PSU Switching, Flyback diode etc

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 Netgear ethernet switch (in a remote location) from any (or all) of three power supplies, and to failover between the devices. Other devices at the location have built in capability to failover between the three power supplies. Trying to rig up the Netgear to make use of them all for redundancy as well. No particular concern about which supply is used… just want continuous operation.

LOGIC: Two DPDT relays stacked would work fine and would keep supplies isolated from one another. OR-ing would probably be even better but makes me nervous. So I have designed the circuit below using P-MOSFETS thinking it might be a tiny bit safer to try to keep supplies somewhat isolated. (Is this a bit like being somewhat pregnant?) Using IRF9540 p-channel MOSFETS circuit is intended to failover from lower to higher numbered supplies. For example, when Supply1 is active it pulls up M2/M3 gates for Supply2 and Supply3 to disable them. So only Supply1 will flow. And then Supply2 if enabled similarly disables Supply3. I thought the logic of P-MOSFETS was most suitable because it hopefully will be simple to use +12v signal from one supply to gates of MOSFETS controlling the others will disable their current flow.

COMPONENTS: Any specific components mentioned are because they are already on hand. Open to other suggestions if any of these are inappropriate. Working from starter kits of components from Amazon.

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.

OTHER ALTERNATIVES: (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 23 secs agoasked 5 hours agoCA_Tallguy111 bronze badge New contributor

  • 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 5 hours 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 5 hours 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 5 hours 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 4 hours ago
  • 1… || 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 4 hours ago
  • what is a DC ethernet switch? … this is what i think of duckduckgo.com/?q=ethernet+switch&ia=images&iax=images – jsotola 4 hours 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 4 hours ago
  • Hi @CA_Tallguy, Welcome and nice to meet you. Ah, let me see if I understand your generic user requirement, as summarized below: (1) You have a couple of PSUs (say, all DC 12V 1A), (2) You would like to have some sort of switches (say, relay, general purpose or power BJTs/MOSFETs. (3) You do NOT have any specific requirements in this feasibility study stage (say, must use CAT5 connector, blah, blah, … which we can deal with later). Please feel free to comment or make counter suggestions. Cheers. – tlfong01 2 hours ago   
  • Questions: (Q1) (a) You are using a power P MOSFET of 20+A, with Vgs(th) 2V > 4V. You might like us to know why P and no N MOSFET (I am asking this, because if we can use a FET as common and cheap as possible, more newbies and poor hobbyists like me would be interested to learn together. (b) Are you using Arduino or Rpi, or others? For same reason above, if we can generalize our project for both Arduino and Rpi, then more future visitors of this Q&A would be interested to read on. – tlfong01 2 hours ago    
  • (Q2) Your TO220 IRF9540 might be an overkill. Have you considered other weaker, less bulky, and cheaper alternatives? Futurlec IRF Power MOSFET Catalog futurlec.com/TransMosIRF.shtml. – tlfong01 2 hours ago    
  • 1Getting there :-). Try doing what I suggested. -> Tell us WHAT you want to achieve and why. It is probably clear to you but not yet to us. | 1: Do you CARE which supplies supply power if 1 or 2 or 3 are operating? || If you just want to share 3 supplies and don’t care if any or all combinations supply power of those that are on then 3 diodes (1N4007 for mild loss, Schottky for slightly less loss.). If all supplies and all loads are identical then adding a 2nd or 3rd diode in a leg will largely more current towaqrds the fewer diode supply. – Russell McMahon 1 hour ago
  • 1Here – let me make that clearer 🙂 : Tell us what you are actually trying to achieve. | Tell us what you are actually trying to achieve. | Tell us what you are actually trying to achieve. | Tell us what you are actually trying to achieve. | … -> Really. We can go from there. It’s usually the best way. – Russell McMahon 1 hour ago
  • (Q3) I know you are thinking of using power MOSFETs to switch/select PSUs. But are your PSUs always on, or do you switch them on when necessary? And how do you know if your system is in trouble (say, freezing, and the watch dog is barking?. If you don’t need fast response non stop switching over, you can boot another PSU when necessary (ie, just switch on/off PSU. I used to play with LM2956 and LM2941 PSU and current switches and found them OK for toy projects. You might like to read my answer to the following question for more details: / to continue, … – tlfong01 1 hour ago   
  • “RpiSE Q&A – Rpi freezes every now and then, how to fix it with a watchdog?”: raspberrypi.stackexchange.com/questions/99584/…. This way you don’t need power MOSFET switching on/off current which might cause scary back EMF problems. The cheapy, popular, LM2596 5V/12V output 3A regulated power supplies usually have over current/voltage protection. – tlfong01 1 hour ago   
  • @RussellMcMahon I have updated my original post to try to be more clear. Nothing complicated…. I have a Netgear 12V ethernet switch that I want to power from any available (or all) of 3 power supplies. The supplies should have 99.99% or better availability and I just want to automatically switch to one of the others automatically should 1 power supply be offline for a few minutes or longer. I need something extremely simple so as not to introduce a weaker point of failure to the system. Even 2 DPDT relays (if a relay fails, whatever position would likely still be useful). – CA_Tallguy 30 mins ago
  • One more thing to clarify. Schottky diode are mainly for flyback, Power diode are for rectifying. (3) Walter Schottky and Flyback Schottky Diode 1N5819 raspberrypi.org/forums/…, /to continue, … – tlfong01 2 mins ago   Edit   
  • (4) Flyback Schottky Diode 1N4001, 1N4004, 1N5819, 1N5822 raspberrypi.org/forums/…. Cheers. (5) AliExpress 1N5408 1N5404 1N5401 1N5822 1N5818 UF5408 UF5402 6A10 10A10 Rectifier Diode nl.aliexpress.com/item/…. Cheers. – tlfong01 1 min ago   Edit   

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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  flagedited 4 hours agoanswered 4 hours agoDKNguyen26.6k11 gold badge3434 silver badges7676 bronze badges

  • Now add a few diodes to get his priority switching of supplies. He still hasn’t said why or even if he actually needs to stage supplies. – Russell McMahon 1 hour ago
  • @RussellMcMahon Well the diagram was just to slap together something that vaguely looks like something he is familiar with to show convention. Doesn’t really represent any real function. – DKNguyen 1 hour ago

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