[sdiy] Power distribution board

[Roy J. Tellason]

On Saturday 04 April 2009 12:20:47 am David G. Dixon wrote:
> I've just taken delivery of a Power-One +/-15V, 1.5A power supply, and I
> need to etch a distribution board for it.  I would like to build something
> like the MOTM board with an array of MTA156 4-pin connectors, and I'm
> wondering if anyone can point me to a PCB image for something like that. 
> If not, I will have to draw my own, in which case I have a question:
>
> I ascertain that it is important to connect each ground pin to the power
> supply ground buss with it's own trace (the so-called "star ground"
> configuration).  Is this important enough to justify the inconvenience?  If
> so, then should one also do the two power rails this way?  Also, what is
> the deal with the two ground pins on the 4-pin connectors?  Are they
> actually different?
>
> Power distribution seems like a fairly important thing -- I don't want to
> screw it up!

Having each and every load in whatever system it is that you're building 
having its own power run to the main distribution point and ground point is a 
nice idea theoretically,  but not doing so isn't going to get you in anywhere 
near as much trouble as ignoring this idea completely and letting things fall 
as they may,  convenient to routing traces and whatnot.

I personally would have no particular problems with connecting power and 
ground to a bunch of modules by way of a number of heavy buses on a board,  
though if it'll make you feel better sprinkle assorted bypass capacitors 
along it,  to deal with whatever bits of noise might end up there.

If you really want to get some idea of the effect,  figure the maximum and 
minimum current that any given load is going to present to the supply,  then 
figure out what voltage drop these are going to create for a given 
interconnect,  whether it's that bit of distribution board trace or the 
connectors you're using (all of which have _some_ resistance) or some other 
aspect of things.  Having all that info on hand the calculations get a little 
cumbersome after a while.

I suspect that the overall noise won't amount to that much.  I also suspect 
that some decoupling at the power pins of each module or board will help this 
some as well.

There's also a fair amount of stuff I've seen that ran two sets of grounds.  
One instance would be a 22 gauge ground lead for something that didn't have 
much of a current draw,  the other would be an 18 gauge wire for stuff that 
did,  like audio power amps and such.  This was in organs.  Another instance 
of separating things out like that is when analog and digital get mixed on a 
board,  there will often be separate grounds which,  while ultimately both of 
them end up grounded the digital side will tend to have (and even generate) 
more noise,  which you wanted to keep out of the analog lower-level stuff.  
And the old c64 had two separate and distinct +5V supplies,  one on-board and 
one coming from the external power brick,  the onboard one mostly feeding the 
VIC and SID chips and not much else IIRC.  (Depended on what version of the 
board you had too.)

How important this is depends on what you're gonna use that PS for.  What will 
it power?

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