Voltage regulator with alternate voltage source...

John Robertson jrr at flippers.com
Fri Apr 8 12:31:39 CDT 2016

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)
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