Voltage regulator with alternate voltage source...

[William Donzelli]

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

Is that the big flat plastic package with the wide flat leads? I might
have a few of those around, but I agree, not an easy variant to find.

> 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?

Yes, but that regulator might get mighty hot! I would not do this for
fear of cooking the poor thing.

> 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?

Yes, you could do this. Pick an appropriate power resistor, or use a
big wirewound rheostat.

> 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?

I would not do this at all.

--
Will