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Need help with a PSU schematic

Discussion in 'Power Electronics' started by solo2racr, Aug 21, 2013.

  1. solo2racr

    solo2racr

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    Aug 21, 2013
    I have been working on this for awhile and really could use another set of eyes to check it over for errors. Amp draw is approx 250mA on each of the 15 vdc outputs and 20mA on the +48vdc output. The PSU in my Soundcraft audio mixer died and was a SMPS. Rather than fix or repair it, I thought it would be cheaper, easier and better to use a linear PSU.

    I am attaching both the schematic and a zip file that can be opened in EAGLE software of the schematic.

    If you see any problems, please let me know. I would really like to get this right the first time.

    THANKS!!!
     

    Attached Files:

  2. davenn

    davenn Moderator

    13,293
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    Sep 5, 2009
    hi there solo
    welcome to Electronics Point :)

    ya got some serious problems in the circuit there. mainly around how you are feeding the 48V regulator

    don't know why you tied the negative lead of the bridge rectifier to the positive rail of the output of the top Brdg rect. ??? doesn't make sense just wrong
    should be going to your lower GND line ( at bottom of diag)

    not sure if you can connect that lower GND line to the transformer centre tap .... it may cause unwanted interaction with the split rail supply
    ( some one else can comment on that)

    you have capacitors in the lines from the transformer to the lower bridge rect. that's also a total no no, get them out of there

    cheers
    Dave
     
  3. solo2racr

    solo2racr

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    Aug 21, 2013
    Thanks davenn for the quick answer. The reason it is drawn the way it is, is because that's what I have been led to believe to be correct. Perhaps I should post a link to where I had been getting help from so you can see the back story. http://www.diyaudio.com/forums/power-supplies/239750-please-check-schematic.html The first page shows the original schematic that was drawn by another person that needed to replace the same PSU as I do. The schematic has gone through a few variations, mostly in the regulation area. The two others that have been helping me in the other forum are talking in terms that are over my head about half the time. So, I thought I would seek help elsewhere. I am trying to learn but being told so many different ways to do this, it's all getting rather confusing. I know this isn't rocket surgery and should be fairly simple to design a working schematic. Once I have that, I know I can implement onto stripboard for a working PSU. Ultimately, I just want to get my Soundcraft Mixer working again.

    THANKS for the help.
     
  4. davenn

    davenn Moderator

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    Sep 5, 2009
    heavens above !!! :(

    there's some bad info on the net some people have absolutely no idea

    your C4 and C6 could be a bit higher the norm would be ~ 10uF

    nothing wrong with the C3 and C5 values

    as said earlier, you MUST remove C9 and C10 ... you can NEVER have caps in series between a transformer and a bridge rectifier. Those ones would last a VERY SHORT time before they exploded.

    You must move that lower bridge - terminal to the lower GND rail

    Dave
     
  5. solo2racr

    solo2racr

    142
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    Aug 21, 2013
    Again, THANKS for the quick response and for going over the back story. What you are saying in regards to C9 & C10 as well as the GND rail make sense to me. I also questioned the caps before the bridge but, I consider myself a noob that doesn't know all that much about electronics. Just enough to get into trouble. Which I really do not need for this project. It's 2:00 am here in the U.S and I will revise the schematic tomorrow.

    I am also considering getting another transformer (a 25.2v CT) for the +/-15vdc rails just to simplify things, even though the transformer in the schematic has a high enough VA to handle everything. Two separate PSUs I understand and can handle.
     
  6. duke37

    duke37

    5,231
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    Jan 9, 2011
    I would think that C9 and C10 are OK. Are they not feeding as voltage doublers?
    The normal voltage doubler is half wave, this would be full wave.

    I know because I have a gold star !
     
  7. solo2racr

    solo2racr

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    Aug 21, 2013
    OK......One person adamant they must not be there (C9 & C10) and another says they are OK. I have no idea. :confused: Can I get a consensus?
     
  8. solo2racr

    solo2racr

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    Aug 21, 2013
    How does this look?????
     

    Attached Files:

  9. davenn

    davenn Moderator

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    Sep 5, 2009
    Duke

    show me one other circuit where there are electros in series with the transformer and the bridge rectifier
    in any normal PSU ????

