Voltage increase after rectifier issues

Discussion in 'The Projects Forum' started by Mike Bedford, Feb 10, 2015.

  1. Mike Bedford

    Thread Starter New Member

    Feb 10, 2015
    18
    0
    I have a power supply design I am working on and I am using switching regulators by CUI (V7805-1000 AND the V7812-1000) instead of a typical linear regulator. So far, in testing, I am happy with these switching regulators. However, I am concerned about the longevity of my design and wanted to know if anybody has any suggestions.

    Both of these regulators I am using now are rated to a max input voltage of 32VDC. I am feeding the regulators both with a 24VAC output transformer into a full wave bridge rectifier. Even though I should get 24VDC out of the rectifier, I actually get more voltage out. I am reading around 34VDC. This exceeds the regulator input by 2 volts. It seems to be working okay but I am not comfortable with this. In reading about full wave rectifiers it seems what I am seeing is normal as I need to factor in the peak voltage and should then be multiplying my rated 24VAC output by 1.4 which comes to 33.6V which is pretty much what I am seeing happen. I am a newbie to power supply design and this is more for a personal/hobby project.

    I was not aware of this and was hoping to feed my regulators with 24VDC to be well under the 32 volt max input. I would like to look at quick and easy ways to bring this voltage back down to 24VDC. I hope to not have to add too many components or cost.

    Any ideas?

    Thank you!!

    Mike
     
  2. #12

    Expert

    Nov 30, 2010
    16,247
    6,744
    How much current will you be using?

    There are several ways to do this. Picking the right transformer is easily the most effective. You could use a regulator rated for more than 40 volts. Failing that, you need to get the peak voltage down below 32. I say don't take it down to 24 volts by just wasting heat.
     
  3. ScottWang

    Moderator

    Aug 23, 2012
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    1. You can in series with a 5Ω/20W resistor between Vin and V7805 and V7812.
    2. Using another transformer as 15Vac,0V/1.5A then it will get about 20Vdc.
    V78xx-1000 datasheet.
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,414
    3,353
    AC voltages are RMS values, not peak amplitude.
    To convert from RMS value to peak, multiply the RMS value by square root of 2, i.e. x 1.414

    Edit: Just to make sure that you do not misquote me. This multiplication factor of 1.414 works a pure sine wave. The factor is different for other wave shapes.

    To be sure, the RMS value of a constant DC voltage is the same as the DC voltage (i.e. conversion factor is 1.00).
     
    Last edited: Feb 10, 2015
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  5. wayneh

    Expert

    Sep 9, 2010
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    And then subtract the diode drop for two rectifier diodes. Adding a few more diodes to drop as many volts as you want is an option.

    There are also 27V zener diodes. I don't think you'd have to drain much current through the zener in order to keep the voltage down at 27V, but that's just a hunch. It depends on the properties of your transformer.
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    If you have a 6VAC transformer of similar current-rating you could connect the secondaries of the two transformers in series, with appropriate phasing, to get 24-6 =18V and rectify that. Clunkier though (and perhaps not much cheaper) than just buying an 18V transformer :).
     
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  7. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,498
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    Maybe not quick or easy, but very cheap and effective is to remove a few turns off the secondary, you can usually reckon on an average of 2turns/volt.
    Very much easier if the transformer is Toroidal type.
    Max.
     
  8. ronv

    AAC Fanatic!

    Nov 12, 2008
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