power supply ripple

Discussion in 'General Electronics Chat' started by ninjaman, Nov 30, 2013.

  1. ninjaman

    Thread Starter Member

    May 18, 2013
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    1
    hello
    I have to calculate the ripple voltage of four different capacitors in a power supply. the power supply is a transformer, bridge rectifier, load resistor and capacitor would go across the load.
    I used multisim to simulate the circuit. I looked at the ripple which has a peak to peak value.
    my question is, to get the mean dc value, what is the formula for this. is it the peak to peak value times 0.636?

    I know 0.636 is the average. but im not sure if the peak to peak part is right?

    any help would be great

    thanks
    simon
     
  2. #12

    Expert

    Nov 30, 2010
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    There is no such thing as the peak to peak value of DC. The peak to peak value of what? The ripple voltage? The transformer output voltage?
     
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    No, I don't think there is a formula. You can do it graphically by drawing a horizontal line through the ripple such that the area above the line just fills in the "open" areas below the line.
     
  4. #12

    Expert

    Nov 30, 2010
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    I would guess that the average value of the DC voltage is the peak value of the DC voltage minus 1/2 of the peak to peak ripple voltage.
     
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    What do you mean by "mean DC value"?

    DC is a constant. A DC value is the mean. It is also the peak, the minimum, the maximum, and every other characteristic name you have.

    Assuming a transformer-bridge-cap configuration a useful first approximation is a cap bank charges to the peak AC from the rectifier, minus the diode drops. Then each cycle (or half cycle) it discharges per:

    V = I * T / C where I is the load current, T is the time between cycles, and C is the cap.
     
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    That's my take on it too. :) Where the ripple on the DC line is typical (probably less than 20% of the DC value, ie 12v DC with say 1v or 2v ripple) it is symmetrical enough to give the DC value as half way between the ripple peaks and troughs.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    No calculations needed. If you are simulating it in Multisim then just run the DC plus ripple through a low-frequency LP RC filter (say 1meg in series with 10uF to ground). That will give you the average DC voltage of the DC plus the ripple.
     
  8. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    this is the assignment that I have done so far. it includes the results, sort of. if any one could offer some advice or point out anything I have done wrong I would really appreciate it.
    thanks
    simon


    Measurement and testing
    Power supply analysis
    The brief is to build a power supply and measure the voltage and current using various testing methods. The assignment hand-out contained the circuit schematic and instructions for construction and testing.
    The circuit was simulated on multisim, components and values were chosen and laid out similar to the given schematic. Simulated test equipment was attached to the circuit before and after the bridge rectifier to show alternating current and variable direct current. After getting the calculated mean DC voltage and current through the load resistor, capacitors were added to get values of ripple.
    The oscilloscope was used to show the voltage waveform and ripple voltage. Using the grapher function on the oscilloscope, the waveforms could be isolated and enlarged to get a more accurate measurement. These measurements showed the ripple peak to peak voltage. Four capacitance values were entered and measured to show varying amounts of ripple voltage.
     
     
    In supply voltage was just under 17 volts and calculated by using the formula:
    12 volts rms * 1.414 = 16.968 volts
    As the voltage would pass through two diodes for each half wave, the diode voltage drop was taken as 0.6 volts per diode and 1.2 volts for each half wave. So subtracting this from the voltage peak gave:
    17 vpk – 1.2 v = 15.8 vpk
    The mean dc voltage was calculated as 15.8 volts * 0.636 = 10.0488 mean dc voltage
    The mean dc current through the load was calculated using ohms law:
    10volts = 0.00213mA = 2.13mA
    4.7kΩ
    Measured values
    Capacitors
    1uf
    22uf
    47uf
    100uf
    Mean DC volts
    15.5
    20.9
    20.8
    20.4
    Oscilloscope AC
    Ripple
    14.25
    1.8
    0.72v
    0.36
    Mean DC current





    The table above shows the values taken from the live circuit using an oscilloscope to measure the ripple voltage and a multi-meter to measure the dc voltage and current.
     
    Calculated values of voltage with capacitors
    Capacitor
    Peak voltage
    Min voltage
    Ripple difference
    1uf
    16.0579
    6.0784
    9.9795
    22uf
    15.8369
    14.8268
    1.0101
    47uf
    15.7200
    15.2349
    0.485
    100uf
    15.6317
    15.4025
    0.229

    The above table shows the calculated values of ripple voltage simulated on multi-sim. The difference between maximum and minimum ripple voltage with each capacitor is shown. As the capacitor gets larger the ripple voltage difference gets smaller.
     
    Limitations of the test equipment

    Capacitor values
    Calculated voltage
    Actual voltage
    Percentage error
    1uf
    11.585
    15.51
    33
    22uf
    15.342
    20.4
    32.9
    47uf
    15.482
    20.8
    34
    100uf
    15.518
    20.9
    34.6

    The table above shows the percentage error for the values calculated on multi-sim and actual values taken with an oscilloscope. The quality of the meter is a factor and circuit simulation software doesn’t take into account real world problems such as component tolerances and quality.
    Capacitor value
    Actual value
    Simulated value
    Percentage error
    1uf
    14.25
    9.9795
    42.8
    22uf
    1.8
    1.0101
    78
    47uf
    0.72
    0.485
    48.45
    100uf
    0.36
    0.229
    57

    The table above shows the actual ripple voltage measured on a live circuit with a multi-meter and the simulated value from multi-sim. The percentage error shows some inaccuracy. There were some issues with capacitor quality during testing that may attribute to this.
     
    The multi-meter used was set to voltage and measurements were taken across the positive and negative rails. There were some issues getting an accurate reading with the higher capacitor readings. Poor readings are possibly because of the quality of the meter.
    The oscilloscope measured peak values of voltage, again across the positive and negative rails. The channels were set to a voltage division that allowed us to get the most accurate reading. The oscilloscope took some practice to use and get accurate results and proved more useful than the multi-meter.
    The simulation software is acceptable for approximate values but not to be relied upon for usable results.
     
  9. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
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    Or get a real Analog Simulator (LTSpice) which does it all for you...

    Note the peak to peak voltage (cursors), the Average and the RMS voltage values.

    Notice that for this waveform, Average and RMS are different. LTSpice knows all this...
     
  10. #12

    Expert

    Nov 30, 2010
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    Grump and grouch. I'm going to have to move into the 21st century if this keeps up.
     
  11. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    It's the 21st century?? When did that happen?? :eek:

    :D
     
    #12 likes this.
  12. wayneh

    Expert

    Sep 9, 2010
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    The OP may benefit from this post and the thread it was in.
     
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