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 <[email protected]> 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 <[email protected]> >>> 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 . <[email protected]> >>>> 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" >
