OT: General power supply question - test-drb@ccmp.vtda.org

10 Nov 1998

Sam:  (It was Sam, wasn't it?)
...

 If I have a center negative power supply (the diagram shows that the
 negative is in the center of the connector) does that mean the center of
 the connector on the device to be powered should be connected to the
 ground plane? Tony:

...
  One of Vonada's Laws is 'There is no such
thing as ground'. This has 2
 distinct meanings, BTW, both of them applicable to classic computers. The
 first (and the one we need here) is that voltmeters have 2 probes and you
 can take the reference where you like. The other is that a ground
 connection has impedance, so all the points you thought were ground may
 in fact be at different voltages.

 Anyway, that said, it is common in most systems to have the ground plane
 at the -ve supply voltage. Exceptions are most ECL systems (which run off
 a -5.2V supply, and thus the +ve side of the supply goes to the ground
 plane), some discrete transistor machines (the HP9100's main supply rail
 is at -15V wrt the ground plane), and probably others.

 But in 99% of TTL machines (and things using TTL-compatible
 microprocessors, etc),  the ground plane is, indeed, -ve. I've forgotten who
else attempted to answer the question - Tony's was by
far the best reply - but one or two things need clearing up.

Person who said check continuity between centre pin of socket and ground on
the machine.  That tells you if it _is_ connected to the ground plane.  Sam
wanted to know if it _should be_ connected - i.e. he seems to be adapting a
machine for which he has no PSU to work with a PSU he has.

Person who talked about safety earthing.  All true, but I think the
question was about the ground plane within the machine, and whether it
should be connected to the negative supply rail, not whether the machine
needs to be earthed.

Tony:  Agreed 99%.   But there are one or two exceptions as you imply -
wasn't it you who eold me about the machine which regulates the negative
rail with a -5V regulator and makes that the ground?  The positive rail is
then +5V and the unregulated negative is around -5V for serial ports and
things.

If the supply you are using is a regulated 5V supply, then + and -
terminals go to +5V rail and ground plane respectively.

If it is not, try and trace supply rails back to a regulator.  It should
then be possible to work out which way the regulation is done.

But as Tony says, in 99% of machines the answer is yes.  If you tell us
what the machine is, there will probably be someone on the list who can
tell you how to wire it up.

Philip.