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My Raspberry Pi 4 project will be powered by a single official Pi power supply with a USB C plug. I want to provide power sockets on both the left and right side of the housing, but this is for convenience and cable management only. Only one input is to be used at a time.

I would only plug in one power supply, but I can't vouch for what other people might do so it needs to be safe. I'm assuming that allowing two power connections at the same time would not be safe?

I was hoping to find some sort of reverse power splitter, but I'm not seeing anything.

My electrical knowledge is sparse, but I am quite adept with a soldering iron. (Worrying I know).

Any suggestions as to how I might do this safely with minimum bulk?

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  • \$\begingroup\$ Put a warning label on it. \$\endgroup\$
    – Andy aka
    Jan 17, 2022 at 12:16
  • \$\begingroup\$ :-) That would work for most people, but there's always one! \$\endgroup\$ Jan 17, 2022 at 13:20
  • \$\begingroup\$ Would a mechanical shutter be too much to implement? - it could use a slider to cover one or the other socket. Or some sort of cable route that goes around the case that the cable can slot in to, so it only has one socket? (Similar to the way some keyboards allow you to route the cable.) Or a physical electrical switch. \$\endgroup\$ Jan 17, 2022 at 19:11
  • \$\begingroup\$ Thanks Andrew. I like the idea of a slider. I was planning on having the sockets on either side, but I could put them both at the back. The cable route is probably what I will do with a right-angled USB C plug. This electricity business, being this complicated, I'm not sure it's going to catch on. \$\endgroup\$ Jan 18, 2022 at 13:24

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Maybe a power-path prioritizer is what you might need: -

enter image description here

It uses internal MOSFETs so there is little volt drop. One of two inputs is automatically selected. Please read the data sheets to ensure it meets your voltage and current requirements. There are other types that are more powerful should they be needed.

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  • \$\begingroup\$ Thank you, really appreciate your response. I am so far out of my depth! I shouldn't have bragged about my soldering skills. I'm going to have to lower my sights... I'm thinking a right-angled USB C adapter and a single power input. With USB C being reversible it would allow the cable to come from the most convenient side. Not elegant though. \$\endgroup\$ Jan 17, 2022 at 14:18
  • \$\begingroup\$ @clarke-baxter you can get small blank PCBs that will allow one of these surface mount chips to be fitted and that will make it easier for you. Obviously you'll need to get someone to solder it to the blank PCB first but, once done, the PCB will offer 0.1" spacing on through hole pins making it easier to solder to. \$\endgroup\$
    – Andy aka
    Jan 17, 2022 at 14:23
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You would want to avoid having one power supply feeding back into the other one when both are connected at the same time for some reason. This might happen because of some voltage variations, even with identical supplies.

What you want is called 'supply or-ing' or just combining two supply rails. I take it they dont need to be selectable/switched? For high current this can be difficult requiring mosfet switching and supply monitoring. Since you are asking for 'minimal bulk' this wont work for you.

You can just use an or-ing diode arrangement with a schottky diode like 1N5822 might work for you. It will prevent the supplies from damaging each other. The Pi would be fed with the supply that has the highest voltage. Just beware that the supply will drop by about 0.5V with this arrangement. Lower voltage drop diodes exist but they are not of the standard trough hole variety. Can you work with SMD stuff? I used a nice one a few designs back that could manage 0.35V at 10A.

enter image description here

(Image source: codemsys.com - Use ORing diodes)

That webpage uses it for another end though, trying to share current between the two supplies, not necessarily blocking backflow into the supply. Either way, the circuit will be the same for you.

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  • \$\begingroup\$ Thanks for this. I read something about one power supply sinking current from the other being a bad thing hence my question really. I don't expect two supplies to be connected at the same time, but somebody just might do it. A physical switch would be safe, but not the most elegant. I can't do SMD. Would the addition of a diode cause a voltage drop even when just one supply was connected? Excuse my ignorance. \$\endgroup\$ Jan 17, 2022 at 13:27
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    \$\begingroup\$ What would be really ignorant is assuming that dual-psu plug in will not happen. With safety related issues you must assume all users are complete idiots, so you are thinking ahead ;) A switch selecting either of the two inputs is easiest, but indeed not the nicest solution. Yes unfortunately the voltage drop will always be there, and its depending on the current that passes trough the diode. at low loads it can be 0.3V at high loads this can creep up to 0.4V/0.5V for a good diode. You can test it with the Pi by just placing a schottky diode in series and checking stability \$\endgroup\$
    – Thijs
    Jan 17, 2022 at 14:46
  • \$\begingroup\$ You could use trough-hole P channel mosfets to do some switching, blocking current if there is a voltage present on the other rail. But that is a bit more difficult analog design, and will take up quite a bit of space if all the components are trough-hole. I can draft up such a circuit if you want, but if the diode works for you it will be a lot easier to put together \$\endgroup\$
    – Thijs
    Jan 17, 2022 at 14:49
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Thanks for your answers.

I found the thing!

https://thepihut.com/products/5v-usb-c-dual-supply-dual-ideal-diodes

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