[sdiy] Power Supply Questions

[ASSI]

On Wednesday 16 March 2011, Oscar Salas wrote:
> 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?

The VA figure includes the core and winding losses, but also depends on the 
max. ambient temperature.  Small transformers are often surprisingly 
inefficient.  Also, the load regulation is getting poorer the smaller the 
core and you may not have enough drop-out voltage left for the regulation if 
you pull the maximum power.  This all depends a lot on the actual 
construction of the transformer, so you'd need to consult the datasheet 
whenever you plan to max out more than about 80% of the rated the power.

> It is 1A... or it can not be considered after the regulator and remains
> 0.83A?

It is 0.83A, or the stuff before your load must not get warm at all. :-)

But note that this is a an RMS current for resistive load.  You will 
actually have a very impulsive current that provokes higher losses, so after 
the regulator you would get a slightly smaller DC current.  So if you want 
1A current from each secondary, you should get a larger transformer.  The 
next size up is probably around 50VA rated and that should give you enough 
room not to worry about the fine print on some data sheet.


Achim.
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