Voltage regulator with alternate voltage source...

[Bill Sudbrink]

If you have a circuit which is normally designed to
operate with an unregulated supply, through a regulator...
say unregulated +8 through a 7805 to a regulated +5 and
you want to test it independent of the +8 supply, if
you leave the unregulated rail unattached and put +5
switcher straight onto the regulated +5 rail, will you
damage the 7805?  Clearly the VIN is open, but the ground
pin will still be attached.  Would this push voltage
back through and screw things up?

Thanks,
Bill S.