powersupply help!

[planescott]

Hi I am building my powersupply. I used the gecko motor basics info to calculate my powersupply. What I came up with is 25 times voltage for my lowest rated motor is 57.5v and 2/3 amps for all my motors is 9.2 amps so I bought a 625va 8.93amp 35+35v output tranformer wiring it parallel gives me 49v output. For my rectifier I choose a 300v 20amp.
The capacitor has me confused, because I need to be 20 percent higher in voltage ( which is 58.8v) than the output of the transformer and I need 15,020uf for my capacitor. The real question here is all can find is 25,000uf 75v capacitor can type is this to much or will this work, and I noticed that some of them say 75v or 75vdc what is the difference here? Now is the time I wish I were an electrical engineer. Also the rectifier I dont understand either but someone told me that is ok to use.

[Al_The_Man]

The rating of electrolytics in power supplies is stated in VDC usually wether it says V or VDC, most have a surge rating on them also, which is generally around 50% higher voltage.
You can use higher capacitance for less ripple, however, in theory, if you were to run at the max current for extended periods, the higher capacitance would cause higher heating effect of the transformer.
Al.

[planescott]

Thanks Al, I found a 35,000uf 35vdc if I run two in series that should be 17,500uf and 70v. Also if my capacitor is rated for lets say 70v does it put out 70v or does it only put out what my transformer puts into it 49v.

I have read this 200 times on this site " AL is THE MAN" and I agree. (lol)
scott

[Al_The_Man]

Although in theory you can series up capacitors for higher voltage operation, it not always a good idea with Power electrolytics due to them not being the most stable of components, they traditionally have very wide capacity tolerance. If you do go this route use a 3watt 750 ohm resistor across each capacitor to assist in the voltage division, and make sure they are the same make/model.
Al.