26 Jul
2000
26 Jul
'00
2:43 a.m.
Geoff Roberts stated thusly:
invidual voltage
rating of the caps.
circuit,
hmmm,
logic
there,
hot
This is starting to sound like it has either a
separate + and - supplies
with a common ground, or
it's some kind of voltage doubler. Since I
can't imagine why they'd need
to do a doubling act
for the primary side of a SMPS I'm inclined to
suspect the former.
Here's a quote from a former Prime engineer (who's been as helpful as his
memory permits):
: These things are much more complicated than that. Functionally the Line is
: directly connected to a special rectifier configuration called a voltage
: doubler. It produces rougly 280 volts DC. There is no isolation
transformer,
: which makes these things potentially lethal. The 280V DC is input to
: switcher, which runs the 280V DC through the torroidal transformers to
: produce the other voltages. The Big Bridge is to convert the output of the
: switch from high frequency (probably about 25Khz) AC to DC.
Howwver, your former remark would also seem to be true, as the two rectifer
outputs are +280vdc and -280vdc, respectively.
[..snippage..]
Yes, but
aren't the 2 capactiors in series, something like :
------------+-------------------- +ve
|
=====
-----
|
------------+
|
=====
-----
|
------------+--------------------- -ve
Well they could be if they were across a potential higher than the That gives you an effective voltage rating equal to
the sum of the ratings
of the caps. The penalty is
that it divides the effective capacitance by the
number of caps in the
series circuit (assuming all are the
same value - it's a bit more complex if they have
different values) Have
you got a + where there should be -?
The above diagram looks wrong to me. If this was a
+/- supply, then the
lower cap should have it's anode
connected to the common (centre) ground point, or
it's effectively reverse
polarised. It would be more
common to find caps in parallel to increase the
capacitance (opposite of
series - the total c of the
circuit is equal to the sum of the values of all the
caps in it.)
> And you're seeing 600V between the 2 points I've marked '+ve' and
'-ve',
> right? That's still twice what I'd have expected -- I'd expect peak
mains
voltage (i.e.
115*sqrt(2)) across each capacitor, or about 320V between
+ve and -ve in that diagram.
The Prime techie mentioned a bridge, so I assume it's not a doubler that leaves some kind of +/- supply but I'm blowed
if I know how they
achieve that
out of a bridge. I'd definitely like to see a
circuit for this one.
--------------------- +
to BBU
|
|
+---------{ MCI 307138 }---------------+--------------------
+ve
/ \ | + |
/ \ ===== / 12k ohm
/ \ ----- \ 5%
/ \ | - /
----* *---------------------------------+----------*
| \ /| | + \
| \ / | ===== / 12k ohm
| \ / | ----- \ 5%
| \ / | | - |
| ----------{ MCI 307138 }---------------+---------------------
-ve
| | |
| | |
| | ---------------------- -
to BBU
| |
| |
| |
115v Load 115v Neutral
I am still puzzled as to how it gets 600V without
either other large
capacitors, or a mains-frequency transformer.
Ditto. I will just
ask again -- this thing does run off 115V, right?
I keep wondering the same thing, if it was multiphase it might make sense, I *wonder* if this was originally 3 phase and been
converted to single?
2 mains rectifiers in series? With a common rail? Sounds ugly but
possible I
suppose.
There is an adhesive label on the front of the PSU that says "115 V". Since
it's
a removable label, that there is some way to change it over for 230v
operation.
With that in mind, I can report that there is a connector on the main PCB
that
has wires that loop back to itself. I'm wondering if it's replaced with a
plug
either without wires or with wires in a different configuration that it
changes
the PSU from accepting 115v input to 230v input. But about phases, I am very
ignorant.
[..snip-o-rooney..]
> > > PALs? Are you sure? It's not at all
common to have significant
amount of
> > > digital electronics in an SMPSU. Most
of the time it's either simple
> > > analogue circuitry either as discrete components, or one of the
well-know
> >
SMPUS control chips.
How old is this box of tricks anyway? I must agree, if there is a lot of from the description of all the led monitoring etc, it
might be more to do
with that than
part of the supply proper. Possibly control logic to
ensure the system
won't power
up if the supplies are out of spec, or shutdown the
whole thing if one
fails etc...
I think this is correct.
> > > In which case look for dried-up (high
ESR) electrolytic capacitors.
This
> > is a
very common heat-related problem.
Indeed. Probably the commonest fault in PSU's, which tend to get pretty anyway. The startup resistor is still a potential
culprit here too.
Geoff, see my other post which is a reply to the same message to which you
were replying in this message to which I was replying. [hope that's clear!]
regards,
-doug quebbeman