[sdiy] L7912ACV puts out -16.3 volts

[Scott Gravenhorst]

I thought about other threads on these parts... that you can
put a resistor between ground and the ground pin to raise the
regulation voltage.  So imagine a poor solder connection
(a resistive connection) between ground and the ground pin.
Voila, a resistor and then the regulator puts out too much
voltage.  Capacitance could cause startup and shutdown problems,
maybe stability issues, but not overvoltage (AFAIK).

"Rob B" <cyborgzero at home.com> wrote:
>Could suspect bad/leaky caps perhaps...
>
>App notes... Prolly a must to troubleshoot this type of thing too... Perhaps
>the load is very capacitive? I wouldn't suspect this as a problem, but you
>never know till you are knee deep..
>
>Rob
>
>
>aim : cybrgzr0 <--last thing is a number
>----- Original Message -----
>From: Scott Gravenhorst <music.maker at gte.net>
>To: <synth-diy at dropmix.xs4all.nl>
>Sent: Sunday, August 26, 2001 6:12 PM
>Subject: Re: [sdiy] L7912ACV puts out -16.3 volts
>
>
>> This has not been my experience.  I have many in operation
>> and they are all within factory spec. for years.  I have
>> 6 in two FatMan synths and another 4 in mods for one of them.
>> I have another 10 or more in various other things.
>>
>> "Dr Strangelove" <phdinfunk at hotmail.com> wrote:
>> >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/>
>> >>
>> >
>> >
>> >_________________________________________________________________
>> >Get your FREE download of MSN Explorer at
>http://explorer.msn.com/intl.asp
>> >
>> >
>>
>> =========================================================
>> - Government: The other religion.
>> - The media's credibility should always be questioned.
>>
>> -- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
>> -- Linux Rex         | RedWebMail by RedStarWare
>> -- FatMan: www.teklab.com/~chordman
>> -- NonFatMan: members.xoom.com/_XMCM/chordman/index.html
>>
>>
>

=========================================================  
- Government: The other religion.
- The media's credibility should always be questioned.

-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
-- Linux Rex         | RedWebMail by RedStarWare
-- FatMan: www.teklab.com/~chordman
-- NonFatMan: members.xoom.com/_XMCM/chordman/index.html