Go Down

Topic: Two 7805s on one source (Read 1 time) previous topic - next topic

tack

Lefty's diode solution also works for multiple battery supplies, if you need more capacity or the ability to 'hot swap' a single battery set whilst your device continues running.

Where you do it with a regulator you could use a variable VR and set the VR output to be 5v PLUS the Vfd of the diode, say 5.7v total. That way you would have your 5 V common rail after the diodes.

cjdelphi

Lefty option 2 sounds intriguing... would a couple of resistors do a similar thing having the resistors tied to give 1 output?


MarkT


Lefty option 2 sounds intriguing... would a couple of resistors do a similar thing having the resistors tied to give 1 output?


Diodes have a fairly constant voltage characteristic without allowing one regulator to back-feed the other - resistors
are not going to do a great job here I think.

To be practical you'd use schottky diodes (forward voltage 0.4 to 0.5V max, not 1N4001's which have a forward voltage of
1.1V or so).  These days schottky diodes are pretty much used for everything (unless very high voltage / high temperature,
or if low reverse leakage matters).  Schottky's have considerable reverse leakage at higher temperature...
[ I won't respond to messages, use the forum please ]

retrolefty



Lefty option 2 sounds intriguing... would a couple of resistors do a similar thing having the resistors tied to give 1 output?


Diodes have a fairly constant voltage characteristic without allowing one regulator to back-feed the other - resistors
are not going to do a great job here I think.

To be practical you'd use schottky diodes (forward voltage 0.4 to 0.5V max, not 1N4001's which have a forward voltage of
1.1V or so).  These days schottky diodes are pretty much used for everything (unless very high voltage / high temperature,
or if low reverse leakage matters).  Schottky's have considerable reverse leakage at higher temperature...


Agreed, schottky would be better. However adding a series diode in the ground pin line of the regulator chip is a very simple trick to 'compensate' for the output diode(s) voltage drop.

Lefty


Go Up