AC Voltage Controller

Discussion in 'Embedded Systems and Microcontrollers' started by ammar shamsi, May 27, 2012.

  1. ammar shamsi

    Thread Starter New Member

    Jun 6, 2010
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    I want to built an AC voltage Controller in which voltage can be adjusted from
    0 volts to 230V very smoothly. which has to feed a step down high current transformer which will feed an heating element.
    I want to control the AC O/P by a microcontroller in atleast 100 steps.

    please help me out.
     
  2. be80be

    Senior Member

    Jul 5, 2008
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    You can use a TRIAC and at zero crossing phase shift it by firing the triac.

    Google Pic microcontroller AC voltage controller
     
  3. strantor

    AAC Fanatic!

    Oct 3, 2010
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    phase angle firing triac or SCR. or you could even use bang-bang type control with a relay if your heater has enough thermal mass.
     
  4. ammar shamsi

    Thread Starter New Member

    Jun 6, 2010
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    How is the PWM technique is used to control AC
     
  5. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    PWM can only be used for DC voltages.
    With AC they are using phase control circuits.
    When using a slow reacting heating element, bursts of AC voltages can control the power using the zero cross circuits.

    Bertus
     
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  6. ammar shamsi

    Thread Starter New Member

    Jun 6, 2010
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    How is the PWM technique is used to control AC
     
  7. bertus

    Administrator

    Apr 5, 2008
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  8. strantor

    AAC Fanatic!

    Oct 3, 2010
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    you could use PWM for AC. I'm not sure if PWM would be the proper term to describe it, but this is the bang-bang type of control I was talking about. If the period was say 6 seconds, or .17Hz, then you could control how many of those 6 seconds the control would stay on. 5.5 out of 6 seconds = 92% duty cycle. 1.5 sec out of 6 seconds = 25% duty cycle. This isn't an ideal solution, and should only be used if for some reason you are forced to use mechanical contactors or relays.
     
  9. be80be

    Senior Member

    Jul 5, 2008
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    I've tried what your talking about strantor but to dim light bulb it works but not very good.
    never tried with heating but that would be doable.

    I would check for the zero crossing cut some cycles out which works great
    I've used this type of control [​IMG]

    There a good write up about it here http://pcbheaven.com/circuitpages/PIC_DCV_Controlled_AC_Dimmer/
     
  10. Tahmid

    Active Member

    Jul 2, 2008
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  11. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Sure you can use the PWM technique to control AC as long as you keep the pulse width a hugely large multiple of the line frequency.

    I use a pulse width technique to control the lighting in my house. When it gets dark, I turn the lights on (enable or pulse high), and when I go to bed I turn them off (disable or pulse low).

    You can get a better control by using a control sensor with some hysteresis, which is how the thermostat works in my house. If I set the temperature for say 72°F the hot water circulation pump gets turned on when the temp goes below 72, and turns off when then temp gets to something above 72, say 74 or such. The AC control element is a relay (I can hear it click on and off).
     
  12. Markd77

    Senior Member

    Sep 7, 2009
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    What voltage and power does the heating element need?
    You mention a transformer, a more conventional way to do it (for a low voltage heating element) would be to step down the voltage with the transformer, rectify it to DC and then use PWM on that to control the heater.
    The advantage is that the control part is at a lower voltage, which is safer and because it's DC you don't need to worry about zero crossings and can use MOSFETS instead of SCRs or relays.
     
  13. Tahmid

    Active Member

    Jul 2, 2008
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    Or, another way could be to rectify the mains to high voltage DC and use that for the heating element. Use PWM and a high voltage MOSFET. This avoids zero-crossing and phase angle control or pulse skipping and instead relies on the easier-to-implement PWM.

    Hope this helps.
    Tahmid.
     
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