Logic level TRIAC control

[sam781]

I found logic level TRIAC BTA08. I want to trigger this TRIAC without optocoupler. i.e, MT1 connected to MCU's ground and Gate connected to MCU's output pin with a current limiting resistor. Is this okay?

Now, For +ve half cycle, when the MCU's output pin is high TRIAC will be on. I can control +ve cycle voltage using respective delay in making high the port pin. This is Quadrant I of TRIAC operation. I'm I right?

For -ve half cycle, how can I control TRIAC? Which Quadrant it will work?

 

[betwixt]

I'm not sure I would call a BTA08 a logic level device and I would doubt some MCU could provide enough gate current to trigger it under all conditions. Regardless of that, you can't provide -Ve trigger voltages directly from an MCU and I would urge caution about a direct connection anyway, particularly if you are using an inductive load or have a noisy AC supply as you could easily shoot something very nasty into the MCU pin.

Using an opto-triac driver is inexpensive and cures all these problems. If you want to avoid them, I suggest a transistor current amplifier feeding a pulse transformer but it will cost more and be larger.

Brian.

 

[mtwieg]

Most MCUs can't directly drive enough current to reliably latch a triac, unless you use a few IO in parallel. The bigger issue is that your control circuit is no longer isolated from the AC line, which is usually problematic in terms of safety and EMI immunity.

 

[sam781]

mtwieg

Thanks for your comment. Actually isolation is not important here. I want to avoid extra component (like optocoupler) to make the circuit as small as possible.

 

[mtwieg]

Well then I would think critically about what I and betwixt said, especially with regards to switching inductive loads, since you will be exposing your control circuitry directly to any transients on the AC line.

 

[sam781]

At least I kept optocoupler in the circuit. Thanks a lot everybody for your valuable feedback...

 

[Ogu Reginald]

Since you don't want to bring in any other component. And some MCUs may not source the required current, I suggest that you parallel two pins or more of the MCU and also remember to correct it in the program.

 

[sam781]

Okay, I can test. Let me know what will be the signal to trigger triac in +ve half cycle as well as -ve half cycle.

 

[schmitt trigger]

Well then I would think critically about what I and betwixt said, especially with regards to switching inductive loads, since you will be exposing your control circuitry directly to any transients on the AC line.

Agree 100%. By all means keep the optocoupler.
I also participate in a microcontroller forum, and the vast majority of issues reported from microcontrollers latching up or self-destructing, are related to lack of isolation from the AC line.
The AC line has all sorts of nasty stuff, and it can couple back into the uC's output pin, causing a latch up that at best the program stops responding, and worst it will draw excessive current that eventually causes a hard failure.

 

[Ogu Reginald]

Yes,I think Schmitt Trigger has a point, it is better to completely and safely isolate/interface the MCU from AC.

 

[schmitt trigger]

With an optocoupler, the gate trigger current is in phase with the main terminal voltage, all triacs handle that.

 

[adeel abid]

hi, i am in doubt about using the triac with micro-controller directly without any isolation. many application notes particularly from microchip indicates that triac can directly be driven by uc. but what i think is that any uc cannot aceept input at its pins that is much greater than its vdd. but there is all time appearing the two cycles of line voltage at the gate of triac. how gate of triac can be connected to the uc pins directly?

 

[schmitt trigger]

Yes, you can drive a logic level Triac directly from a microcontroller. The Triac's gate is an input, it does not produce a voltage referenced to MT1.
Now, between MT1 and MT2, that is another story.

But if you read my post #9, it is better if you use an optocoupler.

 

[betwixt]

Sam781 / Adeel Abid, please listen to the advice you are being given. You can NOT trigger a Triac so it conducts all quadrants with a single polarity signal. You can do it the expensive way with a pulse tranformer or the cheap way with an opto-coupler. Both are safe to use and will work. When you see triacs being driven from logic signals it is usually because they are only working in SCR mode and will not conduct in both directions. For your own safety, if you do not understand how these devices work, please don't use them. Triacs are invariably used in circuits where deadly voltages and current are present and we don't want harm to come to you or your circuits.

Brian.

 

[FvM]

You can NOT trigger a Triac so it conducts all quadrants with a single polarity signal.

There are three- and four-quadrant triacs. Four-quadrant triacs can be triggered with only positive gate voltage, three-quadrant triacs (e.g. BTA08) at least with only negative gate voltage, which is also an option for direct MCU control (when tying MT1 to Vcc). You'll find it often implemented with home appliances.

 

[betwixt]

Sorry, I should have qualified my statement by referring specifically to the BTA08!

I still think an opto is the best solution though!

Brian.

 

[FvM]

I still think an opto is the best solution though!

No doubt about it. I just wanted to clarify that "logic level control" of triacs can work but must consider clearly specified datasheet parameters of different triac types.

 

[sam781]

I found the following doc in nxp site related to logic level TRIAC-
**broken link removed**

I use BT134,136,139 TRIAC as general purpose TRIAC. Are these TRIAC logic level TRIAC? How it operates while it is connected directly connected with MCU port like Fig -1?

 

[betwixt]

The term "logic level" simply refers to the sensitivity of the trigger signal, it does not mean the triac has a magical logic property. A triac which the manufacturer considers could be driven from a logic level voltages is described as a 'logic level triac". Please note that common logic levels these days are 1.8V, 3.3V and 5V, a triac that works on 5V may not work on lower logic supplies.

I am not sure why you resist a safe and inexpensive opto interface, it solves many of your problems in a single component and may also eliminate the need for a zero crosing detector circuit.

What exactly is your application?

Brian.

 

[sam781]

Thanks betwixt to make me understand what is meant by logic level TRIAC. Actually I want to avoid extra opto interface to reduce the space requirement. I'm trying to make switch/dimer in which can be fit into a mechanical switch used in house wiring. As it is not possible to use SMD in my project at this moment, I need to bypass extra opto isolator.