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Measuring Ripple???

Discussion in 'Electronic Basics' started by [email protected], Jun 7, 2006.

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  1. Guest

    I was wondering if there were any way to measure ripple with a DMM or
    something? I built a power supply (AC to DC conversion) from a
    transformer, full wave rectifies, some capacitors etc... and want to
    measure the ripple Any ideas? Thanks, Lucas...
     
  2. The Phantom

    The Phantom Guest

    Which ripple? Ripple voltage on the reservoir capacitor? Ripple current
    in the reservoir capacitor? Do you have an oscilloscope? Does your DMM
    measure true RMS voltage and current?

    I'm sure the people here will have lots of ideas.
     
  3. John Fields

    John Fields Guest

    ---
    Usually, you use an oscilloscope. If you don't have one you can
    measure the AC voltage on the DC output of your supply by putting
    your DMM on "AC VOLTS" and multiplying what you read by 2.83.

    It won't be perfect because the ripple waveform won't be a sine
    wave, but it'll be close.

    For example, I measured 0.443VRMS (which is 1.25VPP) using a true
    RMS meter, and my scope gave me 1.3VPP. Pretty close...


    You can also calculate what it'll be by using:

    It
    Vr = ----
    C

    Where Vr is the ripple in volts,
    I is the load current in amperes,
    t is the period of the ripple waveform in seconds, and
    C is the capacitance of the reservoir cap in farads.


    Let's say that you've built a 12V supply using a transformer
    connected to 60Hz mains, a full-wave rectifier and a 10000µF
    reservoir capacitor, that you're feeding its output into a 12 ohm
    resistor, and that you measured the voltage across the resistor and
    found it to be 12V. That means the current into the resistor will
    be:

    E 12V
    I = --- = ----- = 1 ampere
    R 12R

    Or, you could measure the current directly with your DMM.


    In any case, you now know what the load current is and you have what
    you need to figure out what the ripple is:


    It 1A * 8.33E-3s
    Vr = ---- = --------------- = 0.833 VPP
    C 1E-2F


    That 8.33 milliseconds is because full-wave rectified 60Hz makes
    120Hz ripple.
     
  4. Guest

    Thanks for all the great answers...I really appretiate it. Lucas
     
  5. Or a high impedance analog voltmeter where the analog meter movement
    responds to true RMS.
     
  6. John Fields

    John Fields Guest

     
  7. The RCA "Voltohmist" was reincarnated as the VIZ with solid state
    electronics. Had mine for 30 years and still accurate.
     
  8. Phil Allison

    Phil Allison Guest

    "DecaturTxCowboy"

    ** Huh ?

    Analogue volt meters with *moving iron* movements are scarce as hen's
    teeth.




    ........ Phil
     
  9. Phil Allison

    Phil Allison Guest

    "DecaturTxCowboy"


    ** Ain't " true rms ".




    ....... Phil
     
  10. Phil Allison

    Phil Allison Guest

    <

    ** Just use your DMM on AC volts. .

    You may need to connect a cap ( 0.1 uF ) in series with the meter.

    Multiply the reading by 3 to give the peak to peak ripple voltage.





    ........ Phil
     
  11. Perhaps I should have said "RMS" instead of "true RMS".
     
  12. Phil Allison

    Phil Allison Guest

    "DecaturTxCowboy"


    ** Bout the same difference between a gal being "plump" and "
    pregnant".






    ...... Phil
     
  13. Still not quite sure what you are saying.
     
  14. The Phantom

    The Phantom Guest

    Perhaps he's not quite sure what *you* meant by:

    'Perhaps I should have said "RMS" instead of "true RMS".'

    I'm not either. What's the difference?
     
  15. Phil Allison

    Phil Allison Guest

    "The Phantom"
    DecaturTxCowboy
    Phil Allison wrote:



    ** An "rms responding" meter is also "true rms" - such meters indicate
    the rms value of any continuous waveform within the frequency limits of the
    meter.

    However, most AC meters are "average responding" types that have been
    calibrated to read the *rms* value of a sine wave. Such meters do not
    correctly read the *rms* value of other AC waveforms.





    ......... Phil
     
  16. The Phantom

    The Phantom Guest

    Actually, I already know this, Phil. I was just trying to find out if
    DecaturTxCowboy does.
     
  17. To me, the RMS voltage is the amount of rectified voltage that can heat
    up a resistor, move an analog meter movement, etc., in other words, the
    voltage under the curve that can actually do work.

    What I am not sure of is - Is Phil saying that an analog meter cannot
    correctly read the "heating equivalent" of an AC waveform?
     
  18. The Phantom

    The Phantom Guest

    It's not the "voltage under the curve", but the "square of the voltage
    under the curve" that does the heating (speaking somewhat loosely, you
    understand).
    What he is saying is that a (typical) analog meter movement's needle will
    be deflected (respond) proportional to the *average* current applied. The
    "typical" movement in such meters is the classic D'Arsonval moving coil
    movement. When measuring AC, the input voltage is rectified (usually
    full-wave) and applied to a multiplier resistor in series with the meter
    movement, which then responds to the average current in the movement. To
    get the meter to read correctly on AC, the assumption is made that such
    meters will usually be measuring a sine wave derived from the power grid.
    The ratio of a full-wave rectified sine wave RMS value to its average value
    is used as a multiplier in the calibration of the meter.

    Thus, the meter will read the correct RMS value of an AC sine wave, but
    will be in error if the input waveform is not a sine wave. If, however,
    the meter movement weren't a D'Arsonval movement, but rather what is known
    as a "moving iron" movement, the meter would read correctly on any AC
    waveform (within the frequency limit of the movement), because moving iron
    meters inherently respond to the true RMS value of the applied waveform.
    But I don't think I've ever seen a high impedance voltmeter using a moving
    iron movement. Maybe Phil knows if any such have ever been manufactured.

    The ripple voltage on a reservoir capacitor in a power supply is
    typically not a sine wave, so a meter whose movement is a D'Arsonval type
    won't read the correct RMS value.

    Of course, I suppose nowadays a meter could be made that had an RMS to DC
    converter chip in it rather than just a full wave rectifier, and it would
    then read correctly any AC waveform, even with a D'Arsonval movement. But
    RMS to DC chips weren't available in the days of analog meters, so those
    older analog meters only read the value of an AC voltage correctly if its
    waveshape is sinusoidal.
     
  19. Yep...the the voltage that does the work is "over the curve" to put it
    another more inclusive way.
    OK, that's what I thought he was saying, but something made me think for
    a minute he might have been referring to something I was not even aware of.
    Scientific Atlanta rings a bell. And maybe Edmunds had one.
    Like the biz end of an SCR controller is not a sine wave, then all bets
    are off.

    Bottom line, we're on the same page...just a different way of saying it
    with more or less artistic over simplification.
     
  20. Phil Allison

    Phil Allison Guest

    "DecaturTxCowboy"

    ** I certainly was.

    YOU were NOT aware of the correct meanings of "true rms" or "rms
    responding" .






    ....... Phil
     
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