# Thread: How Much to big.. is to big? [Capacitor's]

1. ## How Much to big.. is to big? [Capacitor's]

Hey-lo.. I'm getting to the final parts of my power supply and looking for a decent CAP for my system. I'm wonder how much to big.. is to big? [as far as oversizing my Cap.]

I'm using the formula c=(80,000*I)/VDC

therefore.. C=(80,000*17A)/58VDC
C=23448uF

So I look on ebay and what do I see.. Capacitor's everywhere.. I've come across several that seem promising.. I could run 2 in parrellel that are rated at 18000uF each... and 100V. Or 1 that is like 60000uF at the same VDC.
They come out to the basically the same price.. I know that BIGGER isn't nesc. BETTER.. but how much "OVERSIZED" is to much "Oversized"?

anyone guessed yet that I know VERY little about electronic's?

JerryFlyGuy

2. General rule 10% ripple is considered adequate, 5% is the maximum you should need to go, for 5% ripple with those numbers comes out to 17,200µfd, 10% to 8,600µfd. Acording to my cap. calculator.
The higher you go the higher the peak charge current resulting in a larger transformer VA needed.
Al.

3. So what your saying is that I'd ok with just one 18000uF Cap? cool... where did I go wrong w/ my numbers?

Jerry

4. The other thing is what percentage of the time do you expect to draw 17amps?
Al.

5. Mr. Al, ya know what.. I have no idea.. if I add up all my stepper amperage needs [4 @ 6.3A] I come up with 25.2A however Mr Campbell recommends we only use 67% of that number so..16.884A anyway.. its not probably going to be used for heavy cutting but it will most likely have some higher level acceleration rates.. All I plan on milling is foam and some MDF once in a while..
However some of the job's I plan to do in foam could run into the DAYS not just hours per run.

I don't imagine that any of that offer's any help in answering your question.
Jerry

6. Ah, you had already de-rated the current. So a minimum of 18,000µfd should suffice.
Al.

7. So do you think I'll be ok at 18000uF or should I get two
10000uF's and parrellel them?

Thanks for the help on this one..

JerryFlyGuy

8. The guaranteed tolerance value for electrolytics is often +100% -0, so it is not an exact science.
Two 10,000 should work.
Al.

9. thanks Al.. sorry to be so persistant..

Jerry