+24VDC circuit being run off +/-16VDC supply, positive rail keeps failing

Discussion in 'Power Electronics' started by danotaur, Sep 8, 2016.

  1. danotaur

    Thread Starter New Member

    Sep 8, 2016
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    0
    Hello,

    Looong time lurker here and it's finally time to come forward for help, so thank you in advance!

    I have built a microphone preamp that requires +24VDC, ~100mA. The power supply for it is +/-16VDC, 3A. The power supply is a standard 317/337 dual regulated circuit that has powered multiple pieces of gear over the years.

    I implemented an LM2585 DC-DC converter to boost the +/-16VDC to +28VDC, then I regulated the voltage down to +24VDC using an LM7824. The circuit I used is straight off of the data sheet, Fig. 50 on page 19:

    http://www.ti.com/lit/ds/symlink/lm2585.pdf


    One of these pre amps work fine, even two or three but any more than that and the positive rail shuts down and goes into about 3.5VDC, the negative rail holds strong at -16VDC. I'm not overloading the supply current wise as I always stay under half an amp and well within the supply's limits.

    The negative rail is connected to the pre amp but goes to a 47uf 63V electrolytic to ground.

    Since there is no load on the negative rail is the positive rail losing some sort of common reference? Since the positive and negative rail are regulated separately I don't see why they would affect each other.

    I don't have schematics unfortunately, my bad habits in design. The power supply and mic pre work just fine until I use the DC-DC booster and even then I only see a problem when I have multiple pre's with the booster circuit being powered by the same supply. I'm fairly confident in everything except how advisable it is to only use the positive rail and basically ignore the negative rail.


    My main question is if it is harmful to the power supply to have this unit loading only the positive rail and ignoring the negative rail?


    I hope that isn't too disjointed of a question and there is enough info to help you get an idea of what's going on.


    THANKS!!

    Daniel
     
  2. #12

    Expert

    Nov 30, 2010
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    Probably not.
    However, I wonder why you designed an audio pre-amp that requires 100 ma when I can do it for 1 ma.
     
  3. danotaur

    Thread Starter New Member

    Sep 8, 2016
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    #12, it is a discrete pre amp modeled after a Neve 1073. The original design runs on +24.
     
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Maybe a picture???
    Seems to me you could put the +/- 16 volt supplies in series, call the -16 ground then regulate what is now 32 volts down to 24. This assumes the +/- supplies have separate windings.
     
  5. #12

    Expert

    Nov 30, 2010
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  6. danotaur

    Thread Starter New Member

    Sep 8, 2016
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    I have multiple solutions that work just fine, a cheap 24W AC wall wart with a rectifier bridge, the psu you have listed, a bench supply. I understand the workings of the pre amp better than the power supply which is why I'm asking about this specific way of doing it and not for alternative ways of going about it. The original design for this piece of gear was designed for one 24V rail and I'm trying to fit it into a situation where my only option is +/-16. The unit works ok until I have multiples hooked up at the same time which causes the positive rail to fail so even though I only need to run one or two at a time and they seem ok, I'm still wondering if there is a deeper issue I'm missing.

    I've done a bunch of reading on power supplies but haven't come across anything like I"m describing. My main concern is that there is something I'm completely missing when using a two rail setup and only using one rail. If it's an ok thing to do and no one see's any problems then great but I'd still be very interested in knowing why or why not something like this is a plausible way of powering something.

    Also, the Neve is very expensive which is why I took a shot at making one myself for a tenth the price.
     
  7. ronv

    AAC Fanatic!

    Nov 12, 2008
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    From your description it looks like this?
    upload_2016-9-8_20-13-53.png
    If so the 317 may be in current or thermal limit with 400ma. load at the output.
    Why 2 regulators?
     
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