transformer pcb ground?

Discussion in 'General Electronics Chat' started by Kelp, Jan 19, 2015.

  1. Kelp

    Thread Starter New Member

    Dec 31, 2014
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    If I have a transformer feeding a power supply circuit on a pcb (rectifier, filter, regulators, etc), do I need a two wire or a three wire connector to accept the wires from the transformer.

    On my breadboard: There are two wires coming from the transformer plugging into the top and bottom of a rectifier (4 diodes). I've then got a positive vdc on the right wire coming out the the rect. and a negative vdc coming out of the left wire from the rectifier. The circuitry on the other breadboard is grounded back to the left wire on the rectifier, and the supply voltage is coming from right wire. So the DC flows out the right side of the rect through the circuitry and then back in the left side of rectifier where it then flows back through the transformer to ground (the neutral wire of the transformer).

    Is this right?


    So if I have a pcb, do I need a two wire connector from the transformer only?
    Or should I have a third ground connection that the circuit grounds/flows to?
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Please post a schematic - computer drawn or hand drawn. Your text is too hard to follow where we can say, definitely, that is what you want to do.

    Also, let us know if you only need a dual supply or single supply (-15, 0 and +15)
     
  3. Kelp

    Thread Starter New Member

    Dec 31, 2014
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    Here is my schematic shown with a 2 wire connector. It shows things flowing to ground but this seems to be misleading because there is no separate ground connection.
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    You are fine. The word ground" and "common" are what the little arrow means in your case. Do Not connect them back to the left side of the transformer.

    Earth or earth Ground would be done on your mains side (no need for that) on the right side of your transformer.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Your drawing shows a DC common, no indication of an earth ground, I see no indication of an indicated neutral on the AC supply side or?
    Max.
     
  6. Kelp

    Thread Starter New Member

    Dec 31, 2014
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    So if I use a 2 wire connector on a pcb the electrons in the circuit would flow back through the transformer to the neutral wire from the mains?
     
  7. Kelp

    Thread Starter New Member

    Dec 31, 2014
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    The wiring in my house is old and I don't have an earth ground in my mains power plug. This is why I started wondering about this.
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Your circuit shows it is isolated from earth ground, the transformer offers galvanic isolation from neutral or mains conductors.
    Although technically you should have the transformer frame connected to a earth conductor.
    Max.
     
  9. Kelp

    Thread Starter New Member

    Dec 31, 2014
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    So I guess my real question is "how are my breadboard circuits, which seem to be working with everything grounded back to the left connection of the rectifier, a closed circuit?" If the transformer is insulated from mains, how is the circuit grounded/closed?
     
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The circuit is a unit unto itself and is isolated from the mains power, the power to supply your circuit is derived from the isolated secondary.
    The circuit look fine and does not necessarily need to be connected to earth ground..
    Max.
     
  11. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Let's start from the beginning. It seems weird but a transformer is just two coils of wire around a block of steel. The primary coil actually connects the two prongs of your wall plug together as a dead short. The coil of wire has enough inductive reactance to prevent any current from flowing so all is well. The only time energy is consumed from the wall (even though those two terminals are connected by the primary coil) is when something is connected to the secondary coil. Then, transformer action caused energy to move to the secondary. The DC circuit is complete on the right side through the bridge rectifier and secondary loop is all you really have to watch. There are some nice animations if you google bridge rectifier. Or transformer animation.
     
  12. Kelp

    Thread Starter New Member

    Dec 31, 2014
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    Yes after I posted that I realized that. Thanks for the reply
     
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