Voltage regulator with alternate voltage source...

[William Donzelli]

Keep in mind that "not powered" and "open" are very different things.
What do the data sheets say?

I will admit that this is all careening towards the academic at this point...

--
Will

On Fri, Apr 8, 2016 at 1:31 PM, John Robertson <jrr at flippers.com> wrote:
> On 04/08/2016 8:54 AM, wulfman wrote:
>>
>> https://www.fairchildsemi.com/datasheets/LM/LM7805.pdf
>>
>> http://www.ti.com/lit/ds/symlink/lm78l05.pdf ( page 8 gives you the
>> internals for a low power version )
>>
>> Nothing in the data sheet saying you cant apply voltage to the output.
>>
>> As per the low power version circuit diagram i cant see how you can do
>> any damage to it
>> I was not able to find an internal diagram for the higher power version
>> but
>> i am sure its close to the same circuit just larger pass transistors.
>>
>> I took a new regulator and measured the output pin to ground with my
>> trusty ohmmeter
>> on the diode setting and had no reading. If you think about it the
>> output on the regulator is a pass transistor
>> and the output to ground will be in effect a diode that will not allow
>> current to pass from output to ground.
>>
>>
>> My last comment still stands. i doubt you will cause any ill effects to
>> your regulator.
>>
>>
>> I personally have done this to some old arcade boards with no ill effects.
>>
>>
>> On 4/7/2016 6:08 PM, drlegendre . wrote:
>>>
>>> "...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:
>>>>
> I archive all data sheets I run across and I found this information on both
> TI and Fairchild data sheets for the 78H05, LM340 and LM78XX devices - it is
> usually under a heading called Application Hints and for some reason
> Fairchild have removed these Hints from the sheets you quote.
>
> In ALL cases the manufacturers caution that putting voltage to the output
> when the input is not powered CAN DAMAGE THE CHIP.
>
> The sheets I have appear to be a bit hard to find so here are my archived
> copies:
>
> http://www.flippers.com/pdfs/LM340_LM78XX_National_2003.pdf (page 11)
>
> http://www.flippers.com/pdfs/LM78H05A_Fairchild.pdf (page 4)
>
> So, yes, you can get away with it for a short time, but you are stressing
> the regulator outside its design limits and that will shorten its life.
>
> John :-#(#
>
> --
> John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
> Call (604)872-5757 or Fax 872-2010 (Pinballs, Jukes, VideoGames)
>                  www.flippers.com
> "Old pinballers never die, they just flip out"
>