Voltage regulator with alternate voltage source...

[Bill Sudbrink]

First, a few quick "whys":

1) The 7805 is actually a Motorola MC7805CP, date
   code 7308 with gold leads.  Very hard to exactly
   replace.

2) As per the design of the unit, the 7805 is soldered
   to the foil side of a single sided, copper clad
   circuit board with no through hole plating.  Lifting
   the traces seems like a significant possibility that
   I don't want to risk.

3) I'm still waiting for the components to build the
   8 volt power supply.  Hopefully they will arrive
   Monday.

4) No, one thing missing from my bench, that I've
   never really needed up to now, is a good adjustable
   DC supply.  My have to ask for one for Christmas.

Three thoughts have come to me since my original post.

1) I have a 12 volt DC supply.  12 volts seems to be
   within the VIN range for the 7805s whose data
   sheets I've now read.  Can I simply apply 12 volts?

2) Could I place a resistor in series between the 12V
   supply and the 7805 to drop the voltage at the 7805
   to somewhere around 8?

3) If I was to "tack on" a jumper between VIN and VOUT,
   would that protect the 7805 and allow me to power the
   circuit with 5 volts?

Thank You,
Bill S.