Newsgroups: sci.electronics
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: want help with power supply
Message-ID: <1989Jul6.164903.1339@utzoo.uucp>
Organization: U of Toronto Zoology
References: <113785@sun.Eng.Sun.COM>
Date: Thu, 6 Jul 89 16:49:03 GMT

In article <113785@sun.Eng.Sun.COM> rom@xor.Sun.COM (Achyutram Bhamidipaty) writes:
>pf	????
>on/off	???? This is NOT a switch, just a bind post
>+s1	??
>-s1	???
>+s2	???
>+v2	plus side of supply 2
>-v2	minus of supply 2
>-s2	???
> ...
>I expected the +/- vn connections, but not the +/- sn connections
>but they didn't teach us about the sn connections at school!!
>So the question: what are the sn and pf connections?

For a guess...  pf is probably a power-fail warning signal, meant to warn
your system that the AC power has failed and the DC outputs are therefore
about to fail.  on/off may be for remote control.  As for the sn connections,
those are probably sensing leads.  Consider:  if you have a fairly long
cable running from the supply to the equipment it is supplying, there may
well be some voltage drop in the cable -- it does have a non-zero resistance,
after all.  So regulating the output voltage at the power-supply end of the
cable won't get it precisely right at the other end.  Worse, the voltage at
the other end will *change* if current demand changes, since the voltage
drop in the cable will be proportional to current drawn.  What to do?  Run
a separate pair of sensing wires.  The sensing wires are connected to the
power wires at the destination end.  At the power-supply end, they go into
high-impedance sensing circuits.  Since virtually no current flows in the
sensing wires, there is essentially no voltage drop along them.  So the
sensing circuitry on the power-supply end gets to see what the voltage is
at the *destination* end of the power wires, and the power supply can
adjust its output so the right voltage is delivered at the destination.
This is pretty standard practice for high-output power supplies.  If your
cables are short or you don't care that much, just connect the sensing
terminals to the output terminals at the supply.
-- 
$10 million equals 18 PM       |     Henry Spencer at U of Toronto Zoology
(Pentagon-Minutes). -Tom Neff  | uunet!attcan!utzoo!henry henry@zoo.toronto.edu