[sdiy] L7912ACV puts out -16.3 volts

[harry]

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< -----