Would this work

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I have a 34v dc power supply, can I do this to achive +/- 15v
 

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Hi Leander,
starting from the bottom your outputs will be 0V, +15V, +30V. You have only 2V overhead before each reg. Look up the datasheet to find what a 7915 needs. Then add on the ripple.
I think you need a new transformer 2 times 15Vac output to generate your +-15Vdc.
If your current requirement is very low you can generate a -dc using a chip.
 
It still won't work.

If you really need this supply to work, and just a few mA, do the following:

Change the regulators to 12V types.

Make a voltage divider on the input out of a pair of 10K resistors.

Feed this ground reference to an opamp wired as a buffer, the output is your ground reference.

If you need more than a few mA: wire the base of an NPN transistor to the output of the opamp, the collector to the positive supply, loop the feedback from the emitter to the inverting input, and the emitter becomes your high power ground.
 
You need to consider Kirchoffs laws. The current through the two regulators must be equal, unless they're supposed to eat current through their ground legs (which they can't). And since the 78-regulators need to have more than 2 volts difference between input and output there's really only margin for one regulator.

So the only real option is to regulate to 30 volts, and then create a virtual ground with a reasonably powerful op-amp.

Rune
 
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