Quick Questions - Transformer to be used for PSU

Discussion in 'Power Electronics' started by Riaan Pretorius, Oct 29, 2017.

  1. Riaan Pretorius

    Thread Starter New Member

    Oct 29, 2017
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    Hi guys. So I currently pursuing the idea of building a variable DC bench power supply allowing for bipolar voltages. I have a bit of trouble understanding what to do in regards with the grounding reference of the soon to be DC PSU.

    So most sources recommend using a center-tapped transformer. My question in regards with the center-tapped transformer is whether or not to connect the center tap of the secondary side to the earth (ground) provided by the wall plug or just use the center tap as a reference to ground (sort of like a virtual ground)? (See the dual-supply image)

    Other sources suggest using a dual output transformer and then to rectify each output and finally connect the two outputs in series and grounding the section of wire between the 2 DC supplies. Again, is this ground only a reference voltage or should it also be physically grounded? (see the other image)

    Thanks
     
  2. dl324

    AAC Fanatic!

    Mar 30, 2015
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    Welcome to AAC!
    You don't need to connect power supply ground to earth ground unless you need that for a common ground reference.
    Same answer regarding whether to connect circuit ground to earth ground.

    Regarding using separate windings as configured in the second schematic, the reasons for doing that vary. It would have been simple to connect the transformer secondaries in series, ground the connection between them, and use the first circuit.

    Reasons for using two transfers could be
    1. a center tapped transformer wasn't available
    2. couldn't find a CT transformer with the correct rating
    3. worried about not being able to determine polarity
    4. had single winding transformers and didn't want to buy a CT transformer
    Using two rectification circuits and then connecting outputs in series could be due to current rating of the diodes.
     
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  3. Riaan Pretorius

    Thread Starter New Member

    Oct 29, 2017
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    Thanks for the reply, this helps a lot. But what you are also suggesting is that if I could not find a CT transformer or a dual output transformer with the correct rating, I could just use 2 separate transformers with the same rating (half of the required secondary voltage as the CT transformer would have been), rectify each transformer and then to connect them is series and Voilà, a DC power supply?
     
  4. #12

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    It's simpler than that. Wire the two transformers as if they were one (dual output winding) transformer and just rectify that. It saves a couple of redundant diodes.
     
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  5. dl324

    AAC Fanatic!

    Mar 30, 2015
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    As @#12 has pointed out. You just wire the transformer secondaries in series and use the common point as the center tap. You just need to get the secondary polarity correct.
     
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  6. crutschow

    Expert

    Mar 14, 2008
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    The advantage of using one bridge circuit with a CT rather than two bridges, is that there is one less diode drop in the output with the single bridge.
    A single bridge rectifier with a CT is actually acting as a full-wave rectifier circuit for the positive voltage, and a separate full-wave rectifier for the negative voltage (if you follow the current path for each voltage, you will see this).
    A full-wave, center-tap rectifier only has one diode-drop in series with the output whereas a bridge rectifier has two diode drops.

    Another small advantage of the single bridge is that each half of the transformer supplies half the current for each output, thus if you were only using one output voltage for example, the current draw is still balanced between the two halves of the winding.
     
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  7. Riaan Pretorius

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    Oct 29, 2017
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    Thanks again @dl324 and for the diagram @#12. Things just got a lot less complicated
     
  8. #12

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    That's what we do here.;)
     
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  9. Riaan Pretorius

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    Oct 29, 2017
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    In essence, you are saying that even if I don't use both rails at the same time, or even if each rail is set to different voltages (say 5V and -12.5V), each transformer will still draw equal amounts of current? If that is true, then that means if I'm going to use 2 separate transformers, can I effectively half the required power rating?
     
  10. #12

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    Not clearly worded but the answer seems obvious if I assume. When needing 10 VA of power and using 2 transformers, each one must be rated for at least 5 VA.
     
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  11. Riaan Pretorius

    Thread Starter New Member

    Oct 29, 2017
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    Yeah sorry, not the best phrasing but yes that was my question and it got answered. Now I think I got all the details sorted. Thanks everybody.
     
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