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11th September 2012, 05:15 AM  #1 
diyAudio Member
Join Date: Jul 2009
Location: MUMBAI

ripple current measurement
hi ,
I want to calculate the ripple current flowing in a given circuit through a capacitor. For example the load requires 40V@10A and I am using a 10000uf capcitor what will be my ripple current through the capacitor. I calculated ripple voltage as = I/2F X C so the ripple voltage comes to 10V from here how do i calculate the ripple current..........?????????? regards Sekhar 
11th September 2012, 05:26 AM  #2 
diyAudio Member

You need to know the charge and discharge times as a proportion of the mains cycle time. As a reasonable first guess, the cap is charging for 20% of the time and discharging for 80%.
Assuming that 10A output current flows all the time then the charging current will be 50A. I'm not sure whether the ripple current is defined as the average capacitor current over time  if so then its going to be (40A * 0.2) + (10A * 0.8) = 16A. The heating effect within the capacitor though is going to be proportional to the mean squared current which is considerably greater than the average.
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'The total potential here must be nothing less than astronomical.' 'Nothing less. The number 10 raised almost literally to the power of infinity.' 
11th September 2012, 05:54 AM  #3 
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lets say the ripple voltage is 10V as shown in the picture how will i get to ripple current from ripple voltage
regards sekhar 
11th September 2012, 05:58 AM  #4 
diyAudio Member

Use the formula that I = C * dV/dt i.e. current is capacitance multiplied by rate of change of voltage. Ripple voltage is not a sinewave as shown in your picture, it has more of a triangular shape with a charging slope and a discharging slope.
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'The total potential here must be nothing less than astronomical.' 'Nothing less. The number 10 raised almost literally to the power of infinity.' 
11th September 2012, 06:42 AM  #5 
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and how to calculate dv/dt ............
regards sekhar 
11th September 2012, 06:43 AM  #6 
diyAudio Member

That's the slope of the ripple voltage waveform  as I said earlier there are two slopes, the charging and the discharging.
You know how to calculate the slope of a graph?
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'The total potential here must be nothing less than astronomical.' 'Nothing less. The number 10 raised almost literally to the power of infinity.' 
11th September 2012, 07:00 AM  #7 
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Join Date: Jul 2009
Location: MUMBAI

this means i need to get a oscillscope to look at the slopes u are mentioning ......and i dont know how to calculate the slope of a graph....if u will help me to learn
regards sekhar 
11th September 2012, 07:04 AM  #8 
diyAudio Member

I suggest you Google that one for yourself  I just tried it and I found a few resources to help with that. Yes you do need to get an oscilloscope to see what the slopes are in practice. Or you can run a simulation program like LTSpice.
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'The total potential here must be nothing less than astronomical.' 'Nothing less. The number 10 raised almost literally to the power of infinity.' 
11th September 2012, 08:40 AM  #9 
R.I.P.
Join Date: Jul 2004
Location: Scottish Borders

psud11
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regards Andrew T. 
11th September 2012, 08:43 AM  #10 
diyAudio Moderator

Some good general PSU info from the Signal Transfer Company that shows ripple current and voltage calculations.
The Signal Transfer Company: Power Output 
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