which one is right - pin outs on a split rail 7912cv

[Deathwish]

Ok this should be easy  , I pulled a couple of diagrams of the net to build a split rail supply from a centre tapped transformer. the 7912 is an ST 7912CV which according to the DS is a TO220 with pins 1=gnd, 2= in and 3= out.  I take gnd to be taken from the centre tap of the transformer and connect up as such, i took In pin 2 to be from the negative of the Bridge rectifier, but oh no, all I get is a hot regulator and naff all else. so I take a look at the 2 diagrams I have and suddenly my mind goes into a spin , which pin outs are the right ones ..... Do they play pass the pins with these things ?

[Paul Moir]

Yes, the negative (79xx) versions have a different pinout. 

The centre pin on a to-220 is normally connected to the centre tab.  This necessarily changes the pinout of the negative regulator since the silicon is directly and conductivity attached to the tab. 

[Deathwish]

The 7812 is playing nicely as it should its the 7912 that is not.but both of those diagrams have different pin outs to different places yet the transformer is still a centre tapped one in both. So which is right.

I lost the plot with it and am now watching serial killers on youtube till I calm down

[MarkF]

Do you have the tabs of the two regulators isolated?  Otherwise you'll short the input to the negative regulator to gnd.

[Paul Moir]

Oh sorry, I think I'm following you now.  I've never seen them switch the pins on the 79xx.  It should be Ground-input-output (1-2-3), so your first diagram following the red numbers is right.

You don't have both of 'em screwed to the same heatsink do you?

[Deathwish]

No both are on seperate heatsinks.

[Deathwish]

Its running ok now. Oh well, stupid pin outs. got to put a 5V rail on next.

[apis]

It's also a good idea to add some protection diodes for different fault conditions:

[ludzinc]

Ok this should be easy  , I pulled a couple of diagrams of the net to build a split rail supply from a centre tapped transformer. the 7912 is an ST 7912CV which according to the DS is a TO220 with pins 1=gnd, 2= in and 3= out.  I take gnd to be taken from the centre tap of the transformer and connect up as such, i took In pin 2 to be from the negative of the Bridge rectifier, but oh no, all I get is a hot regulator and naff all else. so I take a look at the 2 diagrams I have and suddenly my mind goes into a spin , which pin outs are the right ones ..... Do they play pass the pins with these things ?

RTFM.  Read The Formatted Datasheet for the parts

[Deathwish]

"the 7912 is an ST 7912CV which according to the DS is a TO220 with pins 1=gnd, 2= in and 3= out."

Kind of tells you I DID RTFM
My gripe is with common parts such as the 78 or 79xx series devices I would expect them to be drop in replacements for each other, but if you are going to draw a circuit diagram use the pin designator not numbers. The use of G, I ,O on those circuit diags rather than 1,2,3 would of been much more use to a beginner or anyone else as far as I am concerned.