Quoting Daniel Imfeld <dimfeld@SOFTHOME.NET>:

> The behavior sounds like that of a clamper, which employs a diode,
> capacitor, and second voltage source to shift an AC source by a certain DC
> level, in this case about +20 volts.  I suppose it's possible that the cap,
> diode and 5V output on the output side of the regulator are combining to
> clamp the input signal up 20V.  I'm afraid I can't tell you how to fix this
> problem though.  If this is what's going on, it might not be such a problem
> since I think you would still have 5VDC for the output.  I could be wrong
> though.
>

You are right - it wouldn't be a problem since I'm getting +5 anyway (although
specs say 40 MAX on the input... so that might be pushing it a little). BUT..
I need to feed a +12 regulator ALSO, which is just a linear - and the max is
only 30V on that. I tried hooking it up (just for fun since I had a lot), and
it suddenly became a +15V regulator instead of a +12 :-) So that's why I need
to maintain it around +25. If this isn't possible, I will have to drop the
voltage a bit with a resistor - which is fine, I just want to know if there is
another way around.



> Daniel Imfeld
>
> ----- Original Message -----
> From: "Jai Dhar" <jdhar@ENGMAIL.UWATERLOO.CA>
> To: <PICLIST@MITVMA.MIT.EDU>
> Sent: Saturday, February 08, 2003 8:45 PM
> Subject: [EE]: Rectifier Question
>
>
> > Hello all,
> >
> >  For those of you who knew about my little I2C problem that I had, I have
> > officially given up on it until I receive my new sensors from Microchip -
> in
> > which case I will try again :-) Anyway, working on the other parts of my
> > project, I started on getting my MAX787 5V SMPS regulator working. It's a
> very
> > simple connection, so it seems, according to the data sheet on maxim's web
> > site (http://www.maxim-ic.com/quick_view2.cfm?qv_pk=1271). I am using
> their
> > EXACT circuit, and getting a nice solid +5V output. The problem is at the
> > input from the rectifier. Basically, I have a transformer that goes to a
> full
> > wave rectifier, and then the positive part of the rectifier is fed into
> the
> > input of the regulator. As soon as I connect the input of the regulator to
> the
> > output of the rectifier, the voltage at the input jumps up to ~45V.
> Without
> > the regulator connected, the voltage at the output of the rectifier holds
> at
> > ~25V. Checking the negative side, with no regulator connected, it gives
> a -
> > 25V - so it's symmetric, which it seems fine. But as soon as I connect the
> > regulator, when the positive swings up to 45, the negative becomes
> about -5.
> > How come the voltage swings like that? Is this normal? Is there anyway I
> can
> > control it?
> >
> > Thank you,
> >
> > Jai
> >
> >
> >
> > ----------------------------------------
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> >
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