> Is there a way to get regulated 15 V from a 7812 12 V regulator ?

You would be better off with an LM317 which is designed to be used like this
BUT.

- Place a resistor R1 of 4 units resistance from output to 7805 ground
terminal.
- Place a resistor R2 of 1units resistance from 7805 ground terminal to
ground proper.

R1 will drop 12 volts.
R2 being 1/4 the size with APPROXIMATELY the same current in it will drop 3
volts.

As the 7805 passes current from Vin to its ground pin to power itself and
this will also flow through R2, the output voltage will be somewhat higher
than expected. Adjust R1 to suit.

Voltage stability will be better with an LM317 which powers itself from its
Vin to Vout drop.

I gave resistor ratios above and not actual values so you can play. Too high
and regulation gets bad. Too low and wasted current is too high.

Use an LM317 :-)
See its datasheet to get formula for resistors to use. (As above system but
Vout basic is much lower).

The diode trick that someone else mentioned will also work - it has good and
bad points relative to resistor solution.
Also could use a 2V7 zener.




        Russell McMahon

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