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Take the example of the well-known LM78XX voltage regulators: there is the LM7812, and the LM7912. The latter should be used to generate -12V from, say, -24V whereas the former is used to generate +12V from +24V.

I must be missing something because I do not understand why we should use the 7912 instead of the 7812 on -24V rails generated by tying the positive pin of an isolated power supply to GND and the return pin of the same supply to the -24V net. Since the supplies are isolated, why would the 7912 be "aware" that it is in fact handling negative voltages or very loosely said that it is "upside down" with respect to the positive rail?

In a nutshell, why couldn't we use two 7812 to generate +/-12V from two +24V isolated wall adapters tied back to back?

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  • \$\begingroup\$ @JRE: You're right, fixed. \$\endgroup\$
    – user42875
    Commented Nov 28, 2020 at 13:41
  • \$\begingroup\$ I found this question to have a good answer to your question(s). \$\endgroup\$ Commented Nov 28, 2020 at 14:13

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I originally misread your question, or the part about separate isolated power sources was missing. Does the following schematic accurately describe what you are trying to accomplish? (It is always helpful to provide a schematic) If so, the answer to the question of why there exist the 79xx series of regulators, is so one does not need to have two truly isolated (floating, ungrounded) supplies.

schematic

simulate this circuit – Schematic created using CircuitLab

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