[sdiy] L7912ACV puts out -16.3 volts

[Ken Stone]

If two bridges are used incorrectly, a short circuit will result through one
of the diodes in the bridge. I've seen transformers cooked by this.
Overvoltage would not be the expected result.

Ken

>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/>