Non stop power supply

Power Source Switching (my first circuit) — will I blow anything up?

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First post and first attempt at electronics and schematics so please go easy!

GOAL: power 1A @ 12v ethernet switch from three electronic regulated power supplies. Simultaneously or switching between them, does not matter.

LOGIC: Using IRF9540 p-channel MOSFETS circuit is intended to failover from lower to higher numbered supplies.

Initial idea was to use DPDT relays for isolation as I have no experience trying to parallel three delicate electronic power supplies together.

For example, pull up M2/M3 gates for Supply2 and Supply3 from Supply1 when it is on to disable them.

So only Supply1 will flow. And then Supply2 if turned on disables Supply3.

DISCUSSION: All of this is because I’m too terrified to try straight OR-ing (does that simply require Schottky diodes on the positive before paralleling them?)

I don’t know if I need the flyback diode (or even if I did that correctly) or many other things but just trying to be cautious. I am hoping this switching will “break before make” or that diodes will prevent any “bad things” from happening with switching.

TESTING: This seems to work as desired on the breadboard but I have not yet attempted with 3 independent power supplies. I will fuse the power supplies just before input into my circuit. I plan to tie all the negatives together and I think they already share a common ground.


(1) OR-ing the 3 supplies together with a Schottky diode on each positive.

(2) stack DPDT relays that would remain energized most of the time. If power fails on a supply, that de-energizes coils and fails over to remaining supplies. This seems to offer best isolation and switching time is not a problem.

Thanks in advance for your advice and comments.

power-supplymosfetshareedit  follow  flagedited 35 mins ago

asked 2 hours ago


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  • Normal convention is to draw positive rail at the top and negative rail at the bottom. You sort of try and do that and sort of not do it, and break the convention in some places. Just remove the voltage supplies altogether and replace them with +V and GND net names. You don’t need the entire symbol with the + and – right beside each other. Then rearrange everything so current flows top to bottom, especially the lamp and pull down resistors and diodes. – DKNguyen 1 hour ago 
  • It is not clear to me how you actually control the MOSFET gates to switch supplies in and out. If you want to independently switch supplies in and out you need indepent control signals. You can’t have a dominoe effect where one supply switches in the other just by being on. – DKNguyen 1 hour ago
  • My intended logic is to failover from Supply1 to 3, lower number supply takes priority. For example, pull M2/M3 gates for Supply2 and Supply3 to high from Supply1 when it is on to disable them. So only Supply1 will flow. And then Supply2 if turned on disables Supply3. – CA_Tallguy 1 hour ago
  • It’s always a good idea to provide a link to device datasheet IRF9540 datasheet || While we can get a reasonable idea of much of what you want to do the overall plan is not actually stated. Read what you wrote carefully – do you say explicitly what you are wanting to do? It is probably you want to use any one of the supplies if available with a preference for supply 3. It MAY be that they are all available at once. It may be that you don’t mind load sharing if they would. It may be that …. . … – Russell McMahon 1 hour ago
  • … || A clear description of what you want to achieve without anything on HOW will help us help you. || IF you are happy for all supplies to share if available then a diode from each is enough – they will load share if exactly equal and in an uneven manner if one has more voltage at a given load than others. You can shift the load sharing somewhat with 2 or even 3 diodes on a less preferred source or also add a small series resistor. eg 1 Ohm will drop 1V per amp and move loading towards a resistor less supply. || Good to see you trying to design things like this. – Russell McMahon 1 hour ago
  • what is a DC ethernet switch? … this is what i think of duckduckgo.com/?q=ethernet+switch&ia=images&iax=images – jsotola 1 hour ago 
  • Thanks for comments so far. I have edited post and reworked schematic based on your input. @jsotola yes, that’s the type of switch we’re talking about. – CA_Tallguy 50 mins ago

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1 Answer


This was more what I meant when I said top to bottom. I did not mean simply to flip everything upside down.


simulate this circuit – Schematic created using CircuitLab

That way you can trust that down is always a lower potential, and up is always a higher potential. You are allowed to cut out the supply symbol and use net names for this so there aren’t loops where conventional current is flowing up (which is what happens when you use a supply symbol.)

shareedit  follow  flag edited 36 mins agoanswered 43 mins agoDKNguyen26.5k11 gold badge3434 silver badges7676 bronze badges

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