This is my 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 them. There is no special requirements other than to keep the switch powered up. Other devices at the location already failover between the three power supplies. I am trying to rig up the Netgear for redundancy as well. There is no particular concern about which supply is used... just want continuous operation.

LOGIC: I am trying to come up with a better plan than two DPDT relays stacked. 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. Parts on hand:

MOSFETS, Component Starter Kit, Shottkey Diodes, and these pre-made MOSFET trigger boards.

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.


  • 2
    \$\begingroup\$ 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. \$\endgroup\$
    – DKNguyen
    Commented Jul 28, 2020 at 0:19
  • \$\begingroup\$ 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. \$\endgroup\$
    – DKNguyen
    Commented Jul 28, 2020 at 0:19
  • 1
    \$\begingroup\$ ... || 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. \$\endgroup\$
    – Russell McMahon
    Commented Jul 28, 2020 at 0:52
  • 1
    \$\begingroup\$ Getting 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. \$\endgroup\$
    – Russell McMahon
    Commented Jul 28, 2020 at 3:45
  • 1
    \$\begingroup\$ Here - 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. \$\endgroup\$
    – Russell McMahon
    Commented Jul 28, 2020 at 3:46

2 Answers 2




The OP's question is a bit too broad, and involves a couple of engineering trade offs, cost/risk benefit analysis, and user constraints on knowledge/skills background, learning goal and project developing schedule etc.

Anyway, I suggest to eat the big elephant bite by bite, and I now start brainstorming bites randomly.

First, a reminder: I am a long winded guy and so it might take a long while for me to complete a TLDR answer.

Second, a warning: I am just a friendly hobbyist. No guarantee no nothing won't melt down or blow up.

For the impatient visitors, I would first give a very short, simple solution, which is actually originally proposed by @Russell McMahon and the OP himself.

non stop psu

Design Notes

  1. This circuit is very simple, but beautiful, using one power rectifying diode for each power supply unit (PSU)

  2. One disadvantage is that all PSUs must be switched on all the time.

  3. Another disadvantage is that all PSUs are supplying power at the same time.

/ to continue, ...

Discussion, Conclusion, and Suggestions

1. On flyback (freewheeling) diode and rectifying diodes

I recommend 1N5819 for flyback, because it is Schottky and therefore switches fast, with a low voltage drop.

I recommend 1N5408 for power rectification, because it is designed for power supply rectification. Comparing to 1N5819, it can handle larger current, but with larger voltage drop, and switches slower.

Having said that, both can be used for general purpose flyback and rectifying (Well, the marketing guys say so. :)).

2. On preference of PNP/NPN, NMOS/PMOS, Low/High Logical Trigger Level etc

Newbies usually prefer using a High level signal to switch on something, Low level to switch off something. However, the professional EE guys developing industrial and consumer applications almost always prefer the opposite way, ie, Low level (active, trigger) to switch on or enable something; High level signal to switch off or disable something. The main reason is electrical efficiency. Google for details.

/ to continue, ...


(1) 1N5819 Schottky Barrier Plastic Rectifier (free wheeling, polarity protection, 1A, 0.6V) - Vishay

(2) 1N5408 General Purpose Plastic Rectifier (power supply rectification, freewheeling, 3A, 1.2V) - Vishay

(3) Schottky Diode Forum Discussion - rpi.org.forum 2019

(4) Flyback Diode Selection Notes - rpi.org.forum 2019

(5) AliExpress Power Rectifying Diode Product Sheet

(6) Rpi freezes every now and then, how to fix it with a watchdog? - 2019jun14

(7) Futurlec Power MOSFET Catalog

(8) Vishay N-channel Power MOSFET Guide

(9) Vishay N-channel Power MOSFET (TO220) Catalog

(10) IRL540 N-channel Power MOSFET (100V, 20A, Vgs(th)2Vmax, easy parallel, on chip flyback) Datasheet - Vishay Siliconix

(11) Configuring Netgear Ethernet Switch using Raspberry Pi - Rpi SE 2020mar15

(12) Netgear Gigabit Ethernet Smart Managed Plus Switches Catalog

(13) NetGear GS105E — 5 Port Gigabit Ethernet Smart Managed Plus Switch

(14) Rules and guidelines for drawing good schematics - Community Wiki, EE SE, 2012

(15) Automotive reverse polarity protection - TI 2017apr17

(16) Fundamentals of Power System ORing - 2007 Martin Patoka, TI, 2007mar21

/ to continue, ...


