Re: Biggie size that Capacitor.?

Hi Jerry,
Adding TOO big a capacitance can be a problem for a power
supply. When the supply powers up, the capacitor 'looks' like a big
short (PWR to GND) until the capacitor fills part of the way up.

This 'short' can put excessive demands on components within the
supply. Two solutions come to mind when dealing with this problem.

1). A thermistor is used to limit the current to the cap. A
thermistor, when cold has a high resistance that fills the capacitor
more slowly. As the current goes through it, it heats up (actually
LOWERING it's resistance) allowing more current to flow. Thus, a
ramping up of current into the system.

2) Use a power resistor to slowly fill the cap. When the voltage
reaches a predetermined level, a relay is turned on and the current
is allowed to fill the rest of the cap unhindered.

This is not to say that your particular setup will neccesarily suffer
any problems or need these measures, only that you don't want to just
assume that you can throw any size capacitor on the output of the
supply and not have it affect the supply.

Jeff
http://www.xylotex.com


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "fozzyber <jerry@o...>"
<jerry@o...> wrote:

> Is there any thing nwrong with useing a capacitor larger than what

is

> needed by the powere supply. I plan on running a 68V 8-10 amp Power
> Supply. so I should use a 12000uF/100v capacitor.
> Last night I was given an old large line printer. After tearing it
> down I found a few large caps, One was a 75V 24000uF cap. Is their
> any thing wrong with useing it verses useing a 12000uF/100v?
>
> Thanks
> Jerry