PC Power Supply for DIY - Don't try this!

[Magnus Danielson]

>>>>> "TC" == Tony Clark <clark at andrews.edu> writes:

 >> One way to use such supplies to get proper power at say +/- 12V would
 >> be to use 2 PSUs and hooking the +12V of one of the to the ground of
 >> the other. Then would the ground of the other be -12V... but this is
 >> potentially very dangerous cause you migth have a low-ohmic contact
 >> between ground and case which would then cause a short of the low PSU.
 >> I would not recommend such a interconnection unless it has been
 >> verified that the output sections is really floating to the case (and
 >> other potential shortage points). Recall also that the +5V leads of
 >> the lower PSU will get the -7V potential and how the current of that
 >> cursuit will flow is a diffrent story...

 TC>    Yes, definately do NOT hook up two switching supplies in that manner!  
 TC> You can only do this with "floating" power supplies like those old 
 TC> Heathkit variable power supply jobs (which WON'T run an ASM-1 board worth 
 TC> a flip).  I'm not even sure I'd try this with newer linears.

Rigth. I thougth it would be good to say what you migth and migth not
do...

This floating buisness of PSUs is sadly not a well understood subject
by many even for traditional PSUs and I have found how just a little
resistor can sometimes help out.

Just cause there is no DC path between the primary and secondary side
of the transformer(s) this does not imply that nothing will jump that
border. If you take a standard PSU you will find that most
transformers will also act as capacitive dividers and that the
secondary side will glide along with the network hum... I have
measured this potential glide to have an RMS value of about 50V at a
troubled machine (hooked to an 230V/50Hz supply). The impedance of
this "source" is very high, and you migth reduce this "glide" with a
resistor of about 100k - 1M. I would not recommend a too low
resistance cause this will increese the current loop currents with
other devices. So, I think a relative floating is probably the way to
go... what about you other fellows? 

 TC> I'm just very cautious when it comes to plugging in my synth circuits!
 TC> Nothing would ruin my day more than to fry a whole board because of power 
 TC> supply screw ups!

I couln't agree more...

 >> Maybe traditional PSUs isn't to bad after all... :)

 TC>    That's why they still make 'em.  :)

That is not the hole truth, I think lazyness in accepting new
techniques and learning them well is also a major player... :)

Cheers,
Magnus