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Can I use 2 x transformers?

Discussion in 'General Electronics Discussion' started by Balrock, Mar 11, 2012.

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  1. Balrock

    Balrock

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    0
    Mar 1, 2012
    I am building a power supply for my mE analog dev board, The board requirs +10 & -10 volts to power it. So I decided to build a PSU.

    I have designed most of it when I had an idea. My transformer has a secondary of . . .

    12v 0 12v which that will allow me +12 & -12 volts(ish) output

    I have looked at getting a higher output voltage. Instead of using a larger more expensive replacement transformer could I just use another (12 0 12) transformer and use one for the positive voltage and one for the negative voltage? Like . . . .

    Transformer 1 12v 0 12v Use this for +24v

    Transformer 2 12v 0 12v Use this for -24v

    which would get me roughly +24v -24v adjustable PSU.


    Is this a silly idea?

    The reason I though it may be a good idea is cost and size. The tranformer is board mounted and smaller than the 22000uf smoothing cap that im using for each rail.


    Paul
     
    Last edited: Mar 11, 2012
  2. Rleo6965

    Rleo6965

    585
    9
    Jan 22, 2012
    I think you can. You have to use 2 bridge diode for each transformer.
    Bridge1 for +24V and -side of rectifier would be connected to ground.
    Bridge2 for -24V and +side of rectifier would be connected to ground.
     
  3. davenn

    davenn Moderator

    13,836
    1,951
    Sep 5, 2009
    Hi Paul,

    22,000uF ?? what is the current rating of the PSU ? The usual rule of thumb is 1000uF / amp. I'm picking that if the transformers are so small then your transformer current capability is also very small probably less than 1 Amp. You would only need ~ 2200uF at the most any more would be a total overkill and waste so much space on the PCB

    Dave
     
  4. OLIVE2222

    OLIVE2222

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    25
    Oct 2, 2011
  5. Balrock

    Balrock

    39
    0
    Mar 1, 2012
    Thanks everyone for your comments. This is what I have come up with.

    @davenn
    I took the design from an audio power supply so it maybe the reason. Thanks for the guidance.

    @Rleo6965
    Please see my diagram.

    @OLIVE2222
    I have read a bit about the symetrical designs that you kindly recommended. From what I could see there was quite a big current drop. As an additional transformer only cost me a few pounds I stayed with the Positive and negative regulators.

    I have also seen designs using power darlington transistors over the regulators to increase the current output. What are the downsides to using this approach and would I need to increase the power rating of some of the components if I do this.

    I realise these may be fundamental questions but I would rather ask at the risk of looking stupid than not ask than learn less.


    Notes:
    All diodes are suppose to be 1N4004. Just because thats what I have.
    The +V output is +1.3 to +25vdc.
    The -V output is -1.3 to -25vdc

    Please feel free to comment. Thankyou again for you help.

    Kind Regards

    Paul
     

    Attached Files:

    Last edited: Mar 11, 2012
  6. Rleo6965

    Rleo6965

    585
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    Jan 22, 2012
    @Balrock

    This is my first time saw this kind of ps diagram. Is that half wave rectifier for each voltage regulator?
     
  7. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    The diagram looks reasonable to me. The rectifier diodes are only 1A.
    A pass transistor shouldn't need to be a Darlington with a gain of 1000, an ordinary transistor with a gain of say 20 should give all the current that you require.

    Remember that the higher the input voltage and the lower the output voltage, the greater the amount of heat to be dissipated.
     
  8. Balrock

    Balrock

    39
    0
    Mar 1, 2012
    @ Rleo6965
    It's full wave.


    @duke37
    Good point about the diodes. I was planning on some pretty big heatsinks but it maybe a better solution to be able to use a physicall switch to move from a single +12 -12 v transofmer to +24 - 24 v dual transformer setup. I would normally be only using +10 -10 v so it would make sense just to be able to switch the other transformer "in circuit". Not the most elegant or technical solution but it would fit my needs quite well.
     
  9. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    You could leave both transformers in circuit and move the output from the ends to the centre of the transformers. Two pole change over switch required.
     
  10. Balrock

    Balrock

    39
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    Mar 1, 2012
    Sounds like a plan Duke. Consider this one sorted!
    I can get the stripboard designed now.

    Thanks again everyone ;)
     
  11. davenn

    davenn Moderator

    13,836
    1,951
    Sep 5, 2009
    Your output cap the 10uF on the negative rail is reversed in the diagram!!

    Dont forget that the 0V (GND) rail is more positive than the -24V rail

    edit... ohhhh so is C1 in the input to the regulator on the positive rail

    now for your info on circuit symbols, with an electrolytic cap the dark end ie. the diagonal lined end in your diagram is the negative terminal of the cap

    [​IMG]

    Dave
     

    Attached Files:

    Last edited: Mar 11, 2012
  12. Balrock

    Balrock

    39
    0
    Mar 1, 2012
    As I am nearly at the end of my PIC project this seems like the perfect opportunity for my next one. I have already planned the use of a PIC driven LCD display showing each variable supply output. As the PIC side of things is all but assembled together it only seems logical to add a few more components and make the switching in and out of circuit of the transformer and the variable voltages all controlled by a PIC. This way I could have the following;

    Uni-polar mode (+)1.5 - 50vdc
    Bi-polar mode (+ & -) 1.5 - 25vdc
    Auto Power down
    etc.


    Good to have a reason to learn.Best of all I have all the parts and plenty of free time :)
     
  13. Balrock

    Balrock

    39
    0
    Mar 1, 2012
    Thanks dave! Yes that was an example of a of copy and paste error!

    I'll go and repair my errors now :)
     
    Last edited: Mar 11, 2012
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