🚀 go-pugleaf

On Jun 14, 9:29 pm, rebel <m...@privacy.net> wrote:
> On Sat, 14 Jun 2008 07:48:55 -0700 (PDT), MooseFET <kensm...@rahul.net> wrote:
> >On Jun 10, 4:26 pm, rebel <m...@privacy.net> wrote:
> >> On Tue, 10 Jun 2008 06:36:46 -0700 (PDT), MooseFET <kensm...@rahul.net> wrote:
> >> >On Jun 9, 6:32 pm, rebel <m...@privacy.net> wrote:
> >> >> On Mon, 9 Jun 2008 07:55:58 -0700 (PDT), MooseFET <kensm...@rahul.net> wrote:
> >> >> >On Jun 9, 5:42 am, F Murtz <hagg...@hotmail.com> wrote:
> >> >> >> I would like to make an adjustable power supply using a transformer with
> >> >> >> a 24v 5a single wound secondary I would like the voltage to be
> >> >> >> adjustable from lowest to highest possible given transformer I would
> >> >> >> like to have regulated current
>
> >> >> >> There is another possibility I also have a larger transformer with two
> >> >> >> 32v 4.7a secondaries If anyone has a schematic for an adjustable power
> >> >> >> supply with adjustable current regulation making the most of the
> >> >> >> transformer specs I would be eternally grateful
>
> >> >> >How much current do you want?
>
> >> >> >Lets just assume you make DC from the AC and then regulate down.
>
> >> >> >Doing a couple of figures for making 4A into lets say zero volts:
>
> >> >> >24V * sqrt(2) = 34V
>
> >> >> >34V * 4A = 136W
>
> >> >> >The the regulator circuit needs to get rid of 136W.
>
> >> >> Minor points - the transformer isn't rated to deliver 136W, and that 34V is
> >> >> unloaded peak.
>
> >> >When working out the power in the regulator, it is best to assume that
> >> >the peak voltage is what you get.  The rated voltage on the
> >> >transformer is for a normal line voltage.  In the high line case, you
> >> >can get a little more.
>
> >> I appreciate that, but my original comments stand.  There is no valid reason to
> >> state "the regulator circuit needs to get rid of 136W" when the transformer is
> >> not rated to deliver that power.
>
> >You seem to have a poor understand of what the rating of a transformer
> >means.
>
> On the contrary.  A 24V 5A secondary is rated for 120VA.  Your education
> suggests something different?  Please inform us all as to YOUR transformer
> theory.

Yes that is the *rating* in VA and yes that works out to 120/1.5=80W
of power from the rectifier.  It however is far below the limit of
what you can actually get out of the transformer.  It is what the
maker says the transformer can do forever on a normal line voltage and
usually with an air temperature of some 50C.  The transformer does not
contain any significant current limiting.  The output impedance is
fairly low.  The heat sinking on the regulator needs to be designed to
withstand the high line case with a short circuit on the output.  This
is a lot more than the rated power of the transformer.

If the OP goes with less than the amount of heat sinking I suggested,
I predict he will be replacing the transistors very early in the power
supplies life.