Voltage regulator with alternate voltage source...

[William Donzelli]

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

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 :-#(#
>
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