    I have NEVER seen something like that

    Dave
     
    Last edited: Aug 21, 2013
  10. davenn

    davenn Moderator

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    Sep 5, 2009
    yup that looks great, solo. :)

    only comments
    .... the large electros after the BR's, the 470uF's, may need to be a bit bigger ... at least 1000uF ...
    there's a general rule of thumb ... 1000uF / 1A of current drawn, so if you are drawing 0.5 to 1A, at least 1000uF would be best

    .... you need to make sure the input voltage to the bottom regulator is at least 2V higher than the required output voltage
    ie. ~ 50V


    Dave
     
    Last edited: Aug 21, 2013
  11. solo2racr

    solo2racr

    142
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    Aug 21, 2013
    The draw on each rail shouldn't be any more than around 250mA. But I do have on hand bigger caps.

    The 40vac rectified should yield around 56vdc going into the LM317HV
     
  12. duke37

    duke37

    5,231
    718
    Jan 9, 2011
    Dave

    Wikipedia describes the voltage doubler as a Greinacher circuit. I do not see how to down load the diagram, I could sketch and scan if wanted.

    The doubler as normally used contains two rectifiers. Here, two voltage doublers driven in anti-phase are used with outputs connected in parallel.This will double output current and reduce ripple. Electrolytic capacitors will be needed to reduce output impedance.

    I have used this in a power supply but then I am not normal !

    Another transformer would do the job as shown but will be bigger, heavier and more costly.

    Duke
     
  13. solo2racr

    solo2racr

    142
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    Aug 21, 2013
  14. solo2racr

    solo2racr

    142
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    Aug 21, 2013
    I have been doing some more reading about the caps C9 & C10 and voltage doubler circuits. I don't know enough to know if they are implemented correctly here but, I did learn that a doubler circuit will also lower the ripple. Given the purpose of the 48vdc (Phantom power for microphones) ripple, or the lack there of, is important. Noise in the PSU will be transferred to the mic and that is bad when it comes to studio recording. I am sure this is why they were originally placed in the circuit. I also learned that this method of ripple reduction, when followed by a regulator, gives zero or virtually zero improvement. With that in mind, I believe the best course of action for me to take on this PSU is to leave them out.

    Any other thoughts on this??? :D
     
  15. davenn

    davenn Moderator

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    Sep 5, 2009
    yes, for voltage doubling and not using electro's and not in each leg of the transformer
    before any of the diodes

    eg from wiki a basic doubler ... this as others on that page is sure to use non polarised disc ceramics etc

    The Greinacher voltage doubler
    [​IMG]

    but for a standard PSU, no, I don't see the need for voltage doubling here ??

    I'm always willing to learn something new if you can show me something specific :)

    Dave
     

    Attached Files:

    Last edited: Aug 22, 2013
  16. solo2racr

    solo2racr

    142
    0
    Aug 21, 2013
    Here is the schematic that I have worked out, that removes C9 & C10.
     

    Attached Files:

  17. davenn

    davenn Moderator

    13,293
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    Sep 5, 2009
    hi solo
    in a standard PSU non-doubler or tripler etc circuit there is no ripple as such before the bridge rectifier. As you have the full AC voltage.
    Ripple is usually referring to the small amount of AC that is still present on the DC Voltage after rectification and initial capacitor smoothing .....

    [​IMG]
     

    Attached Files:

  18. davenn

    davenn Moderator

    13,293
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    Sep 5, 2009
    I see you note about the only ~ 250mA likely drain on the 15V rails
    you may well get away with the 470uF caps :)

    for a little basic PSU info....
    if you look at that last diagram I did ... you can see how the capacitor fills in the gap between the 1/2 cycles.
    If the capacitor isn't big enough for a given load then it cannot store enough charge (current) to hold over till the next 1/2 cycle charges it.
    As a result the voltage will sag ( drop) and that will be evident in an increase in measured ripple voltage

    Dave

    PS just make sure your transformer is rated for at least 1 amp on the secondary
     
    Last edited: Aug 22, 2013
  19. solo2racr

    solo2racr

    142
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    Aug 21, 2013
    That's how I understand it as well. If the psu has no regulation after the bridge, then I can see where the caps would help. But, from what I gleaned from what I was reading, with a regulator in place (in this instance, an LM317, 337,317HV) any smoothing from a voltage doubler is for naught. The regulator is more than capable all on it's own and there is no realized benefit from the C9 & C10 caps.
     
  20. solo2racr

    solo2racr

    142
    0
    Aug 21, 2013

    I may have some larger ones here. I'll have to look. It's won't hurt a thing to go larger.


    1.2A....... I just re-confirmed by looking at the datasheet again.

    Now I get to lay this all out on Vero.....again.

    DIY Layout Creator is a good freeware program for that.
     
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