Bipolar power supply problem

[Zero the hero]

I'm building a bipolar +-15V power supply using 7815 and 7915 regulators.
Maybe it's me that it's not so smart, but the positive stage works well, and I read +15 volts on the display of my colorful DMM. I've got a problem with the negative supply: I read +31V!!!
I've built it a couple of times: the first one using the same circuit board. Both regulators shared the same diode rectifier stage and had the same ground (of course I used a bipolar transformer).
The second time I've built two separate circuits, they had their own diode rectifier stage. They where indipendent but I obtained the same results.
I knew that 7915 is the negative counterpart of 7815 and they work in the same manner, although they have different pinouts...
I don't know where the problem is!!!
Should I draw a schem of my stupid work?
Superman (or any other super-hero), please, help me!!!

[Boofhead]

You should be able to get either your first or second configuation working.  The first configuration is simpler and is preferred.

First check your pinouts.  Looking at the front of the device with the tab at the top and the leads pointing down, and from left to right:

78xx:   In, Ground, Out
79xx:   Ground, In, Out

Make sure you have a  10uF caps across the output of each each regulator located quite close to the regulator pins.   Occasionally you will get problems with small caps like 100nF on output, particulary with the 79xx.

If you have * ever* connected the 7915 around the wrong way it is extremely likely the device is blown.

[Boofhead]

This might help:

http://sound.westhost.com/project05.htm

[Zero the hero]

On monday I'll buy a couple of new negative regulators... maybe they are blown.
I do'nt understand: I used the same schematic you've posted!!! And I obtain the same results...
Time to go to bed now... I don't want to think about it!
Thanks for the help!

[Boofhead]

I don't know what could have caused the problem.

Another thing about the 79xx's is the heatsink tab is not ground, it's connected to the centre pin.  If you have it grounded or you are connecting things to the tab for ground you will get some strange results.  Just mentioning this in case you have done something inadvertent.

[Zero the hero]

I didn't put any heatsink because now I'm just testing it. By the way, I think I'll not put the heatsink cause I don't plan to use more than 500mA.
I remember that I tried another 79xx in this configuration and I readed -11v in the DMM. The IC was a 7905!
The gruond of the circuit must be connected with the central pin of the transformer, isn't it?

[Boofhead]

I didn't put any heatsink because now I'm just testing it. By the way, I think I'll not put the heatsink cause I don't plan to use more than 500mA.

You will be surprised just how hot the regulators get.  From what I can see your input is 31VDC that means the regulators will be quite hot with only a small current draw - perhaps only 50mA to 100mA.  Under short-circuit conditions the device will burn your finger.  

I remember that I tried another 79xx in this configuration and I readed -11v in the DMM. The IC was a 7905!

You might be doing something wrong - I don't know what!  The negative regulators do work!

The gruond of the circuit must be connected with the central pin of the transformer, isn't it?

Do you mean the centre-tap of the transformer secondary winding?  Yes you do have to connect this to ground - connect to the common of the (large) DC filter caps.

If you mean the mains earth then you should connect main earth to the enclosure but you don't *have to* connect it to the 0V of your DC supply.

[Zero the hero]

Thanks 4 your patience and help but I gave up...
I've buoght another 7915. Even a friend of mine was shocked by my "amazing" unit.
Then (disappointed) I've built to equal positive 15 volts regulator, then I've flipped the output of one unit, so now I've got +-15v.
I DID IT!!!!