power supply Ripple frequency and voltage

Discussion in 'Power Electronics' started by harrison2015, Apr 22, 2015.

  1. harrison2015

    Thread Starter Member

    Apr 22, 2015
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    How do you find out the ripple frequency and ripple voltage of a linear power supply?
    Do you add all the filter capacitors and the power supply load resistor
    Time Constant R X C = All Filters capacitors total X load resistor = the power supply ripple frequency?

    I'm looking for a very simple formula to get the ripple frequency and ripple voltage for a linear power supply but i can't find any
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    Hi harrison2015,

    Ripple frequency is determined by the rectifier circuit; 60Hz for half wave and 120Hz for full wave.

    Using the formula for calculating filter cap value based on max allowed ripple:
    Ripple = \frac{I_L}{C} * 6 * 10^{-3}
    C in uF, Ripple V peak-peak, I amps; this holds for I of 1A or less.
     
    Last edited: Apr 23, 2015
  3. harrison2015

    Thread Starter Member

    Apr 22, 2015
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    So how do you calculate the ripples Peak to Peak voltage will be?

    What is this max allowed ripple?
     
  4. dl324

    Distinguished Member

    Mar 30, 2015
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    Substitute the max current and filter cap size in the equation and it will give you ripple. It doesn't hold for max current over 1A, but it'll give you something. If you already have the supply, you can measure ripple under full load.
    It's something you spec when designing a power supply. Normally you use the formula to calculate the size of the filter cap.
     
  5. #12

    Expert

    Nov 30, 2010
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    I use this one;
    Square root of 2 x C x Eripple (P-P) x Frequency = current in amps.
     
  6. ian field

    Distinguished Member

    Oct 27, 2012
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    Mains frequency filters usually use electrolytics - and electrolytics usually have pretty wide tolerance. So if you arrive at a figure by calculation, you need to work out the worst possible case taking into account the various tolerance limits.
     
  7. harrison2015

    Thread Starter Member

    Apr 22, 2015
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    If i was designing a basic linear power supply , what formula would i use to get the size of the filter caps?

    But how did you get the ripple Peak to peak voltage? what formula did you use

    Yes true, but once you measure the ripples peak to peak voltage of a linear power supply, what formula do you use to verify that it should be normally be and the max and min ripple voltage "range" of the linear power supply should be without going out of tolerance?
     
  8. #12

    Expert

    Nov 30, 2010
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    1) By using the formula I gave you.
    2) The same formula.
    3) The same formula using the high and low limits of the capacitors you buy.
     
  9. harrison2015

    Thread Starter Member

    Apr 22, 2015
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    I'm talking about calculating the ripple min and max voltage range before I build a linear power supply and take measurements

    How would I know the ripples min and max voltages if i don't have a formula to get it?
    And how do you know which capacitor values to use?
     
  10. #12

    Expert

    Nov 30, 2010
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    Algebra.
    Vripple P-P = I/1.414 x C x F
     
  11. harrison2015

    Thread Starter Member

    Apr 22, 2015
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    where do you get the 1.414?

    When you look at schematics of linear power supplies, what formula do you use to get the ripple Peak to peak?

    The fuse rating is 1 amp , do you just use the fuse ratings current as the I in the formula? the schematic doesn't tell you the current draw from the circuit so how would you know what is I current ?
     
  12. #12

    Expert

    Nov 30, 2010
    16,298
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    1.414 is the square root of 2, to 3 decimal places.
    I use the formula I gave you to calculate the P-P ripple voltage.
    When I design a power supply, I already know what current I am designing it for. That is where I get the current.

    You obviously don't know how to do math, but there is no other way to calculate the capacitance needed to achieve the ripple voltage that YOU decided to allow when the current that YOU decided to design for is flowing. I can not fix that for you.
     
  13. harrison2015

    Thread Starter Member

    Apr 22, 2015
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    Ok I understand this

    But when looking at various schematics on google of linear power supply schematics how can you calculate the ripple voltage if you don't know the current? do you just use the fuse current rating?

    1 amps divided by the square root of 1 amp?
     
  14. michael8

    New Member

    Jan 11, 2015
    18
    2
    But when looking at various schematics on google of linear power supply schematics how can you calculate the ripple voltage if you don't know the current?

    I couldn't. I'd look at the output regulator maximum current and use that or some other hint from the circuit.

    The following assumes a capacitance input (only) filter.

    The AC power flow isn't continuous (single phase, either half or full wave rectified). In the USA it's 60 Hz (50 Hz is usual in Europe). For a full-wave rectifier there are 2 pulses of power each cycle. So for 60 Hz power and full wave:

    1 cycle = 1/60 -> 16.6 mS (milli-seconds) in one cycle,
    full wave has two pulses so each is separated by 1/2 of 16.6 mS or 8.3 mS

    Between input pulses the only source of power is that stored on the capacitor which discharges supplying the load during this time. How much does it discharge?

    voltage change on capacitor = (current amps * time seconds) / capacitance farads

    The ripple is the amount the voltage on the capacitor changes. So once you have the input pulse
    timing, load current and filter capacitance you can calculate the ripple (peak to peak).

    And you can run this the other way and calculate the capacitance needed for a specific ripple too...
     
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