voltage-regulator
I'm wondering if it is possible to get three voltages out of two voltage regulators. Let's say I have a 9V and a 12V regulator in a circuit. Would I be able to get the 4 voltages below out of this setup (if I include the input)?
I can not see why this won't work.
Simple answer: You would get 5V in your case, but referenced to -12V instead of ground. In other words, you would have -7V, not -5V. In addition, the regulator would only source current onto the -7V rail, not sink it as would be expected for a negative voltage.
If you want to run some circuitry from 5V between -7V and -12V (the -12V will be the ground for this circuitry), then you can use a positive regulator as you described. If however you want to run some circuitry between ground or higher and -5V, then you need the negative regulator.
No - not without due precautions.
SOME regulators will withstand this.
Many will not or may not.
Place a reverse polarity diode from output to input of each regulator so that if Vout > Vin the diode will conduct and charge the input capacitor or whatever.
This problem can occur in real world applications.
Largish output cap.
Input short of bus to ground or fuse blows and some other load rapidly lowers Vin.
Vin now low, Vout now high, Vout cap discharges into regulator and destroys it
From page 11 of NatSemi datasheet - here.
Pay no attention to the man behind the curtain when he says a 1N4002 is a fast diode. It's OK here. 1N400X.
Best Answer
Regulators normally do not like to sink current.
If you connect a load between the 12V regulator and the 9V regulator, then the 9V output will tend to rise out of regulation (to as high as 12V for a heavy load). If a load (to ground) on the 9V regulator can be counted on to always be sufficient, then it may work in some limited situations \$^1\$.
It's not really an 'output' though, as it's not referred to ground.