Electronic – Is this negative regulator really meant to be an inverted positive regulator

dc/dc converterpower supply

Project: make a power supply that can replace the 240V AC supply shown, so that I don't die when messing with the board that this connects to.

I'm only interested in the part inside the yellow box; the +40V output goes to the power amp section which I'm not working with.

Power supply schematic

So I went out and bought a 7812 and 7815, but then I got confused because of the general consensus that you shouldn't use a 7815 with inverted output pins as a negative voltage regulator, but it seems like that's what they're doing here.

I've ordered a 7915, but my question:

  • (a) Is this circuit doing something smart by connecting the 7815's positive output to ground and using its output as -15V?
  • (b) Is this circuit doing something weird, and I should use the 7915 to source -15V as the gods intended
  • (c) it's impossible to tell, needs more info

Note that I'm sourcing ±30V DC from a preamp circuit in this project, which is higher than what would be at the input in this schematic but well within tolerance for the 7812/7815/7915.

My schematic, using a 7915:

schematic

simulate this circuit – Schematic created using CircuitLab

Version with 'reversed' 7815:

schematic

simulate this circuit

Best Answer

What you have shown in the yellow box is OK, because the 18 V secondary that's sourcing the 7815 is fully isolated from the other rails. This means the 15 V output is fully floating, and any terminal can be grounded.

If you want to use a centre-tapped transformer (with some slight savings in hardware and diode drops) which produces non-isolated rails, or the non-isolated rails from another piece of equipment, then you need to use 79xx series regulators for the negative rails.

The reversed 7815 you've drawn will not provide the -15 V you want.