On Wed, 3 Sep 1997 22:34:19 +0200 Bernd Grunwald <embo@BSE.NETESTATE.DE>
writes:
>
>No, sorry, I do need a transformer because I don`t want to get burned,
>if I
>insert my Headphone in the included amplifier that will be connected
>somehow to
>the digital part of the device (volume control ...)

If your project has any hardwired connections to the outside they must be
isolated from the power line.  Usually the best way to do this is to
isolate the entire project circuit from the power line by using a
transformer-type power supply, especially if it only needs a few watts of
power.  RF connections can be inexpensively isolated by coupling the
signal and ground leads through safety-agency approved capacitors.   Many
TV sets use this method on the antenna lead.  For other types of signals
the cost of isolating them is usually higher than that of isolating the
power supply.

The efficiency of small transformers is low.  They will use a watt or so
of magnetizing power without any load at all on the secondary.  Using a
transformer at less than its full voltage rating should help this a lot,
since the iron will not approach saturation then.  After the transformer,
loss in the regulator can be significant.  Reducing the power demand of
the project is the best way around this.  If most of the current goes to
a relay, try and run it off of the unregulated supply.  The higher
voltage relay coil will use less current.