May 4, 2020 at 7:29 am #65631
I have two OSPi boards (one is 1.42, the other is 1.43 or 1.43+, I didn’t check closely). Both of them have been working fine with the 24VAC transformers that were supplied with them.
I run other software on the Pis these boards are plugged into, and as a result I want to upgrade them to 3B+ boards for better performance. I’ve run into the well-known problem that the OSPi board’s 5V power supply is insufficient for these boards; they often crash if I push the CPUs hard. The boards work fine off of 5V power supplies independent of the OSPi boards, so I’m pretty sure this the ‘insufficient current’ issue Ray has mentioned on the forum multiple times.
I removed the PTC fuses from both boards (on the newer one it was actually a zerohm resistor, not a fuse), and on the 1.42 board I also removed the adjacent diode and resistor (which were not populated on the 1.43 board), so that the feed back from the Pi’s power supply is not affected by them. I’ve powered up both boards using normal 5V USB power supplies and they boot fine, and the OSPi board LED comes on, so I’m fairly certain all the power connections are in order.
However, after reviewing the schematic for the 1.43 board, I see that the GND connection from the 24VAC terminal is connected to the single ground net on the board, it’s not isolated from the Pi’s GND net. I’m concerned now that if I plug in a 24VAC power supply and the ‘GND’ lead from that supply is not actually at ground potential, there will be a current flow between that transformer and the USB power supply which is providing its own GND.
Is powering the OSPi board in a ‘split’ fashion like this actually safe to do?May 4, 2020 at 7:35 am #65661
I think you may be overthinking. Note that 24VAC transformer is always isolated from the mains — the two wires coming out of the transformer, neither of them has galvanic connection to the mains, or the earth ground. So the ‘GND’ on OSPi is just a logic ground, it has no connection whatsoever with the mains or the earth ground. If you are worried that the USB’s ground is earth ground, that’s totally fine, all that this means is it’s going to tie OSPi’s GND to earth ground, but there is nothing wrong about that.
When you have two circuits (such as OSPi and RPi), it’s typical that the ground of both circuits are shared — this is important for all the logic level signals to work. If their grounds are not shared, the voltages would be referring to different reference points, and the two circuits cannot work.
You said “‘GND’ lead from that supply is not actually at ground potential” — I don’t know what you mean by ‘ground potential’ — did you mean logic ground, or earth ground?May 4, 2020 at 8:38 am #65671
You are certainly right, out of an abundance of caution (to avoid frying the boards!) I’m being extra-careful 🙂
Yes, I’m referring to ‘earth ground’, and now I understand that there shouldn’t be any expectation that either of the leads from the transformer would be equivalent to ‘earth ground’; it’s why some transformers have three pins and connect the *actual* earth ground to a third lead, used for safety ground and not logic ground.
To make things more fun, I’ve decided to power the Pi boards using the PoE ‘hat’ boards from Element14 (same as the ones from the Raspberry Pi Foundation), so the ‘other’ power supply doesn’t have an earth ground either; it is also fed from transformer taps, which pull power from the Ethernet cable. Since that technology is also designed to provide isolation, there’s no connection to an actual earth ground (or even a shared logic ground with the network switch) present.
I think this means I’m all good to proceed with the plan. Thanks for the helpful response!June 15, 2020 at 7:51 am #66815
In case anyone had lingering doubts, this configuration works just fine. I now have remote power control of the Pis running OpenSprinkler, and no more issues with the Pis rebooting because of an inadequate 5+ power supply.July 13, 2020 at 6:20 am #67294
I did something similar. I damaged the 5V supply IC on the OSPi due to an error connecting things. I also wanted a way to power the Pi during short power interruptions to prevent SD card corruption (had it happen several times). So I removed the power supply IC and fed the Pi 5V from a small 12V to 5V converter and fed 12V through the network cable from my wiring closed. 12V supply is powered by a UPS. Was able to find off the shelf parts – wall wart for 12V, power injectors for the ethenet cable, and 12V to 5V regulator. Works great.
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