Re: Capacitor Sizing in Power Supplies

Jan, Jon, Mariss: Thanks very much for your replies to my cap sizing
question. I am indeed attempting to make a power supply for formerly
series wound, and now independently engergized DC motors.

I found a nice stout-looking transformer rated 120 VAC in and 12 VAC
@ 30 A out. There were 40 turns of 1/8" x 1/8" square wire on the
secondary which I replaced with 2 turns of #4 copper wire. I don't
know if such big wire was required, but it looks very impressive.
Anyway, I fired it up and 1.2 volts, nice and steady, everything
running cool, no fuses blowing--all at no load.

Surley this thing would not be capable of 1.2 VAC @ 300 A?

I next connected one of C&H's magic 600V @ 40A $4.95 butter pat
rectifiers and a 43K uF cap which happened to be on hand and read
very close to 1.2 * 1.4 VDC.

I doubt the motors' simultaneous amp draw would ever exceed the
nameplate rating of the rectifier, but can full wave lumps be
connected in parallel, or does one just hunt down a bigger unit?

Jon, I'm concerned re your comments about current surges in the
secondary--what would cause such surges and how could they be
controlled?

I hadn't hought about connecting the field coils of the axes motors
in series. Suppose just X any Y were connected in series (Z is
another installment)--how might this affect the voltage requirements?
Maybe a couple more turns of #4?

Jan, would you say the 1k uF per amp independent of voltage is a
working rule of thumb most practical power supply designers would
agree with?

Thanks in advance.

--Jack

--- In CAD_CAM_EDM_DRO@egroups.com, Jon Elson <jmelson@a...> wrote:
>
> jmw@c... wrote:
>
> > I seem to recall a formula in some Gecko docs re capacitor sizing
on
> > roll your own power supplies (the xformer, bridge, cap variety).
And
> > I think this formula was of the form (80,000*amps)/volts = uF of
cap.
> > (The 80,000 is related somehow to the number of millisecs per half
> > cycle of 60 cycle AC and the time it takes the cap to charge?)
> >
> > Suppose you wanted to build a 2VDC @ 20A supply. Without getting
into
> > why one might want to do that, this works out to 80,000*20/2 =
800k
> > uF. That's a lot of capacitor, albeit at pretty low voltage.
> >
> > Is this a non-linear situation or would 800k be the proper sized
cap?
>
> If you need good regulation, these numbers might be OK. I think
this
> is for shunt motor field excitation? The inductance of the field
windings
> smooths the current such that current fluctuations will be much
reduced.
> Putting the fields of several motors in series would increase
inductance
> and lower current requirements, while raising the voltage to a much
> more reasonable range. I think you will find the rectifier loss
and the
> current surges in the transformer secondary would be a major problem
> in such a low voltage power supply.
>
> Jon