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Old 11th September 2012, 05:15 AM   #1
sekhar is offline sekhar  India
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Exclamation 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..........??????????


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Old 11th September 2012, 05:26 AM   #2
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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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Old 11th September 2012, 05:54 AM   #3
sekhar is offline sekhar  India
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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





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sekhar
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File Type: gif ripple.gif (57.1 KB, 105 views)
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Old 11th September 2012, 05:58 AM   #4
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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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Old 11th September 2012, 06:42 AM   #5
sekhar is offline sekhar  India
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and how to calculate dv/dt ............


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Old 11th September 2012, 06:43 AM   #6
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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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Old 11th September 2012, 07:00 AM   #7
sekhar is offline sekhar  India
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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


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Old 11th September 2012, 07:04 AM   #8
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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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Old 11th September 2012, 08:40 AM   #9
AndrewT is online now AndrewT  Scotland
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Old 11th September 2012, 08:43 AM   #10
Mooly is offline Mooly  United Kingdom
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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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