[sdiy] L7912ACV puts out -16.3 volts

[Dr Strangelove]

People have said, on the EFM list, that 78xx and 79xx series voltage 
regulators just plain suck.  I've used 7815 and 7915 regulators in a power 
supply from a Paia kit.  I built the thing exactly right and it puts out 
something more like a +/- approx 18v (+/- .5v error).  Whats really strange 
is that when I FIRST built it it put out close to 15v pos and negative.  It 
has "drifted" over time and is close to 18 volts top and bottom.  I always 
just assumed that this was anticipated in these types of regulators.

         -=<Jonathan Pratt>=-
       (Phdinfunk at hotmail.com)




>From: sasami at blaze.net.au (Ken Stone)
>To: synth-diy at dropmix.xs4all.nl
>Subject: Re: [sdiy] L7912ACV puts out -16.3 volts
>Date: Mon, 27 Aug 2001 09:16:29 +1000 (EST)
>
> >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< -----
> >>
> >
> >
> >
>_______________________________________________________________________
>Ken Stone   sasami at blaze.net.au
>Modular Synth <http://www.blaze.net.au/~sasami/synth/>
>Catgirl Paradise <http://www.anime.net/~kens/>
>Australian Miniature Horses & Ponies <http://www.blaze.net.au/~sasami/>
>


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