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esteban
Joined: 01 Sep 2007
Posts: 13
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| Probably off topic... |
 Posted: Thu Sep 20, 2007 3:09 am |
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Hi.
I might be asking in the wrong place, but i don't know where to ask this question. Maybe somebody could point me in the right forum. Sorry for the inconvenience.
I have a triac to control a PSC (Phase Split Capacitor) motor. Not phase control, not speed regulation... just forward and reverse. I'm driving the triacs (BTA16-600) with an optocoupled triac driver (MOC 3083). The PIC, then drives one MOC or the other (PIN_C1, PIN_C2).
Pretty simple.
For testing, i've even added a 1 sec delay between switching one way or the other.
The problem is that the "inactive" triac (whichever it is at any moment) doesn't open. It even starts conducting. Both of them. Even disconnected from the PIC board (no connection at all).
With the scope i see they're conducting half cycle (positive one). Tried with a lamp (instead of the motor, to avoid inductive load problems), with/without snubbers, and even with led's in series with a 22k res. Allways the same.
Can anybody give me some ideas? Where to look for help? What to do? Thanks a lot. |
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Guest
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| Posted: Thu Sep 20, 2007 8:25 am |
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It sounds like you did a decent job of troubleshooting the problem (i.e. using a light bulb, etc) Call the applications engineers for the semiconductor switches - fax or email them a circuit diagram - I'm sure they can help.
HTH - Steve H. |
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libor
Joined: 14 Dec 2004
Posts: 288
Location: Hungary
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| Posted: Thu Sep 20, 2007 10:48 am |
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It even starts conducting. Both of them. Even disconnected from the PIC board (no connection at all).
TRIACs can begin conducting 'by themselves' if their dV/dT limit is reached. (a too fast changing voltage). I don't think it can be the case with normal AC mains freq. Even if you apply voltage at the peak of a sinewave they must switch off at the next zero-cross. ...just thinking. don't know. |
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Guest
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| Posted: Thu Sep 20, 2007 12:17 pm |
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| Quote: |
TRIACs can begin conducting 'by themselves' if their dV/dT limit is reached
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Yes, i was aware of that. That's why i mention that i've tried with AND without snubber network on them.
The news are that they seem to be working out of the board. I connected the circuit with mere cables (this was not an intended solution, just for trying to find the problem). When i do this, they cut. I still see the false start at the beginning (as libor says, probably because the dv/dt) but they shut off in a half cycle. And stays off as expected. Weird.
Two different boards. Same behaviour.
Any ideas? Any data about the dielectric constant of a pertinax board?
And thanks a lot for the input libor, and guest. |
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Humberto
Joined: 08 Sep 2003
Posts: 1215
Location: Buenos Aires, La Reina del Plata
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| Posted: Thu Sep 20, 2007 12:59 pm |
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The point is that TRIACs can begin conducting if the Vg or Ig meets the condition trigger.
This behaviour is predictable and assuming to your comments, it seems to be uncontrollable
in your project.
It is very important to analize the instantaneous voltage biasing after each zero-crossing, for sure there are some important issue that had been misunderstood or not considered. For example, you didn´t mention what criteria are you using to triggering, Zero-crossing firing or NonZero-crossing firing. Also it is very important to know if the control used is referenced to Neutral and in which quadrant the firing is done.
Humberto |
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SET
Joined: 15 Nov 2005
Posts: 161
Location: Glasgow, UK
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| Posted: Fri Sep 21, 2007 7:35 am |
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I've used triacs for motor drive and it can be tricky - I agree with Humberto, it sounds like you are (unintentionally) providing gate current during one half of the mains cycle.
Any way you can post a schematic?
Cheers
Vic |
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