Voltage regulator with alternate voltage source...

-----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? Thanks, 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