Voltage regulator with alternate voltage source...

[John Robertson]

On 04/07/2016 4:41 PM, William Donzelli wrote:
> Does this warning assume that there is something on Vin, and not open?
> With a quick look at the internal schematic of an old-school 7805, it
> seems like what Bill suggests (leaving Vin open) will not actually do
> much.
>
> --
> Will
This warning is for someone who is thinking of adding a large filter 
capacitor (>10ufd) on the output of the 78XX device and the manufacturer 
is warning against this. Typically the reason is when the input power 
drops below 2VDC above the ouput (power removed) then the device can 
conduct backwards to ground - if you provide enough current it will 
probably self destruct.

To me it is simple, the manufacturer says don't do it as you will damage 
the device. That is enough for me. If you want to check the internal die 
design you can see why this is so.

John :-#(#
> On Thu, Apr 7, 2016 at 7:23 PM, John Robertson <jrr at flippers.com> wrote:
>> On 04/07/2016 2:18 PM, Bill Sudbrink wrote:
>>> drlegendre wrote:
>>>> Not saying it's going to smoke-out, but it does
>>>> seem like a wonky thing to do.
>>> I disagree about "wonky" let me try with more
>>> diagram and less English:
>>>
>>>     (+8)
>>>       |
>>>    VIN|    ceramic cap
>>>       |-----][-----
>>>    ___|____       |
>>>    | 7805 |----------GROUND
>>>    --------       |
>>>       |           |
>>> VOUT|(+5)       |
>>>       |           |
>>>    ___|____       |
>>>    | LOAD |-------|
>>>    --------
>>>
>>> +8 is not currently available (no pun intended).
>>> I would like to test LOAD without removing 7805
>>> as it is soldered in place.  Is damage to 7805
>>> likely if alternative regulated current is applied
>>> at (+5) and (+8) is left open?
>>>
>>> Bill S.
>>>
>>>
>>>
>> Yes, you can damage the 7805 - READ the data sheets...
>>
>> Raising the Output Voltage above the Input Voltage:
>>
>> Since the output of the device does not sink current, forcing
>>
>> LM340/LM78MXX Series 3-Terminal Positive Regulators LM340/LM78MXX Series
>> 3-Terminal Positive Regulators
>>
>> the output high can cause damage to internal low current paths in a manner
>> similar to that just described in the “Short- ing the Regulator Input”
>> section.
>>
>>
>> LM340/LM78MXX Series 3-Terminal Positive Regulato John :-#(#