see here, the junction J2 is reverse biased and J1 and J3 are biased forward. Thus, permitting the flow of only small amount of leakage current from another to the cathode. An avalanche breakdown is initiated when the applied potential gets to the break-over value for J2. J2 starts to conduct at this break over but below that the current used to face high resistance from it and the SCR is known to be in off state.

Forward Conduction Mode

An SCR can be converted to Conduction mode from Blocking mode in 2 different ways.

• At the gate offer a positive pulse.
• Uplifting the potential difference between the cathode and the anode over the break-over voltage. As soon as the SCR starts conducting, to maintain it in the ON state no gate voltage is needed.

Reverse Blocking Mode

The SCR is said to be in reverse blocking mode when a positive potential is provided to the cathode and a negative potential at the anode. Thus, J2 is biased forward and J1 and J3 are reverse-biased. The equipment acts as 2 reverse-biased diodes associated in series. A small amount of leakage current flows through it.

So this was all we have about SCRs. Hope you enjoyed reading it. Brands that make SCRs are Littelfuse, STMicroelectronics, and NTE. Refer to them through the links as provided.

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