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I am aiming to power 2 Raspberry Pis via their GPIO pins. As there is no protection on GPIO, I would like to add one on my PCB (that will deliver the 5 V thanks to a TRACO, which can deliver 6 A maximum).

I looked on the internet how to do it, but I am a bit lost in the dimensioning part. I am following this tutorial, and I don't know how to deal with point 2 in section 4 in my case:

Calculate the load current that is linked across the circuit of overvoltage safety. This is 50mA for our example discussed above. Other than the load current, biasing current is required by Zener diodes. Therefore, the total current, plus the Zener diode biasing current, should be equal to the load current. For the above-mentioned example, it can be total current=50mA+10mA=60mA.

How can I know the load current of my RPi? Should I use the maximum (2*3 A)? Here is the diagram of what I planning to do.

Power circuit with protection

Thanks in advance.

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    \$\begingroup\$ power 2 Raspberry Pis via their GPIO pins Why would you use the GPIO pins for powering the devices? The GPIO pins are not designed for that. I do hope you mean that you will be using the GPIO connector and power the devices via the VDD and VSS pins on that connector. \$\endgroup\$ Jun 14, 2021 at 6:47
  • \$\begingroup\$ You should be adding a series resistor like you added R1, the current consumption of an RPi varies a lot so you'd need to use a low value resistor but then you'd need a very high power zener diode. You would do much better by replacing R1 with a fuse so that when the input voltage gets too high, the fuse will blow. Suffice to say that you should be using a good quality 5V power supply and then no protection is needed. \$\endgroup\$ Jun 14, 2021 at 6:51
  • \$\begingroup\$ Sorry, yeah through misuse of language I called them GPIO. Yeah, I am planning to use the VDD and VSS of the GPIO connector, the +5 V and GND pins. \$\endgroup\$ Jun 14, 2021 at 6:53
  • \$\begingroup\$ Oh, I didn't think about a fuse instead of resistance! I read that TVS diode can absorb more power than Zener Diode, should I use that instead? I don't know how the TRACO is good to avoid additional protection. \$\endgroup\$ Jun 14, 2021 at 6:55
  • \$\begingroup\$ I'll check the TRACO datasheet \$\endgroup\$ Jun 14, 2021 at 6:56

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I did it several years ago, using a Traco to power a Pi via the GPIO connector. There is no overvoltage protection, but I used a pair of power schottky diodes to protect against reverse polarity, like this:

schematic

simulate this circuit – Schematic created using CircuitLab

In fact this was a nicety, as with a PCB it's pretty hard to reverse power. If I did it again I'd probably use a MOSFET to protect against reverse. But anyway that design has been in use for several years and survived multiple power cycles without a single RPi failure. The Traco seems to be quite well behaved when used like this (which you would expect it to be, given its intended function).

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  • \$\begingroup\$ One of my RPi will be powered directly from the PCB's track (and a full GPIO connector), so I guess I can avoid reversing current protection. For the second RPi, which is connected with only 2 cables, so I added MOSFET protection before the connector. With the TRACO you never used overvoltage protection then? So maybe I can avoid it, and just using a fuse. \$\endgroup\$ Jun 14, 2021 at 8:37
  • \$\begingroup\$ no, as stated above, no overvoltage. If in doubt, load the Traco you intend to use, and check output with a storage scope. I maye well have done this but it is long ago. Obviously if you intend to use long cables where ringing might be an issue, it's different, and there you would want protection on the Pi itself, implying some kind of board mod, which isn't easy. In my case, there was only about 10cm of PCB track, and I had absolutely no issues. \$\endgroup\$
    – danmcb
    Jun 14, 2021 at 8:41
  • \$\begingroup\$ In any case I'd avoid using a zener as they just aren't accurate enough and the V/I graph has too soft a knee for anything other than the crudest form of OV protection. Also speed might be an issue. You'd have difficulty blocking transients of say 5.7V without the zener drawing significant current at 5.5V. \$\endgroup\$
    – danmcb
    Jun 14, 2021 at 8:44
  • \$\begingroup\$ @DarkPatate by the way, if this answers your question, please mark it as such. \$\endgroup\$
    – danmcb
    Jun 14, 2021 at 8:58
  • \$\begingroup\$ In the case of a long cable, just the decoupling capacitor could be useful for the ringing, right? But in my case, the cable would be short (the PCB track as well). \$\endgroup\$ Jun 14, 2021 at 9:02

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