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Triac Question

Discussion in 'Electronic Design' started by Dan, Apr 15, 2007.

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  1. Dan

    Dan Guest

    I have an conventional AC motor control circuit that uses an MOC3012
    driving a BTA06 sensitive gate triac. The design has been working well
    for several years, however it has a random problem where the motor
    "bumps" or rotates slightly and randomly upon initial power-up. The
    motor drives a gearbox whose output shaft must not move unless
    commanded to do so. I've made sure the MOC3012 is not being turned on
    during power-up. I've also tried loading the triac's gate to gnd, A1,
    A2, etc. with a variety of resistors with no improvement. I swapped
    the MOC3012 for an MOC3043 zero-crossing driver and the problem
    improved considerably, but not completely. As you might have guessed,
    the power-up 'bumping' of the motor occurs more frequently if there is
    a noisy load on the same circuit as my controller. It really
    misbehaves with flourescent lights and soldering irons. My customer
    will no longer accept any movement of the gearbox shaft, however
    slight.

    Any suggestions?

    TIA,
    Dan
     
  2. Phil Allison

    Phil Allison Guest

    "Dan"

    ** Which version - there are lots.


    ** Read the data sheet for your triac.

    http://www.datasheetcatalog.com/datasheets_pdf/B/T/A/0/BTA06-600SW.shtml

    Add a good snubber, use a "snubberless " version.



    ......... Phil
     
  3. Dan

    Dan Guest

    Thanks for the quick replies and good advice guys.

    I put in a 100 ohm/0.1uF snubber and so far so good. The glitch is
    random, but seems to have ceased. Spehro, I think you nailed this one!
    I did consider the snubber, but discounted the idea thinking the
    problem was gate related rather than across the anodes of the triac.

    Thanks again for your help.
    Regards,
    Dan
     
  4. Swap the MOC3012 for something like a MOC3082, add a snubber if that
    doesn't work. And make really sure that your drive circuit isn't
    triggering the optoisolator.


    Best regards,
    Spehro Pefhany
     
  5. john jardine

    john jardine Guest

    Sounds like you've got a single half cycle switch through. Most likely time
    for it to happen, is when the supply is switched on at the peak of the
    incoming voltage waveform.
    In the UK this can amount to a +/- 340V step edge turning up at the triac
    main terminal over the space of a few uS. The triac's dv/dt rating is
    massively exceeded and it switches on by itself for the rest of that single
    half cycle. Hence the twitching of a motor or lamp etc. This problem is
    completely separate from the normal action of the gate terminal and trigger
    pulse.

    As suggested, you need a snubber across the triac. Something like 0.1uf or
    0.22uF in series with maybe a 100 ohm resistor. Best to just buy the snubber
    item as a standard part. 1nF may be capable of doing the job but better
    safe than sorry.
    Idea is that the capacitor instantly passes the nasty voltage edge across to
    the other main terminal of the triac. Essentially both sides of the triac
    are lifted by the voltage edge at the same time, hence it cannot crowbar due
    to excessive dv/dt across it.
    For the same reason you may also need another snubber across the opto triac.

    One other technique is available which is to "prewarn" the triac of the
    incoming waveform, so it never sees a switch-on transient. This method
    simply bypasses the contactor/relay with a high value resistor.
     
  6. Fist thing you need to figure out is:
    1) Is the big triac firing by itself or
    2) Is the controller sending pulsens when it should not.

    In case (1) make sure you also have overvoltage proctection across the triac
    (VDR perhaps), proper RC snubbers, the works.
    In case 2 you will have to scope for supply noise, pickup by wires, etc,
    use RF (metal) shielding perhaps.
     
  7. Guest

    A triac is not the best device for the application, the current and
    voltage waveforms are out of phase so when the triac turns off at zero
    current there is an instantanious voltage across it which can fire it
    again. Hence the need for a big snubber network. There are "special"
    triacs made by Telcon that are supposed to address this problem. A
    pair of thyristors is the best way. A zero crossing MOC shouldnt be
    correct for the same reason. Your problem seems to be EMI. You should
    have a filter/surge limiter on the AC supply, a MOV accross the triac
    and a triac that has a voltage rating of at least twice the MOV
    voltage.
     
  8. Phil Allison

    Phil Allison Guest

    ** Groper FUCKWIT Alert !!!

    ** It is perfectly OK.


    ** Probably not by very much.



    ** No serious issue.


    ** Yeah - called snubberless.

    Stale news here - fuckhead.



    ** Not true at all.


    ** Zero crossing is OK - less volts at switch off.


    ** No it is not - you fucking tenth witted prick.

    The OP's main problem occurs at switch on.



    ** Go SFA to do with the fast voltage rise ( dv/dt ) at switch on.


    ** No need with an AC motor load and a simple snubber on the triac.




    ....... Phil
     
  9. It's far more likely to be related to the optoisolator than the triac.

    Both the breakdown voltage rating and the dv/dt rating of the
    optotriac in the optoisolator are far less than the WORST version of
    the BTA06. It's rated for only 10v/usec (less at higher temperatures
    than 25'C- down to less than 2V/usec at 100'C) and 250V Vdrm, whereas
    the MO3083 is rated for 800V Vdrm and guaranteed not to turn on with
    dv/dt of 600v/usec (1500v/usec typical, both at 25C).

    Best regards,
    Spehro Pefhany
     
  10. Dan

    Dan Guest

    The production boards are using MOC3012 drivers which are spec'd at
    12V/uS typ @ 25C. Because of UL approval, swapping the '3012 for
    another driver such as the '3083 as you suggest is a less desirable
    option. Even though they are both UL approved components, the cost to
    update the UL paperwork for the part change is around $1,500. I
    know... it sounds crazy to me too.

    My BT06-400C datasheet shows a dVdt of 200V/uS. I have not tried a
    snubber on the driver's output yet. I did however short the opto's
    input to guarantee that the driver is not getting turned on
    accidentally via the micro.
     
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