[sdiy] Switching power supply transformers

[Roy J. Tellason]

On Thursday 13 January 2005 11:28 am, Karl Ekdahl wrote:

> > You should be alright in terms of the 5 volt supply for the logic, Though
> > it would be prudent to observe the rules for mixed signal PSU routing
> > overall. That is. Separate termination for analogue and digital grounds.
>
> This is one of the issues i was thinking of. I'm doing a analog drummachine 
> with a digital sequencer and the only signal i need to connect from the 
> digital part to the analog is the triggers (8 of them). How do i realize 
> this without mixing the grounds? Optocouplers? Or is there any even simpler 
> method?

You can connect the two grounds together,  just do it at *one* point to avoid 
ground loops.  The main thing with that is not so much that you don't want 
them connected as that you don't want the sort of nonsense that digital 
signals can do (including "ground lift",  where transient currents actually 
raise "ground" by a bit) getting into your analog circuitry.  Just thinking a 
bit about where that current is flowing should make it clearer.

> What i found is a wallwart transformer on a flee market. I bent it open and 
> it's shielded and everything so i'll probably just glue it back together and 
> use it as it is. If 20% of usage is the requirement, then out of 2A i have 
> to use 200mA, right? My application is allready consuming well over 500ma so 
> that should be O.K. i guess..

The 20% figure is a guess,  some of the real early stuff needed closer to full 
load,  some supplies can go as low as 5% without any trouble.  Best thing to 
do is to hook up some sort of a dummy load to it (use light bulbs,  handy 
power resistors,  whatever) to it in stages,  testing the output voltage at 
various loads to see how well it works out.  Or just test it with something 
that's close to the load you plan to use it at,  to see if it works.