Let's say I have a supply of 12v, and a 7805 5v voltage regulator. If we use this 7805 between the 12v and GND we get an additional 5v line. Now can we use the 12v and the 5v to power a load that requires 7v (=12v-5v)?
I'm asking because this would mean that 7805 has to deal with current going in the opposite direction than in the more "usual" circuits. If this is possible, does that hold generally for most voltage regulators or is that highly dependent on the particular model?
Looking at its internal schematic, 7805 doesn't have a push pull output, only one pass transistor (outlined in yellow) so it can't sink current.
Generally manufacturers won't waste silicon area on a power transistor that is not used in the application, so most voltage regulators don't have this feature.
Regulators that do have this feature are, for example: shunt regs, DDR termination regs, and synchronous buck regulators (forced PWM mode is probably required).
The answer can be inferred from the datasheet.
Not only does it provide a full circuit diagram, it provides several example circuits which make it clear that if reverse current may be present, a clamp diode should be fitted to bypass the regulator because current flowing in reverse in the regulator may damage it.
Very few regulators are symmetrical, you'd need one specifically designed for it.
