How to Turn Off a Triac?

[Suraj143]

Hi all I’m new to triacs

I want to know how do I turn off a triac?

It will turn on when it has logic high on its gate. But how do I turn it off
I’m speaking about BT136 triacs.

Thanks

 

[dknguyen]

You can't. Once a triac is turned on it stays on until the voltage/current across it reaches zero and there is no gate signal appliuedIt can be due to the AC in the triac crossing zero or some other device stops the current from flowing (like a relay somewhere else that disconnects the current).

 

[kchriste]

You need to remove the logic high on the gate and reduce the voltage across "main terminal 1" and "main terminal 2" to zero for the triac to turn off. In other words, TRIACs are designed to control AC and NOT DC.

 

[Suraj143]

Ok Thanks for that

let say I'm giving a short pulse to the gate then it will turned on, when it moves to zero cross point it doesn't flow any current then the triac will be off condition is that correct?

 

[dknguyen]

yes. to get it to conduct on the next half cycle you need to apply another short gate pulse. after the zero cross.

You can also just apply a continuous gate signal as long as you want the triac on. When you remove it, the triac will turn off at the next zero crossing.

 

[Suraj143]

Thanks but how do you determine the duration of the short pulse?Whats the minimum needed?

 

[dknguyen]

I don't think it matters too much as long as it's not ultra short.

 

[Suraj143]

Another thing i forgot to ask I'm operating my triac on every positive cycle & in negative cycle it doesn't do any operation.Is it ok if i control the triac only in positive cycle to dim the light?

 

[dknguyen]

There are four quadrants of triac operation: Every possible combination of positive/negative gate pulses and positive/negative current flow through the triac.

If the gate has the same polarity has the primary current flow, then the turn-on is most effective and efficient. But you can use a single gate polarity to trigger the triac. The triac will operate in 3 out of the 4 quadrants. So if you use single polarity make sure to find out which quadrant is the one that won't turn the triac on.

From
https://www.st.com/stonline/products/literature/anp/3578.pdf

It seems that if you use a unipolar gate pulses that are POSITIVE then it will work find. If you use negative gate (why would you do that anyways?) pulses then the triac will only trigger for positive half cycles but not negative ones.

It's not too a big deal usually...it's AC so everything is symmetrical. You can just flip the triac around to make the negative gate pulses positive.

 

[Suraj143]

Hi thanks for the wonderful answer. But I don’t know what are quadrants until I study.

I just connect a light bulb to the TRIAC & give pulses to the gate of it on every positive cycle of the wave form. Don’t know how many quadrants it is.

 

[dknguyen]

They are called quadrants because you can chart it out on cartesian coordinates and there are various combinations of positive X and Y axis. THe 4 quadrants are:

+ Gate Pulse + Triac Current
- Gate Pulse - Triac Current
+ Gate Pulse - Triac Current
- Gate Pulse + Triac Current

It is drawn on figure 1 of the PDF I linked:
**broken link removed**

Notice that it looks a bit like cartesian coordinates with +/- X-axis and +/- Y axis. THat's why it's called quadrants (in English math each of the 4 sections in cartesian coordinates is called a quadrant. Each quadrant is different from the other because the X and Y coordinates in that quadrant have the same combination of +/- signs.

https://en.wikipedia.org/wiki/Cartesian_coordinate_system

Notice the similarity of the PDF and the cartesian coordinate quadrants:
https://en.wikipedia.org/wiki/Image:Cartesian_coordinates_2D.svg

 

[Suraj143]

Hi dknguyen thanks for explaining that quadrants i got a quite understand of that I'll study that.

when giving ( + ) pulse only in the positive side of the wave form, the output bulb fading looks good.I mean i can fade a bulb from fully dim to full bright.Its working.
But when i fade both sides from fully dim to max bright & after max bright to fully dim I got a small problem. I'm using 255 steps to fade the bulb.

the problem i get is
when the bulb is in the max brightness the bulb gets off for one cycle i think.I tried many ways but still the problem is there.

 

[dknguyen]

You can go from dim-> bright just fine, but you have problems going bright-> dim?

Or you have a problem when you trigger the triac for both positive and negative half cycles? I don't understand your description.

Is your gate pulse actually positive? I don't mean a positive voltage relative to ground, I mean a positive voltage relative to the AC voltage on the triac.

How is your circuit switching the triac? Is there any isolation? THe ground for your gate pulse has to be somehow shared with the triac itself through isolation or through direct connection of the ground to the AC neutral (which is unsafe since the AC is at high voltage).

 

[Suraj143]

You can go from dim-> bright just fine, but you have problems going bright-> dim?

Let say the bulb is doing dim -> bright ,working good.(one side)

Let say the bulb is doing bright -> dim ,working good. (one side)

But when i combine these two dim -> bright -> dim the bulb is turning off in one moment its happening when the bulb in the maximum brightness.

after turning off again its fading nicely & when it reaches bright again bulb going off for one moment

this is the problem i get

 

[Suraj143]

Triac driving through 390R resister & no any isolation.

 

[voodoo]

you can turn off the TRIAC

you can turn off the TRIAC by short circuit TRIAC main terminals, connect a BJT in parallel to the TRIAC MT1 and MT2, just pulse the base of the BJT for a very short time to short the TRIAC then the TRIAC is off, remember if the current is very high use a power BJT.
good luck.