> Hi there once again
>
> Correct me if I'm wrong, but I though that the 78XX voltage
> regulators can only take 3X the output voltage on their input,
> so the maximum supply that a 7805 can have as its input
> is 15V.

There are two input-voltage limitations on 78xx regulators:

[1] Maximum input voltage; this may vary with manufacturer and product line
    (7805 vs 78L05, etc.) but is independent of the output voltage.

[2] Maximum power dissipation: since any voltage between the input and the
    output is simply converted into heat, a 7805 will have to dissipate four
    times as much heat converting 25 volts to 5 at a certain output current
    than it would converting 10 volts to 5 at that same current.  If you need
    only a few tens of mA, a heat-sinked 7805 should be able to go from 35 to
    5 without problems; if you're drawing a full amp, the chip will overheat
    if it tries to drop that much voltage.