ac virtual ground..help plz

Discussion in 'General Electronics Chat' started by thehaders, Jan 28, 2009.

  1. thehaders

    Thread Starter New Member

    Nov 14, 2008
    6
    0
    Hi all,

    This is my first time posting a question here. So, if i do anything wrong, please point me into the correct direction

    I was wondering how can I split a 24V ac into +12V ac and -12V ac. I think I have to use the virtual ground circuit don't I? I found some stuff on how to split the dc voltage but do same methods apply to ac as well?

    If anyone wondering, I need to build a fully controlled phase angle controller ac-dc rectifier by using thyristor.

    Thank you all. Really appreciate your help.
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Perhaps you should start by reviewing your AC and DC theory?

    There is no such thing as + or - AC.

    If you were to give more detail of your application it might be easier to help.

    You cannot control both positive and negative half cycles of AC with a single thyristor, you need at least two thyristors to do this, or a triac.

    You can also feed the AC to a bridge rectifier, which inverts alternate half cycles of the supply, and then control the whole wave by placing a single thyristor across the output of the bridge.

    If you actually need the centre ground, as asked, for some reason you can create this with a centre tapped transformer, but you will still need two thyristors to achieve control.
     
  3. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    A center tapped 1:1 transformer would do it, no extra needed.

    In addition to what Stud said, you might ask yourself why you need it.

    Makes me wonder though, if you did the diode bridge and filter, creating a DC power supply, then used my article, wouldn't that work? Overkill and a kludge, but it would be an apparent ground to the AC also.
     
    Last edited: Jan 28, 2009
  4. thehaders

    Thread Starter New Member

    Nov 14, 2008
    6
    0
    i am teribbly sorry for not making the question and specs clear

    basically, i have to build the full wave(4 thyristors) phase angle controller for controlling the speed of the small dc machine in the lab,the problem is i cant get the full wave controlled rectifier to work. i only managed to make the half wave controlled rectifier to work by using only 1 thyristor in the lab.

    so i thought i want a different approach. instead of using centre tapped transformer, i would like to have something like centre tapped virtual ground that produces two 12v ac that 180 degree out of phase each other( resembling the centre tapped transformer operation).

    so i supposed, how can i get 2 equal sine voltages that are 180 degree out of phase form each other from a single sine wave supply by not using the centre tapped transformer.

    thanx again. im so sorry for any confusion caused
     
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Well

    Two sine waves, rectified, equal and opposite or otherwise, does not make full wave rectification.

    You only need two thyristors in a bridge to effect FW rectification. The other two arms can be simple power diodes.
    Controlling four thyristors can be difficult as you have found out.

    Does your application require the output to be DC or rectified AC?
    Do you know the difference?

    What you are asking for is known as a class B power amplifier and is perfectly possible at power levels up to several kilowatts.
    However this is overkill on a grand scale and totally unecessary.
     
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