Voltage regulator with alternate voltage source...

[John Robertson]

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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