Re: What size capacitor should I use?

Steve wrote:

"Now I have a question:
When discussing the sizing of a supply, people seem to use "rated
current" and "max current" interchangeably. So, which is it? I have
three motors (and may add a fourth and possibly a fifth) that have a
continuous current rating of 9.2A, and max instantaneous current of
37A
(which will be limited by the Gecko to 20A). So, assuming the 2/3
rule,
is that 2/3 of the maximum rating (40A), or 2/3 of the continuous
rating
(18.4 A)? And of course I assume that we take 2/3 of the total for all
of the motors expected to run off the supply...
-Steve"


This is a very good question, and i am assuming your are talking
servos. The peak torque of a servo can be used like 5% of the time,
i am not sure a bank of capacitors will fill in a current slug this
long...

How about taking this question over to gecko groups and ask Mariss...


Wally

P.S. as for the fuse thing...Another very good question...I would
like to know as well...I may go over to the bussman fuse site later
and see if i can find anything...









--- In CAD_CAM_EDM_DRO@yahoogroups.com, Stephen Wille Padnos
<spadnos@s...> wrote:

> caudlet wrote:
>
> >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 (:-)
> >
> >
> Hmmm. Smaller caps shouldn't end up being smaller overall than a

single

> large one. Capacitance is proportional to the area of dielectric,
> divided by the distance between conducting plates. The breakdown
> voltage is inversely proportional to the separation distance

(thinner

> dielectric will break down at lower voltages) Given a particular
> dielectric material, you need some thickness of dielectric to get

the

> necessary voltage rating, and some area to get the necessary
> capacitance, so you ned the same amount of dielectric no matter how

you

> slice it. (there are edge effects and packaging questions that

might

> change this a couple of percent one way or the other)
>
> As for filtering, it's pretty common to see two or more different

types

> of capacitor in parallel, to filter different frequencies. A small
> value tantalum cap alongside a large value electrolytic will filter

out

> high frequency noise, while the electrolytic provides low frequency
> filtering.
>
> >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.
> >
> >
> Oversizing the supply is one thing, using higher rated caps is

another.

> There are several specs that aren't usually discussed: temperature,
> lifespan, ripple current, and tolerance. Most electrolytic caps

are

> rated for a capacitance, maximum voltage, maximum temperature, and
> lifespan at max volts/current. "Long-Life" caps are rated for 1000

to

> 5000 hours, some are available up to 8000 or more hours. The

lifespan

> is inversely related to the voltage/current, so the lower the

operating

> voltage, the longer the cap will last.
>
> So, the necessary operating current and voltage need to be

overspecified

> for longer lifespan, and the tolerance on capacity suggests that

you

> overspecify just to get the capacitance you want (go over by

whatever

> the tolerance is, ie 20% over for a +- 20% cap).
>
> >Watts is watts. The average current is a factor of the

instantaneous

> >current times the duty cycle.
> >
> Now I have a question:
> When discussing the sizing of a supply, people seem to use "rated
> current" and "max current" interchangeably. So, which is it? I

have

> three motors (and may add a fourth and possibly a fifth) that have

a

> continuous current rating of 9.2A, and max instantaneous current of

37A

> (which will be limited by the Gecko to 20A). So, assuming the 2/3

rule,

> is that 2/3 of the maximum rating (40A), or 2/3 of the continuous

rating

> (18.4 A)? And of course I assume that we take 2/3 of the total for

all

> of the motors expected to run off the supply...
>
> Additionally, how does one go about choosing fuse ratings for the
> Geckos. Some of the schematics show a 5A fuse in line with each

Gecko,

> but that's obviously too small for my motors (or is it?). Slow

blow,

> fast blow, etc. etc.
>
> If I can get answers to questions like this, I can write a more

explicit

> power supply sizing document for the files.
>
> - Steve
>
> (funny how the EE background and the real world don't actually

relate to

> each other sometimes :)