Using a centre tapped transformer for 2 power supplies... possible ?

Discussion in 'The Projects Forum' started by roycruse, Oct 29, 2014.

  1. roycruse

    Thread Starter New Member

    Oct 29, 2014
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    Hi - Im new here and also just a noob at electronics.

    I know centre tapped power supplies are usually used in audio applications to provide + and - power rails but can they be used to drive 2 separate power supplies in the following config and if so how would they interact if i used them on the same circuit ie could they be configured to give +5v and -5v or how about +12v and +5v etc.

    This is what I had in mind - the two lm338 circuits were actually going to be a couple of velleman kits.

    IMG_20141029_143533.jpg
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,501
    380
    hi Roy,
    It would be unsafe to connect combinations of those two supplies, due to the common connection of the CTap.
    Whats the spec on your transformer.?
    E
     
  3. crutschow

    Expert

    Mar 14, 2008
    12,993
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    You can use that circuit to generate two separate outputs, but they can't be connected together with a common ground.
    The way to connect for a dual supply is to use one bridge connected to the top and bottom winding output. The center tap then becomes the common point for the two outputs as shown here. In effect, the bridge becomes two full-wave rectifiers, with two diodes generating the plus voltage and two diodes generating the negative voltage.

    Edit: Note that the output voltages of the above scheme are twice the voltage of your schematic.
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    It depends on the transformer rating. What is the AC voltage rating, end-to-end and current rating?
    What voltages and current do you need?
     
  5. roycruse

    Thread Starter New Member

    Oct 29, 2014
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    its a recycled transformer from an old cheap sub woofer with +12v -12V centre tapped coil if i use just one half of the coil, after being rectified and running through the lm338 i get about 17volts max out which is enough for what im doing.

    it would be ok to use them as 2 separate supplies for separate circuits though yes ?
     
  6. crutschow

    Expert

    Mar 14, 2008
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    As long as there's absolutely no common connection between the separate circuits, such as between any inputs or outputs.
     
  7. roycruse

    Thread Starter New Member

    Oct 29, 2014
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    Yes understood - thanks :)
     
  8. crutschow

    Expert

    Mar 14, 2008
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    You can get a plus and minus output by connecting one bridge to the top and bottom transformer connections and grounding the center tap. The two bridge outputs will then be the plus and minus voltages.

    An interesting alternate to give two plus voltages, one approximately 1/2 the other, is to again connect one bridge to the top and bottom connections, but ground the negative output of the bridge. Adding a filter to the transformer center tap will give a DC output at the center tap that is 1/2 the filtered bridge output.
     
  9. roycruse

    Thread Starter New Member

    Oct 29, 2014
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    If i connect the same setup as follows instead then - will i be able to use the two power supplies either independently or as a + and - supply if i need one.

    IMG_20141029_163224.jpg
     
  10. crutschow

    Expert

    Mar 14, 2008
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    With limitations. The transformer center tap is common between the plus and minus outputs so the circuit must be used as a plus and minus supply if the grounds of the two circuits are connected, but can power two separate circuits if they are completely independent and have no common connection. Otherwise you could zap the supply.
     
    Last edited: Oct 29, 2014
  11. tcmtech

    Well-Known Member

    Nov 4, 2013
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    If it was me I wold be tempted to just pop the wrapping off the transformer where the secondary windings go to the output leads and split the winding connection at the center tap thus giving you to independent windings to work with.
     
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