Re: [CAD_CAM_EDM_DRO] Re: What size capacitor should I use?

As a general rule, I use several smaller caps in a bank, with bus bars.
Why? well like most everyone else
here, I have to scrounge, I have a supplier that has 10,000 uf 150v caps
and cases of them, at a fair price.
I usually buy 50 or so at a crack, and when I'm getting low, order more.
66% of max load is conservative,
heck, I've got a 60 v 10A ps on my router right now, with 5600 uf
capacitance, by the numbers waaay to small,
yet scoping the rails during operation shows less than 1v p-p ripple,
with 3 201's set at 5.5A each. For a customers machine, it would
probably have 20,000 uf or so, but I haven't seen any issues here yet.
As usual, YMMV.

erie

caudlet wrote:

>--- In CAD_CAM_EDM_DRO@yahoogroups.com, Andy Wander <awander@v...>
>wrote:
>
>
>>Dave:
>>
>>The OP asked if he needed to do this "per drive, so that is where
>>
>>
>the
>
>
>>multiple power supplies came from.
>>
>>Several of us have commented that multiple caps will work just
>>
>>
>fine; one
>
>
>>person did say multiple caps in parallel would be better; I am not
>>
>>
>sure why
>
>
>>that would be, but I am not prepared to dismiss it out of hand
>>
>>
>without
>
>
>>hearing the reasons why it might be better.
>>
>>Andy Wander
>>Verrex Corporation
>>
>>
>>
>>
>There are two reasons to parallel caps (well actually three). Cost.
>In our world of scrounged parts it's ususally easy to find caps at
>the voltage required at smaller values. When you start looking for a
>35,000 to 80,000 mfd cap with a 100 volt rating they get more scarce
>($$$$). Next is the physical size. Smaller value caps are typically
>shorter and have less cross section per MFD so three 15,000 100VDC
>caps will probably end up being somewhat smaller in total. The last
>thing is ESR (Effective Series Resistance). All caps have
>capacitance (duh), resistance and inductance. The larger the cap the
>more ESR it has. Take 3 smaller caps in parallel each have smaller
>ESR each and in parallel they have 1/3 the ESR of one. ESR in linear
>DC power supplies is of little concern since it would limit the
>ability to filter higher frequency waveforms but are used extensively
>in switchers as filters. Besides, I wanted to give everyone
>something else to stew about (:-)
>
>Oversizing the power supply for the load offers no tangable benefits
>other than making the builder feel better. I think even the 66%
>number is conservative. I find in the actual world the total worst
>case current draw (all motors under normal load) is closer to 50% of
>the nameplate values on a chopper system.
>
>Watts is watts. The average current is a factor of the instantaneous
>current times the duty cycle.
>
>
>
>
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