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I have a large transformer and what I need is the formula to figure output ripple. Like the transformer is 20amps out at 50 volts out. Is there a formula to figure the capacitor needed verses the ripple out. With just the transformer and bridge is there a way to figure the ripple before the cap..? Thanks
What is the formula
- Thread starter thedoc8
- Start date
Nope, you need to know the load, not just the capacity of the transformer. Once you have the load, there are all sorts of ways to estimate the ripple. (A precise calculation is fairly tough but few people worry about that level of precision.)
Oh wait, what do you mean "ripple before the cap"? Without the filter cap, the ripple is the full AC voltage out of the transformer, less two diode drops in the rectifier.
Thanks, forget the ripple off the bridge question, was not thinking. Going to check out the post below.
It's a good one - I just thumbed thru it. It'll every question you might have and a lot you haven't thought to ask. Kudos to @bertus.
The test load is 2.5 ohm 20amp, roughly. Thanks
Thanks for the link, reading now.
Very good doc, a keeper. Though it was not going into ripple, but it is there plus loads of good stuff. Thanks a mil
The section on filter capacitors in the doc bertus linked to is frankly terrible.
e.g. time to charge capacitor is 5RC - pretty much irrelevant for a power supply filter capacitor and omits the fact that the input is a sinusoid.
Takes longer for bigger caps to charge - not in any practical terms; the diode conduction angle will be reduced and the ratio of RMS to average current increased (diode begins to conduct later in the cycle and only the energy removed by the load is replaced), requiring greater derating of the transformer for the same winding loss (lowest winding loss is with no filtering).
Don't have time at the moment for more.
e.g. time to charge capacitor is 5RC - pretty much irrelevant for a power supply filter capacitor and omits the fact that the input is a sinusoid.
Takes longer for bigger caps to charge - not in any practical terms; the diode conduction angle will be reduced and the ratio of RMS to average current increased (diode begins to conduct later in the cycle and only the energy removed by the load is replaced), requiring greater derating of the transformer for the same winding loss (lowest winding loss is with no filtering).
Don't have time at the moment for more.
Hello,
Perhaps the wiki article will be better:
https://en.wikipedia.org/wiki/Ripple_(electrical)
Bertus
Perhaps the wiki article will be better:
https://en.wikipedia.org/wiki/Ripple_(electrical)
Bertus
Almost every formula you find will be approximations at low percentage ripple.
You can use this formula:
Ripple Voltage = I / (2fC)
or C = I /(2f x ripple voltage)
Hence if you want ripple voltage = 2V at 20A load at 60Hz line frequency
C = 20 /( 2 x 60 x 2) = 0.08F
i.e. about 80,000 μF
Reference:
http://www.zen22142.zen.co.uk/Design/dcpsu.htm
You can use this formula:
Ripple Voltage = I / (2fC)
or C = I /(2f x ripple voltage)
Hence if you want ripple voltage = 2V at 20A load at 60Hz line frequency
C = 20 /( 2 x 60 x 2) = 0.08F
i.e. about 80,000 μF
Reference:
http://www.zen22142.zen.co.uk/Design/dcpsu.htm
This is the one I was looking for and close enough for what I wanted. Just wanted to see how close the math is to what I see on the scope. Plus got some good reading to boot. Thanks to all.
Just started this one and it is very good. Love to read as much as play. Thanks
MaxHeadRoom
- Joined Jul 18, 2013
- 30,658
For a given current the larger the cap required also increases the required Va of the transformer.
Maz.
Maz.
Thank you all. What I was doing was. Had big transformer that I am using to power a 20amp buck converter. The transformer is rated for that and more. I wanted to have very low ripple out of the transformer to feed the buck converters. (Don't even know if it matters after the mosfet switch), The problem was, I was using 3 10000 uf caps to filter the transformer and thought the ripple was too high. With all your help, the formulas showed me what I was getting was ball park correct. So does it matter to have the feed dc to the buck converter that smooth..? Does it make much difference at the output of the buck. Going to scope it out tonight. Thanks all, Love getting answers to questions that I can't find anywhere else.
