78L05 - quick question

[evandude]

anyone know how a 7805/78L05 voltage regulator behaves when there is no voltage applied to its input, but there is a voltage on its output?

I am using an L05 in an RS232 level shifter. it connects to the target circuit via a 4-wire cable (TX,RX, Power, and GND)
the idea is that the level shifter can be powered off the target circuit if the power pin is connected to +5v in the target, and then the +12v supply to a 2-pin header on the level shifter is left unconnected. This means the output of the regulator is connected to the +5v supply but the input is floating. If, however, the target circuit does not have the power line connected for whatever reason (battery power consumption issues, minimizing external connections, etc) then I would simply plug 12v onto the header on the level shifter, which powers it via the voltage regulator.

I am just wondering if I need to put in a jumper or switch to disconnect the regulator output from the 5v line when the regulator input is not connected.

 

[John Sorensen]

What does the datasheet say? Some regulators will tolerate it, others will not.

j.

 

[Nigel Goodwin]

I am just wondering if I need to put in a jumper or switch to disconnect the regulator output from the 5v line when the regulator input is not connected.

Just add a reverse blocking diode in the output, then it can't possibly feedback into the regulator!. You can compensate for the 0.7V drop by adding another diode in the ground pin of the 78L05.

 

[evandude]

thanks for the help nigel. my original reason for asking was that I was unable to find my DMM so I couldn't easily test it out. But now I found it, and hooked it up to my power supply and crossed my fingers. the regulator is behaving just as I'd hoped... there is no change in voltage across the power supply when the regulator output/gnd are connected, so it looks like i'll be all set. the regulator draws about 4mA with the input floating, which is acceptable, and expected given the equivalent circuit in the datasheet, which places a resistance across Vout and GND.