Allowable Ripple Voltage

Discussion in 'General Electronics Chat' started by gargrahul277, Apr 20, 2016.

  1. gargrahul277

    Thread Starter New Member

    Apr 5, 2016
    11
    0
    Hi all : )

    Im designing a 15 V DC regulated supply.
    Im calculating filter capacitor value for it using following formula,
    C= (0.7*I)/(100*Vr)
    where, I is load current
    Vr is ripple voltage
    0.7 because capacitor supplies the current for 70% of cycle

    i want to know how much ripple voltage should i take for my calculation ??
     
    Last edited: Apr 20, 2016
  2. hp1729

    Well-Known Member

    Nov 23, 2015
    1,956
    219
    You are looking for a precise calculation for a reservoir capacitor that probably has a -20%/+50% tolerance? Figure 1,000 uF per Amp. The regulator will smooth it out.
    (I know. I'm a barbarian.)
     
  3. gargrahul277

    Thread Starter New Member

    Apr 5, 2016
    11
    0
    i want to know standard value of ripple voltage that one takes to calculate capacitor filter value. i have read that there is trade-off between capacitor value and ripple value. u cannot keep capacitor value high to minimise ripple value because large capacitors get heated quickly.
    So one always keeps some ripple value to lower the capacitor value.
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,140
    3,054
    That's specified by you the designer. Some applications require no filtering, some require more.

    That cookie-cutter formula is imprecise, by the way, because it includes a number of assumptions to arrive at such a simple formula. It's probably fine if you're shooting for, say, a 5% ripple.
     
  5. #12

    Expert

    Nov 30, 2010
    16,313
    6,817
    So much depends on what you are not telling us. The amount of voltage between the peak charge of the capacitor and the minimum required by the regulator is where I work this problem. We don't know the peak voltage, the type of regulator, the output voltage needed, the amount of current...and you want the answer.:rolleyes:
     
  6. gargrahul277

    Thread Starter New Member

    Apr 5, 2016
    11
    0
    :p sorry for providing less inputs

    i want 15V output so im using LM 7815.
    maximum load current 2A.
    i want to keep the minimum input voltage for regulator at 22 V.
    maximum input voltage for regulator (hence ripple voltage) is what i want to know.
     
  7. #12

    Expert

    Nov 30, 2010
    16,313
    6,817
    That still doesn't make sense. You want to keep the minimum voltage applied to the regulator at 22 volts and you want to know the maximum input voltage for the regulator.
    The maximum voltage the chip is rated for is 40 volts. That's 18 volts of ripple.
     
  8. gargrahul277

    Thread Starter New Member

    Apr 5, 2016
    11
    0
    yes
    i know the maximum input voltage for chip is rated at 40 volts.
    i was expecting the ripple voltage from capacitor.

    i guess with 5%ripple i can take Vr(Ripple voltage) as 1V.
     
  9. crutschow

    Expert

    Mar 14, 2008
    13,023
    3,236
    Worst case you want to keep the ripple voltage below the maximum voltage the 7812 can tolerate and above the minimum it needs to regulate.
    Anything in between is fine.
     
  10. #12

    Expert

    Nov 30, 2010
    16,313
    6,817
    What is the peak voltage at the filter capacitor?
    The 7815 chip needs about 3 volts of excess voltage.
    Calculate your ripple to be the maximum capacitor voltage minus 18 volts.
     
  11. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Why the min of 22V? Even if you have zero ripple, you will be losing 14W in heat, which will be a problem even with a pretty large heatsink.
     
  12. wayneh

    Expert

    Sep 9, 2010
    12,140
    3,054
    We still don't know what the input is, either. I think we're all assuming full-wave bridge rectified AC, but the TS has not said that.
     
  13. hp1729

    Well-Known Member

    Nov 23, 2015
    1,956
    219
    Correct, for what it is worth. Let me know what your calculations come to.
     
  14. recklessrog

    Member

    May 23, 2013
    338
    102
    Depends too on the quality of the capacitor, last year I repaired a regulated wall wart stabilised psu for for a vocal effect unit. The ESR of the reservoir/smoothing capacitor had risen to the point that the ripple trough fell below the drop out voltage of the regulator at half the rated output current. A nice new low esr capacitor rated to work to a higher temperature has given it a new lease of life. One thing that I have noticed is that there seems to be a poor understanding of what a regulator is being used for. If you need 5v at 1amp, you are going to need a big heatsink and fan if you feed it with 45volts!!! That unwanted 40volts has to be lost somewhere! That's why manufacturers spec sheets are published for correct design and operation.
     
  15. hp1729

    Well-Known Member

    Nov 23, 2015
    1,956
    219
    Does your formula suggest for 1 V of ripple at 2 Amps you need 14,000 uF? Did I do something wrong? If you change that 100 to 1,000 you just about get 1,000 uF per amp.
     
    Last edited: Apr 20, 2016
  16. recklessrog

    Member

    May 23, 2013
    338
    102
    Don't forget that the esr of the capacitor is important when designing power supplies, and as they age, the esr tends to increase.
     
  17. #12

    Expert

    Nov 30, 2010
    16,313
    6,817
    I'm getting the idea that this is 100 Hz for the ripple. Not as critical for ESR as a high speed switching supply is.
    Just calculate what you need and boost it by 50%. You'll be OK at power line frequencies.
     
Loading...