[sdiy] L7912ACV puts out -16.3 volts

[jh.]

Nothing wrong with two bridges.
Description sounds like missing minimal load to me. (If there is
no blatant circuit error.)
Connect two 1k resistors from +12V to GND and from GND to -12V
and see if voltages will be right then.

JH.

-----Ursprüngliche Nachricht-----
Von: harry <harrybissell at prodigy.net>
An: Dave Krooshof <krooshof at xs4all.nl>
Cc: <synth-diy at dropmix.xs4all.nl>
Gesendet: Sonntag, 26. August 2001 06:15
Betreff: Re: [sdiy] L7912ACV puts out -16.3 volts


> Hi Dave: inline
>
> Dave Krooshof wrote:
>
> > Hello,
> >
> > I just made a powersupply that is supposed to put out -12 and +12 volts.
> > The + side is OK, it gives 12.1 volts.
> > The - side is wrong, as it puts out 16.3 volts.
> >
> > The powersupply consists of a 230 volt to 2x 15 volt transformer.
> > It puts out 2x 18volts AC. *
> > The circuit has 2 diode rings (bridge cell in english?),
>
> two diode rings ?   (two full wave bridges i.e. 8 diodes ?)
> That isn't right. You should only need one bridge ... the center tap
> of the transformer goes to DC common. the positive output of the bridge
> is the positive (raw) supply. The negative is the negative....
>
> > 2 elcos 1000µF,
> > 4 104 caps, a L7812CV and a L7912ACV for + and - voltage regulators
> >
> > What has gone wrong?
> > - Was my 7912 off when I bought it?
> > - did I make a wrong connection that causes it to shift? (I checked and
> > rechecked, but who knows...) The 7912 connects differently from its
> > positive brother: 1=left=ground, 2=middle=in, 3=right=out.
> > - Has my over-enthousiastic trafo just fried my 7912?
>
> probably not.
>
> > - Is 18 volts to much for a 7912? I couldn't tell from the data sheet.
> > (which is here:
http://www.st.com/stonline/campaign/gold/linear/stpos.htm )
>
> no... up to 35V is OK. I think its a problem with the two bridges myself.
If
> you
> used two bridges you might use two positive regulators.... then thie them
in
> series.
>
> H^) harry
>
> >
> >
> > Thanks a lot in advance for the responses.
> >
> > Dave
> >
> > * Why the hack can't they make stuff like trafos that behave
> > according to the given specs? (or boxes that say what the inside will
do)
> >
> > Pierce: War is hack.
> >
> > ----- >^v< ---------------------------------------
> > Dave Krooshof http://www.xs4all.nl/~krooshof
> > geluidstechnicus @ http://www.ahk.nl/the
> > webmaster: http://www.popronde.nl
> > -------------------------------------- >^v< -----
>