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Power supply voltage step down and regulation

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by ADWSystems, Sep 1, 2017.

  1. ADWSystems

    ADWSystems Member

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    I have purchased a (Chinese) 12VDC 20A power supply from eBay.

    One of these

    http://www.ebay.com/itm/DC-Power-Su...2A-15A-20A-30A-Plug-Pigtail-lot-/401103056399

    I don't have an oscilloscope to see how bad the voltage regulation and noise output is. I presume for the price, pretty bad. BUT if the power supply is used to "convert" 120 VAC to 12VDC (as it does) and the 12VDC is then used to drive a linear step down voltage chip (such as a 7805, NCP1117, or similar). Would the poor regulation and/or noise be passed to the output of the second stage and the final voltage?


    P.S. Yes this is the same power supply as listed in the diaphragm pump thread, but I'd like to discuss its use in a different application. The power supplies are pretty inexpensive as long as they are usable.
     
  2. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    From datasheet.
    If the 12 volts varied from 12V to 13V the output of 5 volts will change 0.01% max.
    If your load changed 1A the output voltage will change by 0.3% max. (100mA load change = 0.03%)
    120hz noise (60hz) will be reduced by 80db.
    upload_2017-9-1_20-16-21.png
    Noise on your 12V will be reduced by xdb as shown. This is dependent on capacitors and type of caps.
     
  3. Flyback

    Flyback Well-Known Member

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    I woudl just add some capacitors, (less than the amount which will make the psu go unstabl) unless its a really sensitive app like a radio receiver receiving a weak satellite signal.
    Ive had so many cases of micro's not working because of "psu noise", which ended up being faulty software, or floating input pins on the micro, etc.
     
  4. dave

    Dave New Member

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  5. Flyback

    Flyback Well-Known Member

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    In fact, dont even bother adding caps, as there will be enough output caps in the output of the psu.
    Another point is that adding a linear reg to the output wont do anything to help common mode noise.
     
  6. JonSea

    JonSea Well-Known Member

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    Bear in mind, dropping 7 volts across a 7805 will generate a lot of heat.
     
  7. ADWSystems

    ADWSystems Member

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    It was only an example, used to describe the setup to help everyone in the evaluation of the effects of how well a "secondary" voltage regulator may clean up the output of a power supply that isn't very pretty.
     
  8. JonSea

    JonSea Well-Known Member

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    And I was only pointing out that if you think you're going draw an amp at 5 volts for example, you will be dissipating 7 watts in the regulator.

    I'm posting this for others who may read this post and not be aware of this fact. From your question however, I would not assume you have an extensive knowledge of linear regultors - perhaps my mistake.
     
  9. ADWSystems

    ADWSystems Member

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    (a) perhaps a little rusty; but more so (b) never in this application, of trying to correct a poor power supply output. I have "cascaded" them to when connected to a good power supply, but not to (what may be) a poor supply.
     

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