Voltage regulator with alternate voltage source...

Henk Gooijen henk.gooijen at hotmail.com
Thu Apr 7 16:07:38 CDT 2016

-----Oorspronkelijk bericht----- 
From: Bill Sudbrink
Sent: Thursday, April 07, 2016 10:38 PM
To: 'General Discussion: On-Topic and Off-Topic Posts'
Subject: Voltage regulator with alternate voltage source...

If you have a circuit which is normally designed to
operate with an unregulated supply, through a regulator...
say unregulated +8 through a 7805 to a regulated +5 and
you want to test it independent of the +8 supply, if
you leave the unregulated rail unattached and put +5
switcher straight onto the regulated +5 rail, will you
damage the 7805?  Clearly the VIN is open, but the ground
pin will still be attached.  Would this push voltage
back through and screw things up?

Bill S.

It is a special situation I have never seen, but I would try to disconnect 
the output of the 7805 before connecting a foreign +5V to the circuit.
It is good practice to have a diode reverse connected from output to input 
to prevent damage to the 78xx in the case that the output voltage
gets higher than the input voltage. Normally that would not happen, but if 
you have a high capacitance at the output and the load is minimal
it could be possible when power is switched off that the input voltage 
dropped, but due to the big capacitor at the output the output voltage
did not yet drop. And then that diode in reverse from output to input comes 
into play.

Now the input is not connected, but GND and the output are ... I would have 
a look at a data sheet of the 78xx series to see what could happen
if the input is not connected, but the output is.
So, if possible, simply disconnect the 78xx output lead first.

- Henk 

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