23 Jun
1997
23 Jun
'97
1:18 p.m.
Hello Tony,
Actually 34vac 2amp center tapped
transformer. The center tap goes
I was wrong. It's 17-0-17V, then. That's what I will use.
From memory the central pin is chassis ground and
the outside 2 are 18V AC
at about 3A. to ground, what else how can negative voltages can be
generated so
it needs this ground. The both outside 2 pins for 17v ac lines goes
Well, it could use a switching regulator. OK, I know it doesn't do so for
the negative voltages - obviously I wasn't thinking straight earlier...
there. The card rectify it to generate 3 dc voltages,
two is
positive and other one is negative. Bulk of the components is used
to generate 5vdc, and a wimpy 12vdc source for the floppy drive and
The 5V regulator seems to be a switching one, which is what I'd have
expected.
fan. And last one is in very small current negative
current -12v dc
which takes this voltage and go through a 7905 regulator to get -5v
dc, both voltages are strictly for serial use and little use for
else.
Funny, instead of 60hz, you get the same type of circuit design in
secondary side in lots of switching power supplies with few minor
differences.
Yes, is that suprising? After all, a switching supply is nothing magical.
You still need to transform the AC output into DC and regulate it. Of
course you can always perform some regulation on an SMPS by varying the
drive waveforms on the primary side.
Talking of bad design (I was earlier...) I've seen some _crazy_ SMPS's in
my time. In a Zenith monitor there's a supply that combines the
reliability of a switcher with the efficiency of a linear. It rectifies
the mains, feeds it to a free-running chopper (no regulation applied
here), shoves that into a little transformer, rectifies the output and
feeds it (about 18V DC) into a linear regulator that uses the power-on LED
as the reference. It's the only monitor that _requires_ a green power-on
LED. When my blew up (chopper failed, etc), I replaced the entire mess
with a little torroidal transformer. It was cheaper than a new chopper
transistor and a lot simpler.
Then there are the Boschert 2-stage supplies that are used in Sanders
printers and PERQ 1's (and probably elsewhere). A shorted chopper in one
of those (which is a very common failure mode) will blow up 2 more
expensive power transistors and then take out a number of small
transistors, the chopper control IC (a good old 723), a few passives and a
couple of PCB tracks. I had to sort out such a mess once - I can provide
the full story if anyone's interested.
The manual
doesn't give the schematics of the transformer unit (it does
for the PSU card in the main unit), and it's not clear from the
description whether the AC input is centre-tapped to ground or not.
Looking at the schematics, I think that it is _NOT_
Oh yes, I did saw the techref for the outside PSU transformer is pretty simple just a disconnectable center tapped
transformer.
One thing I hated that they did not give us the that schematics for
that power card module which I revsere engineered instead!
But that is. The PSU card schematics (starting from the 3 pin connector)
were included.
Jason D.
--
-tony
ard12(a)eng.cam.ac.uk
The gates in my computer are AND,OR and NOT, not Bill