Re: [CAD_CAM_EDM_DRO] Re: cap question

cadcamcenter wrote:

> Jon Elson <elson@p...> wrote:
> > I think I have 250,000 uF on my 10 A 69 V power supply. I think it
> is adequate,
> > but not overkill. Especially when we had a short power failure,
> the system
> > kept running right through it.
>
> I can understand why 250,000uF is better than 25,000uF. But I
> remember the comment of a power engineer friend who mention large
> initial surge current on start-up when you have such large capacitor.
> What is the start-up current for 69V through 250,000uF? and for how
> long a period will the current be "dangerously" high?

It depends on the characteristics of the transformer primary and secondary
windings, leakage inductance and series resistance, mostly. I have
a power supply a friend made for another project, but ended up in my
CNC system. It has two HUGE transformers, and big stud-mount
rectifiers. It runs from the same power strip as the computer and other
stuff. There is no big blink of the lights, no big hum or anything. So, the
surge is not only limited by capacitor size, but by how much the transformer
will draw. If the resistance and leakage inductance are very small, the
currents will be higher. In that case, you have to put a little resistance,
maybe .1 to .25 Ohm in series with the rectifier output to limit the charging
surge. The surge generally will be done with within one or two line cycles.

250,000 uF is 1/4 F. 25 A will charge that at a rate of 25 * 4 V/Sec, or
100 V/sec. So, it will take less than 1 second to charge to full voltage
at rated current, anyway.

Jon