Voltage regulator with alternate voltage source...

[drlegendre .]

"...if you leave the unregulated rail _unattached_ and put +5
switcher straight onto the regulated +5 rail..."

My error, I read that as "attached".

In any event, just lift both the 7805 IN and OUT pins, and then supply
known-solid +5DC between the OUT and GND pads on the board.

No, you can't feed the IN pin with +5V, for as others have mentioned,
the 7805 has a minimum dropout of 2V or so.

On Thu, Apr 7, 2016 at 4:03 PM, William Donzelli <wdonzelli at gmail.com>
wrote:

> Per his description, the 7805's input will be open. It will not try to
> source any current, as it will have none to give.
>
> I suppose there might be a little leakage.
>
> --
> Will
>
> On Thu, Apr 7, 2016 at 4:58 PM, drlegendre . <drlegendre at gmail.com> wrote:
> > Err.. unless the voltage of the switcher is identical to that of the
> 7805,
> > then one device will source current, and the other will sink it.
> >
> > Like putting two 6V batteries in parallel, where one is fresh and the
> other
> > weak. Current will flow until the potentials are equalized. But with two
> > regulated circuits, I don't see how equality can be achieved.
> >
> > Not saying it's going to smoke-out, but it does seem like a wonky thing
> to
> > do.
> >
> > On Thu, Apr 7, 2016 at 3:41 PM, wulfman <wulfman at wulfman.com> wrote:
> >
> >> You should be just fine.
> >>
> >> On 4/7/2016 1:38 PM, Bill Sudbrink wrote:
> >> > 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.
> >> >
> >> >
> >> >
> >>
> >>
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