Re: P.S question

Dear Mariss,

First let me say that I have enjoyed reading your posts
very much. I am starting at groud zero with CNC and am
very interested in it.

Sizing transformers for electronic power supplies is indeed
a tricky question. It was answered very well a long time ago
by a Mr. Schade (O. H. Schade,"Analysis of Rectfier Operation",
Proceedings of the I.R.E., July 1943, pp. 341-361.) This
paper may be hard to find now but I have seen his graphs
reprinted several times--one being in one of Motorola's books
on rectifiers, I believe.

Your volt-amp analysis does not apply here because the current
in the transformer is a train of rather sharp pulses--the
diodes only conduct during the brief time that the secondary
voltage exceeds that in the capacitor and that voltage does
not drop very far. The heating effect on the transformer is
proportional to the square of the current so we need to first
square and then average and then take the square root. (I
like to show this to my calculus students as a place where
they really need to know how to integrate :-) ).

There is a simple rule of thumb that comes out of all this:
for a full wave bridge rectifier looking into a capacitor
for the filter multiply the DC output current by 1.6-1.8 to
get the rms current rating for the transformer. Of course
the transformer people know this very well. You can see the
last page of Signal Transformer's catalog for this and other
rules of thumb. In pdf form it is at:
http://www.signaltransformer.com/

The upshot then is that I would not draw more than 6 amps out
of the circuit in question. It will put out 10 amps for a while
but you are running the transformer way out of specs. This will
show up in excessive temperature rise and early outright failure.

Sincerely,
Barry

--- In CAD_CAM_EDM_DRO@y..., "mariss92705" <mariss92705@y...> wrote:
> Hi,
>
> That's kind of a tricky question. Your transformer is rated at 24VAC
> and 10A. That is 240VA (volt-amps). Your DC voltage is 1.41 times 24
> or about 34VDC. That would be 340 watts at 10A. There is usually
> enough reserve in the transformer to come near the 10A rating. What
> matters is how warm the xformer gets. If it gets hot, you are asking
> too much of it. The voltage will stay near 34VDC though.
>
> Mariss
>
> --- In CAD_CAM_EDM_DRO@y..., "afogassa" <afogassa@y...> wrote:
> > Hi, All
> >
> > If I have a 24v trans. and a bridge ret. and a 10000uf elet.
> > it gives me about 36v.
> > now if the transformer is rated at 10amps. should I get 36v. up
to
> > 6amps. or only 24v no matter how much courrent is being pulled out
> of
> > it?
> > In other words, will the Voltage be steady at 36v. till I get to
> > the maximum courrent of 6 amps? then it starts to go down to 24V?
> >
> > Thank's
> > Fogassa.