Voltage regulator with alternate voltage source...

John Robertson jrr at flippers.com
Thu Apr 7 16:15:41 CDT 2016

On 04/07/2016 2:03 PM, William Donzelli 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

If his intention is to bypass the 7805 then it should have both input 
and output legs cut and lifted.

John :-#)#
> 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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