[sdiy] Power Supply Questions

[Oscar Salas]

OK, and theoretically, what would happen with a big transformer, say 100VA? What confuses me are:

1.Power-One supplies, for example the HBB15,that has trimmable voltage output, specifies 1.5A output current for 15V and 1.7A for 12V.

2.I tested a Myrra 44383 transformer, rated 15VA per secondary, thus 0.83A at 18VAC with LM317/337 regulators.  Adjusted output to 15V and placed a load resistor, output to ground, that gives 0.83A. I noticed that with same load resistor, if I change the voltage, changes the current output and wonder if before the diode ring there is running the same current regardless settings of the regulator.
My multimeter don't read AC current so I'm asking for the theory.

--- On Wed, 3/16/11, Harry Bissell <harrybissell at wowway.com> wrote:

> From: Harry Bissell <harrybissell at wowway.com>
> Subject: Re: [sdiy] Power Supply Questions
> To: "Oscar Salas" <osaiber at yahoo.es>
> Cc: synth-diy at dropmix.xs4all.nl
> Date: Wednesday, March 16, 2011, 7:29 PM
> VA rating of a transformer is usually
> thermal limited and assumes some typical voltage drop. In
> the case of a SMALL
> transformer (like 30VA) the transformer regulation is poor
> (not much core there) and you should use the .83A rating.
> Draw more and the transformer output voltage will probably
> drop (quickly), and become very non-sinusoidal.
>
> For a transformer that small, cut-and-try is the way to
> go...
>
> H^) harry
>
>
> ----- Original Message -----
> From: Oscar Salas <osaiber at yahoo.es>
> To: synth-diy at dropmix.xs4all.nl
> Sent: Wed, 16 Mar 2011 12:55:20 -0400 (EDT)
> Subject: [sdiy] Power Supply Questions
>
> Hello,
>
> In a 2X18 30VA transformer, so each secondary (15VA) is
> current rated 0.83A at 18V, how much current rated will be
> in each secondary after a regulator that provides 15V?
>
> VA=V*A; VA/V=A; 15/15=A=1
>
> It is 1A... or it can not be considered after the regulator
> and remains 0.83A?
>
>
>
>
>
>      
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> --
> Harry Bissell & Nora Abdullah 4eva
>