Re: What size capacitor should I use?

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Simon Arthur <chroma@m...>
wrote:

> You may end up with higher ripple on the output voltage if your

capacity

> is too large.
>
> A. G. Eckstein wrote:

The higher the value of capacitor the *lower* the ripple. The ripple
frequency on a standard linear supply is 120hz. Motors don't need
ultra low ripple DC (like audio) but do need the ability to supply
current for motor peaks.

I most circumstances builders are over designing the power circuits
by several factors. To simply take the motor plate rating then
multiply by the number of motors then add in some "fudge factor" is
to oversize and overpay for the supply. On chopper type circuits you
will find the average current draw to be a lot less than the max
ratings on the motor. In additon not many people run all 3 (4)
motors full out at the same time. Most people feel good that they
have extra capacity in their power supply just sitting there for the
day something shorts and there is enough reserve power to weld
components to the chassis (:-)

Things like transformers have the ability to supply several times
their average current rating in short bursts. Semiconductor circuits
have to be designed using instantaneous peak ratings while passive
components are based on average values over time.

I think the 100% overvoltage rating is extremely conservative. It
works fine for small caps where the price difference is a few pennies
but in big filter caps you pay a lot for that wider margin. The
manufacture arrives at those ratings based on worst case tests of
ripple, temperature and time. The voltage rating is not an "explode
now" level. Most engineers are comfortable with 30 to 50% ratings
over the high line voltage.

That's my story and I'm sticking to it!!