Power supply design

Discussion in 'General Electronics Chat' started by DigitalReaper, Sep 25, 2010.

  1. DigitalReaper

    Thread Starter Member

    Aug 7, 2010
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    I'm trying to build a small power supply for general experimentation. The plan is to have a fixed 5v output and a +/- supply that can be switched to 5, 12 or 15v. I picked up a transformer at a car boot sale that has isolated 8v (1A) and 14-0-14v (150mA) outputs. I've simulated the circuit in LTspice and the 5v side works fine.

    I was expecting to be able to draw up to 100mA from either side of the dual supply and stay within 150mA maximum input current per winding, however this seems to require ~193mA. The maximum output current I can draw while staying within the transformers limits seems to be 75mA (using 250 ohm dummy loads as in the simulation). I've not included the regulator section in the simulation as LTspice doesn't have the required 3 terminal regulators but they should have a negligible effect on current.

    My reasoning for expecting a 100mA max output is that if the output voltage increases by √2 (peak voltage) the available current should decrease by the same ratio.

    Is my reasoning wrong here or am I doing something wrong in my circuit?

    [​IMG]

    Also attached is the LTspice file.
     
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    Last edited: Sep 25, 2010
  2. Ghar

    Active Member

    Mar 8, 2010
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    I'm not entirely clear on what you're trying to do with this simulation.
    That circuit right there will show about 70 mA of output current because the load resistance is so high.
     
  3. DigitalReaper

    Thread Starter Member

    Aug 7, 2010
    70
    2
    Removed. Re-worded original post instead.
     
    Last edited: Sep 25, 2010
  4. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Your AC peak voltage is not correct. For 14V r.m.s the peak is only 19.8V but you have used 21.64V as peak. Using the correct peak value will lower your load current.

    Have you looked at the ripple current in the two 470uF filtering capacitors?

    That's where most of your current goes.
     
  5. DigitalReaper

    Thread Starter Member

    Aug 7, 2010
    70
    2
    I was using the measured no load voltage and winding resistance. I've switched back to using the rated voltage. That increased the current draw to 250mA for 100mA out.

    As for the capacitors drawing current, surely that current is then returned to the circuit via the load?
     
  6. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    It might help if u use 1000uF filter caps and 1N4001 for bridges
     
  7. DigitalReaper

    Thread Starter Member

    Aug 7, 2010
    70
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    1000uF caps increased the current draw by ~10mA, 1A silicon 200v diodes (ES1D, closest I could find to 1N4001's) increased current draw from 260mA to 340mA. :eek:

    Can someone confirm my reasoning for being able to draw 100mA from the output side is right?
     
  8. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    To tell u the truth I really donno what the heck u are trying to do.
    Can u explain in lamen's term
     
  9. DigitalReaper

    Thread Starter Member

    Aug 7, 2010
    70
    2
    I'm trying to make a linear regulated power supply with +/- rails. My circuit works in that regard, but I was expecting a higher output current capability.
     
  10. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Then U need a higher current capacity transformer.
    See it this way.

    If you use a fullwave bridge then
    Vdc = 1.414 X Vac
    Idc = 0.707 X Iac
     
  11. DigitalReaper

    Thread Starter Member

    Aug 7, 2010
    70
    2
    Idc = 0.707 x 0.15 = 0.106 = 106mA

    The problem is the transformer is maxing out (150mA) when i'm drawing only 75mA from the DC output.
     
  12. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Ur figures are almost correct.

    U need a another transformer for higher current
     
  13. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Are u drawing from dual polarity.
    If so, ±75mA will give you a total load current of 150mA which is the Tx rating.
    Problem is ur understanding

    U cannot draw 100mA from either polarities simultaneously.

    Dual polarity current is 75mA.

    Single polarity current is 150mA.

    Calculations and Experiment figures are in order.

    You are confusing the single and dual polarity current summing
     
  14. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
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    First of all 1N4148 is a signal diode, not a rectifier.
     
  15. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    That's not much of an issue since it can handle 200mA
     
  16. DigitalReaper

    Thread Starter Member

    Aug 7, 2010
    70
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    I didn't realize there was a difference, that would explain a few things :)

    I'd like to add some (fast blow?) fuses to this for safety, where is the most appropriate place to put them? on the transformer output? after the regulators? (not shown on the schematic).

    I'll probably add a slow blow fuse to the mains input anyway.
     
  17. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Hope u understood what u want.


    edit.
    corrected on below post
    Thanks potato
     
    Last edited: Sep 25, 2010
  18. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    Fast Blow can be susceptible to inrush currents from turning the supply on and charging the capacitors.

    I would take that into account and allow the primary slow blow to carry the supply faults protection.

    Place the fast blow fuses in a screw top -quick replace- mounting next to the outputs and let them protect the supply from overloading.
     
    R!f@@ likes this.
  19. DigitalReaper

    Thread Starter Member

    Aug 7, 2010
    70
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    What rating would you recommend for both primary and secondary side fuses?

    The transformer is 240v input, 8v (1A) and 14-0-14 (150mA) output. Inrush current at full load appears to be 3A and 14A and lasts 6ms.

    I'm planning to put the mains side fuse in a quick replace holder, but only for safety (I can heatshrink the connections). I don't mind opening up the case to replace the others.
     
  20. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Use a 100mA slow blow primary.

    Use Slow blow 150mA between Tx secondary and bridge.

    Use 100mA Fast blow at supply outputs
     
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