l7915c regulator has 18v output???

[Gaftbart]

Hi there!

I really could get a lot of information out of this board. But i am really stuck at the moment with a +/- 15 Volts regulated power supply...
My centre tapped transformer gives me about 19 volts at each winding.
So i used a L7815 for the positive rail, and a L7815c from Semiconductor to get a nice regulated output for my next opamp filter experiments.
In the end it looks pretty much like this :
Die Elektronik Hobby - Bastelecke
My values:
C1 3300uf
C2 3300uf
C3 33uf
C4 470uf
C5 1uf
C6 1uf
All of them are Aluminium Electrolytes. Thats why i chose the bis values.

Now on the positive rail i get about 14,85 Volts which is alright.
But on the negative one i get about 18,5 Volts!!!??? How does it come to that?
I checked the pinout. Exchanged the regulator.
And i just have no idea why this is happening to me...since regulators always worked fine for me, but as this is my first symmetrical power supply, i might misunderstood something in the theory of building it...

So if somebody has got an idea, how this error can occur, i would appreciate some help!

 

[TroelsM]

Hi.

Do you have any load on the positive regulator? Some regulators demand a minimum current on the output. 5mA perhaps.

Kind regards TroelsM

 

[AndrewT]

some of the -ve regulators have a higher minimum current to allow regulation.

In 317 vs 337 reg the out to adj resistor can be 240r in the +ve and 120r in the -ve. These resistor values ensure the 5 & 10mA minima.

Something similar may be required for the 78xx & 79xx series.

 

[pinkmouse]

Have you got the pinout on the regs correct? I've made that mistake before.

 

[Gaftbart]

Alright I think the minimum current that has to flow was the problem, as i put a 1k resistor between the minus and ground which suppose to be not more than 0,5W (and no burning resistor) and the meter shows 15,1 volts at the output.
Thanks for your suggestions!!!

As well i checked the pinout with several datasheets from the supplier of the regulator before, and that should be alright too.

So i suppose i should put the resistor as guarantee, so my op amps wont freak out, right?

Again this was really helpful and furthermore a proof that experianced people are around
Thanks for that!

 

[Gaftbart]

So i used a L7815 for the positive rail, and a L7815c from Semiconductor to get a nice regulated output for my next opamp filter experiments.

Of course i used the L8915c for the negative rail...

 

[Mooly]

I remembered this old thread... not quite your problem but you may find it interesting.

http://www.diyaudio.com/forums/anal...r-supply-problem-need-help-2.html#post2096989