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ps filtering and regulation

Discussion in 'Electronic Basics' started by Jon Slaughter, Apr 17, 2007.

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  1. suppose I have a ps that has about 35VDC after filtering. I'm trying to
    determine what minimum filtering caps I can get away with and still have
    good regulation for ic's.


    I know that VR = I/f/C is my P2P voltage ripple size.

    Now can I just take that and subtract it from my total voltage and then make
    sure my regulator is a few volts below that or will it sill cause problems?

    e.g., I have 35VDC non-filtered. If say I use 3300uf cap for smoothing that
    gives me a ripple of about 5V at max 2A. This means I have a minimum voltage
    of ~30VDC. If I keep my regulator below about 25 volts then will I be fine
    and have good regulation? or do I really need to add the extra capacitance
    to improve regulation signficantly?

    I would imagine that the regulator can easily keep up with the ripple and I
    just need to use enough filtering to keep the minimum above the max voltage
    + headroom that I need and that there is no need to waste the extra
    capcaitance since it won't really be used? (obviously will probably double
    it just in case)

    e.g., it seems for the case above that if I have atleast 3300uf that I'll
    have approximately the same regulation as anything greater than 3300uf?

    Just trying to save some caps ;)

    Thanks,
    Jon
     
  2. Phil Allison

    Phil Allison Guest

    "Jon Slaughter"


    ** Means what ?

    The average rectified DC value is 35 ??

    That makes the peak voltage 54.

    You GOTTA get this right.

    Plus know what the regulation factor of the AC tranny is with diode and cap
    load.



    ......... Phil
     
  3. No, not average. Don't worry about that because it doesn't matter. Its just
    a number. As long as I'm consistant and don't mix then its ok. The numbers
    are not whats important at this point.
    Not really. Those numbers are not important. (I don't even remember what the
    exact rms/peak voltage is. I think its a 10:1 transformer)

    I didn't know what this means? You mean it might not be optimal and I'll
    need to increase the filtering?

    I'm not so concerned with the details as I have a lot of wiggle room with
    the actual numbers. Since its a power supply I'm building for myself I can
    pretty much do what I want with it within reason. I'd like to get atleast 2A
    at 12V which I think I can do pretty easy. Hell, I might go for a split
    supply.

    Just wondering if the regulators have any issues with a badly filtered input
    as long as the minimum is well above the regulated voltage(within reason of
    course). I'll figure out the details once I know this and I actually sit
    down to decide what I want. (busy doing a few other things right now)

    Thanks,
    Jon
     
  4. Phil Allison

    Phil Allison Guest

    "Jon Slaughter"

    ** You do NOT get to decide what doesn't matter - fuckhead.


    ** You do NOT get to decide what doesn't matter - fuckhead.



    ** You do NOT get to decide what doesn't matter - fuckhead.


    ** You do NOT get to decide what doesn't matter - fuckhead.



    ** The actual numbers and ratios are the ONLY things that matter.



    ** No. The tranny has to be rated and sized to do the job.



    ** You do NOT get to decide what doesn't matter - fuckhead.



    ** The reg ICs need at least 2 volts more input than output - at every
    instant in time and under all operating conditions - plus they must not
    overheat.

    The AC tranny is the crucial issue.

    Not the damn filter caps.



    ......... Phil
     
  5. "Jon Slaughter" () writes:

    As long as the bottom of the peak is above the minimum voltage of the
    regulator (ie the output voltage plus whatever the head room that the
    regulator requires), it can't make a different.

    Michael
     
  6. Linear regulators do have a frequency response curve. They
    reject low frequencies a lot better than high frequencies.
    Ripple with sharp corners will bleed through a bit more
    bounce in the regulated output than ripple that consist of
    less and lower harmonics of the line. Whether the
    difference matters depends on the use of the regulated output.
     
  7. Try away. But electrolytics change capacitance over time
    and temperature, so something that works one day mat become
    marginal on another.

    If you have excess voltage to get rid of, I think a better
    way (than using the smallest capacitor that produces the
    largest usable ripple) is to replace the capacitive filter
    with an RC filter. It lowers the RMS load on the
    transformer, for a given DC output current, lowers the
    ripple peak to peak magnitude, as well as lowering the
    higher harmonic content in the ripple, for a given size
    capacitor. It also lowers the capacitor RMS current and so,
    its internal temperature rise and cools the regulator by
    shifting some of its heat to the resistor. If you have a
    transformer that idles hot and this is all it is doing, it
    works even better if you move the resistor upstream of the
    transformer, because its drop reduces the core losses in the
    transformer.
     
  8. Sure it can. If we think of a regulator as an opamp then it has finite slew
    and so it will make a difference. Ofcourse will it be significant? Thats
    what I'm trying to find out but it seems not. I suppose the only drawback
    is the extra heat generated in the regulator.

    Jon
     
  9. This is not a a ps that will be used in expensive equipment or anything and
    the ps itself is not expensive so its not a big deal. There is a point
    though when adding more caps does not add anything significant to the
    regulation and I don't want to go to far over that point.
    I was just reading on this but not sure if its a good idea for my circuit
    because of the large current. Better to use an LC? I do have room for the
    inductors.

    Thanks,
    Jon
     
  10. If you don't mind the cost and size of the inductor, it is
    definitely the way to lower voltage efficiently, while
    improving ripple.
     
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