N00b power supply question

[lazarusr]

I am working on a project based on a rather flaky book I have found from the 1980s. I have already established that there are quite a few errors in the book, so I am having to try to work round these. My latest problem relates to the power supply requirements.

The project requires both a +5v rail and a +12v rail. The +5v supply is taken from a port on a retro 80s 8-bit computer The +12v supply is taken from a wall wart.

In the schematic in the book, the +5v from the computer port is connected to the +5v supply, the GND from the computer port is connected to GND and the +12v from the wall wart is connected to +12v (all, obviously, as expected).

However, GND from the wall wart is connected to +5v. I do not understand this at all. So my questions are this:

• Am I correct in thinking that tying the GND from the 12v wall wart to +5v on the schematic is likely to be an error?
• Is it OK to for the two separate power sources to share a common GND or should the +12v rail have it's own dedicated ground that?

I attach a copy of the schematic. A few points should be noted. The text confirms, rather surprisingly, that the +12v wall wart is centre -ve. Also, the +12v supply isn't meant to be doing anything in this schematic, it just goes off to a header for supplying other projects. Hence the outer connection from the wall wart isn't connected to anything else in the schematic and, rather confusingly, isn't labelled. Also, it's the centre connection (which is GND on the wall wart) which is shown connected to the +5v rail.

Any help working this out would be greatly appreciated.

[lazarusr]

I should point out that all the projects that interface to this require that power (whether it is +12v or +5v) and data share a common GND.

[jeroen79]

Am I correct in thinking that tying the GND from the 12v wall wart to +5v on the schematic is likely to be an error?

If you attach the - of the 12V supply to the +5V rail then the + of the 12V supply would be raised to +17V.

Is it OK to for the two separate power sources to share a common GND or should the +12v rail have it's own dedicated ground that?

Yes, you can connect rails from different supplies together as long as they are floating. If the suplies are already referenced to different potentials then you could create a short.

[lazarusr]

Thanks.

I am pretty sure there was no intention for there to be a +17v supply.

Both power supplies have floating grounds. So I will proceed with a common ground. Indeed, it is apparent from the other projects in the book that it is intended that +12v and +5v share a common ground. The other projects simply wouldn't work if they didn't.