Appendix A - Manual / Auto switchable LM2956 / LM2941 PSU

(Ref) Rpi freezes every now and then, how to fix it with a watchdog? - 2019jun14

switchable psu

Appendix B - Ethernet Switch Power

ethernet switch power

Appendix C - Oring Three PSUs at High Side (5V PSU Example)


(1) The OP is ORing three 12V PSUs using P-type power MOSFETs. This preliminary design is using N-type power MOSFETs (with built in flash back diode) to OR three 5V PSUs.

(2) The three inputs will be used to switch on/off the PSUs. For now, 7V (5V + Vgs(th) = 2V max) will switch on, 5V will switch off (need to shift down to perhaps 3V/0V)

oring psus

Appendix D - Oring Three PSUs at Low Side (5V PSU Example)

oring psu low side

/ to continue, ...

End of answer

  • 1
    \$\begingroup\$ all 3 supplies do not always need to be on - he is wanting one or more supplied to power the load. \$\endgroup\$
    – Russell McMahon
    Commented Jul 29, 2020 at 12:44
  • \$\begingroup\$ @Russell McMahon, Ah perhaps I misunderstood what he said below: "@tlfong01 ... PSU's are up 99.9% of the time but this is remote location so if one were to fail it is bigger problem and device could be down for long period. Other devices at the location already use all three power supplies for redundancy but this device only has one power input. ..." – CA_Tallguy yesterday. CA_T seems to say that there are two locations, one location has 3 PSUs, but another location has only one PSU. Perhaps I should ask him to draw us a schematic to clarify. \$\endgroup\$
    – tlfong01
    Commented Jul 29, 2020 at 12:54
  • 1
    \$\begingroup\$ To clarify sources, I am tapping into another device with 3 different +12v rails and 3 corresponding grounds. I assume grounds tie together. These three rails power three sub-devices BUT go up and down with them. I am tapping them to feed two other switches that CAN take all 3 supplies. Then I have Netgear switch that only has single input. All work fine right now keeping them isolated but Netgear device can take only one supply. It seems essential that I do not back feed anything because I don't know if it will damage anything or cause malfunctions. \$\endgroup\$
    – CA_Tallguy
    Commented Jul 29, 2020 at 20:01
  • 1
    \$\begingroup\$ I tested 1N5822 Schottky diode today and I was surprised to see a few volts on reverse. This is why I was looking to switch instead of combine with simple diodes.... because I do not know enough to prevent this without using electromagnetic relays. I have no fear about tapping into the other circuits for power sources if I know there will be no backfeed. I will look at data sheet and check 1N5408 next. \$\endgroup\$
    – CA_Tallguy
    Commented Jul 29, 2020 at 20:19
  • 1
    \$\begingroup\$ This video from YouTube influenced me to use MOSFETS for reverse protection youtu.be/IrB-FPcv1Dc -- it seems that by switching I can have a lot more peace of mind and perhaps efficiency by perhaps not even using diodes on the switched circuits... and then just use them on the LOGIC portion. Then the risk seems mostly in if there is any possible "make before break" due to switching time where "bad things" might happen instantaneously? \$\endgroup\$
    – CA_Tallguy
    Commented Jul 29, 2020 at 20:51

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.)

  • \$\begingroup\$ 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. \$\endgroup\$
    – Russell McMahon
    Commented Jul 28, 2020 at 3:41
  • \$\begingroup\$ @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. \$\endgroup\$
    – DKNguyen
    Commented Jul 28, 2020 at 4:19
  • \$\begingroup\$ @RussellMcMahon I am defaulting to switching/staging supplies to avoid any unexpected problems... for example perhaps they could start fighting with each other doing their regulation. I don't know enough about them or electronics so figure it is safer to switch than to try to combine. \$\endgroup\$
    – CA_Tallguy
    Commented Jul 28, 2020 at 6:30
  • \$\begingroup\$ @DKNguyen I'm a novice with this and with CircuitLab so learning. Thanks for the tips. If I use your method, will circuit simulation still work in CircuitLab? I believe my drawing is exactly matching my breadboard and the CircuitLab solver/simulator seems to work accordingly. I thought CircuitLab's system requires use of their power source item(s) to funcion? \$\endgroup\$
    – CA_Tallguy
    Commented Jul 28, 2020 at 6:37
  • \$\begingroup\$ By the way, that post is a must read for drawing good schematics : electronics.stackexchange.com/questions/28251/… \$\endgroup\$
    – Blup1980
    Commented Jul 28, 2020 at 7:09

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