Ac pwm

Discussion in 'General Electronics Chat' started by chrisw1990, Mar 1, 2014.

  1. chrisw1990

    Thread Starter Active Member

    Oct 22, 2011
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    Hi,
    mission is:
    to PWM control an AC line, the theory is up to mains. (i would like to point out that this is theoretical wonderings, not being put into action, one of my many daily wonderings)
    i have read into TRIACs, but i wonder if they can handle PWM control?
    how would one drive the TRIAC from a microprocessor? there have been mentions of optocouplers (For obvious reasons) but looking at this datasheet : http://www.fairchildsemi.com/ds/MO/MOC3010M.pdf it makes me wonder whether say driving a light, standard filament light requires additional circuitry.
    also, is there any difference to driving an led bulb?

    Like i say, just a theoretical question! hope you have a good weekend!
    Chris
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Triac dimmer switches work by delaying the on-phase to some point into the AC wave. It is PWM of the 60 Hz carrier. You don't need a Microcontroller, a capacitor is fine to cause the delay ( although a micro will work too).
     
  3. chrisw1990

    Thread Starter Active Member

    Oct 22, 2011
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    Thanks =] The original query came from the fact that these new dimmable light controls are available (controlled from smart phones etc) and it made me wonder how they work.. iv seen some powerline communication modules which allow low baud rates over L and N, but i wondered about the actual dimming process.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    PWM of an AC sine wave usually involves switch devices such as IGBT's or Mosfets.
    VFD's use this principle to not only vary the frequency but to also modulate the sine wave.
    Triacs and SCR's can usually only phase angle switch, not PWM unless you devise a method for commutation turn off.
    Max.
     
  5. chrisw1990

    Thread Starter Active Member

    Oct 22, 2011
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    how would you go about using a MOSFET? iv had a oogle on google, see this on instructables http://www.instructables.com/id/safe-and-simple-AC-PWM-Dimmer-for-arduino-Raspberr/
    how does that work? because the MOS potential difference/threshold voltage was why i looked at triac's to solve the problem. clearly that circuit has been tried/tested, so works, but id be interested learning why it works. from what i see it seems to create/bodge a DC voltage from the mains?
     
  6. MaxHeadRoom

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    What that circuit does is the Mosfet gate is fed from a PWM switching source and the Mosfet actually switches across the output of the bridge this causes current to flow in the series load, (Lamp), I am assuming that the switching circuit has enough charged energy to carry it through the time the bridge supply is shorted out.

    The Fairchild app note AN-3006 points out the Triac route.
    Also Picmicro AN521.
    Max.
     
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Obviously due to the TOS here I will provide no details but...

    I did a PWM control of the AC line using a micro for a DXM controlled (basically serial data) light dimmer using a TRIAC as the control element.

    While the math to convert desired voltage into a time delay is quite complicated I was able to find a sight where someone did the math for me; always a happy event!

    A PWM of an AC line is achieved on a half cycle by half cycle basis by delaying the turn of of the AC wave. A micro can easily achieve this control if it has a synch signal to determine when the AC zero cross occurs. Then after a delay an output pin enables the TRIAC.
     
  8. MaxHeadRoom

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    That is essentially what the AN-3006 shows, I tend to call it phase angle control, rather than PWM.
    Max.
     
  9. chrisw1990

    Thread Starter Active Member

    Oct 22, 2011
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    Thanks for the insight! seems like quite a complex solution compared to the traditional pwm controlled (in low voltage scenarios anyway).
    And yes, due to the TOS i have explicitly stated that its a theoretical query, because i would like to know, not do! :)

    id be interested in this DXM thing you mention, is this Serial over the mains? or serial cable or what?
     
  10. chrisw1990

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    Oct 22, 2011
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    i must ask, simply how one would go about dimming an LED lamp, i posted a link yesterday to an instructable, and it says it doesnt dim PL lights.. ( i assume PL is LED )
    perhaps not.. im not sure how one would control an inductive load with pwm.
     
  11. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    For one implementation of DXM type of serial communication over AC power, look up the Microchip application note on X-10 protocol. This was gaining some popularity in consumer products in the late 1980s and early 90s. I haven't followed it since (it might be called DXM by now).


    LED brightness is normally controlled with a constant DC voltage Pwm of the LED itself. Your eye interprets the dimming behavior.

    The other option is to control the power supply with Pwm and feed different average currents to the LED.

    Please clarify your comment on inductive load for an LED.
     
  12. chrisw1990

    Thread Starter Active Member

    Oct 22, 2011
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    Ah, so it is that protocol! i had a look, apparently it doesnt handle cross-branch networking very well over RCDs, (i.e. your plug circuit and your lights..)

    what other mains communication could one use? i found small serial modules (serial => mains => serial) but i must admit any research i do seems to turn up the powerline ethernet modules!
    I guess wireless is easier for the majority of applications and i dont want to get this thread closed down so perhaps leave this for personal research!


    i got confused with the website i posted a link to previously:

    and im unsure of the acronym PL, which made me wonder whether it was some shorthand for LEDs.
    and then i mixed inductive loads and lighting up haha (i think because i wondered if the LED lamps were inductive loads)

    I must admit, this is a hugely interesting subject, more so than i thought it would be!
     
  13. chrisw1990

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    Oct 22, 2011
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    Also, what would be more efficient? a pwm MOSFET? or a TRIAC? obviously ones more complex than the other, but, i wonder which is more energy efficient
     
  14. MaxHeadRoom

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    Jul 18, 2013
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    You are mixing up two methods, Triac is usually limited to full wave AC loads or circuits, PWM is traditionally used for DC and is used all the time for inductive loads, DC motor control is an example.
    Max.
     
  15. chrisw1990

    Thread Starter Active Member

    Oct 22, 2011
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    youre more than likely right (re confusion) but i showed a MOSFET ac pwm circuit for incandescent bulbs, and the circuit wasnt for inductive loads (says on the page)
    but it mentions this PL bulb, and i dont know what that is

    [Edit - Correction for lack of google use]
    a PL bulb is the same as a compact flourescent tube in england (CFL) should have used google!
     
  16. MaxHeadRoom

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    Yes the Mosfet is controlling an AC load but it is via a DC bridge, this allows the Mosfet to be place in series with the AC load, so to speak.
    Another example of Mosfets switching an AC inductive load is using 6 DC fed Mosfets in order to produce 3 phase for a 3ph inductive or PM motor, servo motors for example.
    Max.
     
  17. bertus

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  18. chrisw1990

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    Oct 22, 2011
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