Author Topic: Logic level triac switching BT139  (Read 4541 times)

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Offline caffeinatedbard

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Logic level triac switching BT139
« on: June 02, 2012, 08:58:15 pm »
Hi all,
I am working on a side project and would want to try out the part below.  Aside from using an optoisolator and some simpler driver chips that have the zero crossing circuit built in,  I have no idea how this part detects zero crossing and how do you trigger the triac on the negative cycle of the AC if it's controlled with 1.5V to trigger the gate? The datasheet claims you can just control it straight from a micro pin... I am very skeptical.  Would there be a need for some intermediary hardware between the micro and this part?

Any help would be greatly appreciated.  If any one knows of any books or app notes that deal directly with logic level triacs, please share them here!  I have spent quite some time looking for example appnotes but they all use driver chips that have optoisolation, zero crossing detection and diacs to control the gate leaving the triac very "dumb"if you will.


Offline codeboy2k

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Re: Logic level triac switching BT139
« Reply #1 on: June 03, 2012, 06:40:33 am »
That part doesn't detect zero crossings at all.  You are thinking that it is more than it is, but it's still just a TRIAC, and behaves like a TRIAC, it's nothing special.
The gate happens to be enhanced to give extra sensitivity, thus it can be triggered with a very low voltage and current. That's what they call "logic level". ugh.

I think it's a little reckless for the datasheet to say that it's intended to be directly triggered from a micro-controller without giving an example circuit.
Sure it can be directly triggered from a microcontroller, but that microcontroller will need to be on the same AC side that you are switching,
because the TRIAC must get it's gate current from the MT2 side of the device.

If you are using it to switch AC loads, then you should still isolate it from your micro.  I suppose , in some embedded applications, beyond human contact,
where the micro is directly on the AC mains side, non-isolated, you can directly trigger off a micro I/O pin.

Other than that, you will still need an opto-isolator, diac, and optional zero-crossing detector to trigger it.


Offline digsys

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Re: Logic level triac switching BT139
« Reply #2 on: June 03, 2012, 11:40:44 am »
Hi, First I'd strongly agree with isolation, for many reasons. There are 2 methods -
1st as suggested, using an optocoupler - this requires that you source the gate energy from the AC side.
2nd method is to use a tiny pulse transformer and drive it with a pulse train waveform from the micro.
You then pick off a tiny amount for the 0-xing detect. The BT139 is a nice TRIAC.
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