Eliminating a ripple on a DC signal

Discussion in 'The Projects Forum' started by maxi, Mar 22, 2013.

  1. maxi

    Thread Starter New Member

    Jan 17, 2013
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    I'm designing a power supply with a 120V AC input and an output of 5V DC ..
    I'm using an AC to DC step-down converter (RECOM RAC02-05SC) which outputs 5.6V instead of 5V
    followed by a capacitor which should take out any ripples. (image attached) and a zener diode to limit the voltage to 5V.
    the problem is my output signal is not exactly DC! it contains a periodic ripple of around 300 mV!
    I tried increasing the capacitance across the output to about 1.2 F and that ripple still existed.
    what is a good way of getting rid of that ripple?
    also what would cause the zener diode to get really hot?
    I'm very limited in circuit space so hopefully small size parts would do.
    thanks in advance
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    are you using low esr caps?
    how much ripple can you tolerate?
     
  3. russ_hensel

    Well-Known Member

    Jan 11, 2009
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    1.5 F is a big cap. Sure value is right? A zeener is not useful for much current, and needs a series resistor. I would use a higher input voltage and a three terminal regulator.

    Of course if you had a spec for what you wanted to do we could be of more help.
     
  4. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Did you see the schematic on page 2 of the data sheet for the part? It shows how to remove ripple if needed.

    Is your 5.6 volts under load? If not it will probably be the rated 5volts when loaded.

    Data sheet - http://www.mouser.com/ds/2/468/RAC01_02-SC-16262.pdf
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Here is the datasheet:
    http://www.mouser.com/ds/2/468/RAC01_02-SC-16262.pdf
    The maximum capacitive load for that supply is 2200uF; that's both your external capacitor AND the load together.

    What is your load's requirements? This supply (RAC02-05SC) can output 5v@400mA maximum; I suggest that you don't try to exceed 320mA.

    The regulation is specified to be within 6%, which gives a range of 4.7v to 5.3v; so your supply is out of specifications, and is faulty. You should obtain a replacement and not attempt a "band-aid" approach.

    A Zener won't be useful to limit the output voltage; however it'll make a nice small space heater.
     
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    funny... :D
     
  7. maxi

    Thread Starter New Member

    Jan 17, 2013
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    I'm using an 330 uF electrolytic capacitor recommended in the data sheet..
    I forgot to mention that im designing a power supply to power an ACS712 hall effect current sensor which would provide current measurement for a digital watt meter.
    I can't have much ripple since that would affect my reference voltage and probably would give inaccurate measurements!
     
  8. maxi

    Thread Starter New Member

    Jan 17, 2013
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    The original capacitor value was 330uF.. I tried adding higher capacitance (1.2F) just for testing purposes to see if the ripple would be eliminated but no luck.
    What do you mean by higher input voltage?!
    And yea I beleive a voltage regulator would be A good idea, but that can still drop my voltage below the range needed!
     
    Last edited: Mar 22, 2013
  9. maxi

    Thread Starter New Member

    Jan 17, 2013
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    I added the capacitor recommended in the data sheet, but it didn't do the job!
    I tried putting a load of 12KΩ and the voltage drooped to about 5.4V
     
  10. maxi

    Thread Starter New Member

    Jan 17, 2013
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    Im powering an LCD as well as an ACS712 current sensor, so it won't exceed the specified current output.
    Im trying to finish the project by the end of next week so Replacing the part would be the last thing I need!! Neither the Zener nor the out of range voltage are the main concerns here.. Getting rid of the ripple is!!
    Thanks
     
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