555 Triac MAC15SM

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I wrote an earlier topic about my microcontroller creating a pulse (3 msec or so) after my actual gate pulse. The gate pulse width was 100usec and the small pulse has a width of 5-10usec.

After connecting a triac MAC15SM, I've noticed that it doesn't behave as I have learned in school. In other words once you gate it, it does not hold until the line commutation. Instead, it fires on an actual gate pulse and on those small width pulses. ( I am also using a MOC3023 M with 560 ohm resistor on pic side and 180 ohm resistor on line to neutral side (MT2) node as specified in MOC datasheet)

So I have two questions:

1. I would like to use a 555 timer in one-shot mode to trigger on the external pulse of width of 100usec. That way I would eliminate the false triggering.
Question: What is the minimum external trigger pulse width that the 555 will detect? I have been all over the datasheet but I could not find this information.

2. I've learned that triacs have a holding current and that once you reach the holding current the triac will remain in conduction mode even if gate pulse is removed. Now, the triac (MAC15SM) seems to conduct only if ONE gate pulse is present during a half cycle. (I also tested two of these triacs using multimeter and when anode and gate are shorted the R=339 ohms and as soon as I remove the lead from gate, R=infinity between MT1 and MT2)

Does anybody have a clue about operation of MAC15SM? Am I reaching the holding current with a 100usec pulse?


Any help will be greatly appreciated as I'm reaching the point of madness.

Thanks

P.S. I did go through the PIC code several times more, and I can't find anything responsible for this 5-10usec (width varies) pulse. In other words, there is only one triggering per half cycle and that is 100usec long.

 

[Russlk]

What is the load on the triac?

 

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The load is going to draw a maximum of 11 A at 120V. It is a self regulating heater so the load will vary. MAC 15SM can handle 15A. I have a 180 ohm resistor to protect optocoupler as per triac datasheet.

It is interesting that this problem of a dual firing pulse only occurs during the positive cycle. The negative cycle has only the actual (100usec) pulse.

But I think I might have a solution to this problem. I'll place an AND gate at the output of microcontroller, that will provide a logic HIGH only during the actual gating pulse. In that case the false gate trigger should be removed.

Anyway I would still like to know if anybody has had a similar problem with spurrious triac gating.

Thanks.

 

[Nigel Goodwin]

Try removing the triac, and see if you still get the spurious pulses on the PIC pin?.

 

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Thanks Nigel. I found out that my scope didn't trigger correctly and that is why I kept seeing an additional pulse on the microcontroller output. (scope that I'm using is pretty old and beat up.).

The problem was with the triac gate signal coming out of optocoupler being too low (0.4V) on positive cycle. (It was about 0.7 on negative cycle and that is why the triac triggered correctly on that cycle).

Anyway, MAC 15 SM triac is a gate sensitive triac (it has pretty high (339 ohm) gate impedance). So it needed a resistor between optocoupler's gate lead and its MT1 node. This made a whole world of difference as triggering pulses started to have an even amplitude and the triac started triggering correctly.
It's a shame that the motorola never released any application hints for the MAC15XX triac series. Fortunately enough, Fairchild's AN-3006 had couple of sentences on the subject.

Thanks for the replies.