What is a Triac | Triac Switching Circuits

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What is TRIAC 

A TRIAC is a device that behaves like two SCRs connected back with a common gate. It can conduct in either direction once its break over voltage is exceeded. The break over voltage of a TRIAC decreases with uncrossing gate current. Once it is turned on a TRIAC remain on unit its current falls below IH.

Because a sine TRIAC can conduct in both directions it can repel a more complex pair of back-to-back SCRs in many control circuit. However, TRIAC generally switch more slowly then SCRs and are available at low power ratings.

TRIAC Symbol

the circuit image recognises the means during which the TRIAC operates. Seen from the skin it’s going to be viewed as 2 back to back thyristors and this is often what the circuit image indicates.

Fig : TRIAC Symbol

Like a thyristor, a triac has 3 terminals. but the names of those are a touch harder to assign, as a result of the most current carrying terminals are connected to what’s effectively a cathode of 1 thyristor, and also the anode of another inside the device. there’s a gate that acts as a trigger to show the device on. additionally to the current the opposite terminals are each known as Anodes, or Main Terminals These are sometimes selected Anode one and Anode a pair of or Main Terminal one and Main Terminal a pair of (MT1 and MT2). once exploitation triacs it’s each MT1 and MT2 have terribly similar properties.On the TRIAC image, there are 3 terminals. These are the Gate Associate in Nursing 2 alternative terminals are typically noted as an “Anode” or “Main Terminal”. because the TRIAC has 2 of those they’re tagged either Anode one and Anode two or Main Terminal, MT1 and MT2.

How does a TRIAC work 

Before viewing, however, a triac works, it helps to possess AN understanding of however a thyristor works. during this method, the fundamental ideas will be grasped for the sophisticated} device so applied to a triac that is additionally complicated. The operation of the thyristor is roofed within the article during this section and accessible through the “Related Articles” box on the left of the page and below the most menu.

On the TRIAC image, there are 3 terminals. These are the Gate Associate in Nursing 2 alternative terminals are typically noted as an “Anode” or “Main Terminal”. because the TRIAC has 2 of those they’re tagged either Anode one and Anode two or Main Terminal, MT1 and MT2.

Fig: TRIAC Equivalent Circuit

Triac Phase Control

Another common kind of triac change circuit uses part management to vary the quantity of voltage, and sopower applied to a load, during this case a motor, for each the positive and negative halves of the input wave. this sort of AC motor speed management offers a totally variable and linear management as a result of the voltage may be adjusted from zero to the complete applied voltage as shown.

Triac Phase Control

Another common kind of triac change circuit uses part management to vary the quantity of voltage, and sopower applied to a load, during this case a motor, for each the positive and negative halves of the input wave. this sort of AC motor speed management offers a totally variable and linear management as a result of the voltage may be adjusted from zero to the complete applied voltage as shown.

Fig: Triac Phase Control Circuit

This basic part triggering circuit uses the triac asynchronous with the motor across AN AC curved provide. The resistor, VR1 is employed to regulate the quantity of part shift on the gate of the triac that successively controls the quantity of voltage applied to the motor by turning it ON at totally different times throughout the AC cycle.
The triac’s triggering voltage comes from the VR1 – C1 combination via the Diac (The diac could be a two-wayconductor that helps offer a pointy trigger current pulse to totally stimulus the triac).
At the beginning of every cycle, C1 charges up via the resistor, VR1. This continues till the voltage across C1 is spare to trigger the diac into physical phenomenon that successively permits condenser, C1 to discharge into the gate of the triac turning it “ON”.

Once the triac is triggered into conductivity and saturates, it effectively shorts out the gate triggering sectionfeedback loop connected in parallel across it and also the triac takes management for the rest of the half-cycle.

As we’ve seen higher than, the triac turns-OFF mechanically at the top of the 0.5-cycle and also the VR1 – C1 triggering method starts once more on consecutive half cycle.

However, as a result of the triac needs differing amounts of gate current in every switch mode of operation, as an example Ι+ and ΙΙΙ–, a triac is thus asymmetrical that means that it’s going to not trigger at the precise same purpose for every positive and negative 0.5 cycles

his simple triac speed feedback loop is appropriate for not solely AC motor speed management except forlamp rheostats and electrical heater management and in reality is extremely just like a triac lightweightdimmer employed in several homes. However, a poster triac rheostat shouldn’t be used as a motor speed controller as typically triac lightweight dimmers ar meant to be used with resistive hundreds solely likeincandescent lamps.

Triac applications

Triacs area unit utilized in several applications. they’re typically utilized in low to medium power AC ship necessities. wherever massive levels of power ought to be switched, 2 thyristors / SCRs tend to be used as they will be controlled additional simply.

Nevertheless, triacs are widely used in many applications:

General AC control

Control of fans and small motors.

Lighting control – especially domestic dimmers.


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