Power distribution to a network of ESP32 dev boards

[Swake]

We have these ESP32-WROVER based boards distributed all over the building and now they need to be powered from a central power source.

CAT6 wire is present at each board and goes to a central patch panel. Longest run is about 30m away from the central patch. Peak consumption is 220mA but the boards are 95% of the time in deep sleep mode consuming only +/- 100µA. There are 18 boards for the moment but that amount will grow. The boards have USB, this means a 5V to 3V3 regulator is locally present too.

So... What power supply would you implement ?

Was thinking about using a laptop power supply (got a couple 19V/90W from Dell) as central power source and using a buck converter locally at each board. Unless there is something more energy efficient.

[mariush]

PoE is an option. You could get an old switch with power over ethernet ports and then you just need a poe to regular dc converter at each PI.

For example a 24 port 100 mbps switch for 32$ , adtran netvanta 1234 poe : https://www.ebay.com/itm/164302111135


If you don't mind reducing your bandwidth to 100 mbps and having non-standard ethernet cables, you could just use 4 of the wires to send DC voltage through the unused pairs.

Basically, do passive poe, see https://en.wikipedia.org/wiki/Power_over_Ethernet#Passive

In a passive PoE system, the injector does not communicate with the powered device to negotiate its voltage or wattage requirements, but merely supplies power at all times. The common 100 Mbit/s passive applications use the pinout of 802.3af mode B (see § Pinouts) – with DC positive on pins 4 and 5 and DC negative on 7 and 8 and data on 1-2 and 3-6.

You'd have to buy passive poe injectors or make some "adapters" to inject the voltage into the unused pairs at the switch / patch panel  and take it out when it comes to the pi ... you know the 100 mbps ethernet only uses 2 pairs, wires 1,2,3 and 6  ... so 2 unused wires can be ground, 2 unused wires can be some voltage...   

You can figure out the voltage drop over 30 meters easily.. for example you know AWG24 wires typical in ethernet cables have around 85 ohm per km resistance, and you use two in parallel so the resistance will be half, or around 50 ohm per km,  or 0.05 ohm per meter, and you'll have 60m of wire (to the device and back)
Assuming you'll do 19v input and the device consumes 100 mA  (you'll use a dc-dc converter to convert 19v to 5v so your peak 220mA is basically less than 100mA peak on higher voltage)  ...
So 19v in, 100mA , 60 m of wire at 0.05 ohm  ... v = i x r = 0.1 x 60m x 0.05 = 0.3v drop  ... instead of 19v you'd have 18.7v

Here's an adapter which takes power from passive poe (as explained above) jack, 18v..60v input , and exposes a standard 100mbps ethernet and a barrel jack with 12v at up to 1.2A : https://www.ebay.com/itm/202914660285

There's also  kits of the injector and the splitter  : https://www.ebay.com/itm/121100493562

Another example of set of injector and splitter, 5 pack for $13 : https://www.ebay.com/itm/282049940669

[Swake]

Thank you both for your extensive tips.
A fuse; yes of course, you're totally right this needs fuses. And I'm embarrassed to admit that I would have forgotten this without you mentioning it.

Local USB power supply would be easy indeed but is not possible as there are no sockets available. Btw, there are good ones out there but I found some of these things consume over 2 Watt with nothing connected. Very ugly.

The CAT6 cable for the sensor nodes is not used to transmit data, it is not part of the 'normal' Ethernet network so all wiring options are still available. Will probably go the easy route and send power in a passive PoE like way and stick with the PoE 802.3af using wires 4/5 (=blue) for positive and 7/8 (=brown) for negative. Active PoE is nice stuff but does not really bring advantages in this case.

There are plenty of small cheap Chinesium buck converters available, I can't make them for that price. Will order a couple different ones and measure efficiency at very low load. Doing that analysis is a good excuse to ask for a DC-Load, inst it :-).

Found 2 reels of the exact same cable that is installed. Old building is Cat6 AWG26 seemingly CCA. Will have to give that some attention and check the losses in the cable. Active PoE for is working just fine for IP-cameras on the same cable so it should work but better safe than sorry. The newer building extension is equipped with good stuff: all yellow CAT7 S/FTP Ø 0,56mm (=AWG23) pure copper.

It is at initial power-up that the most power will be drawn, all sensor nodes start at the same time. Might split it over several power supplies if the startup current is too high. Will need some way to monitor that as the system grows.

The idea for a local cap to 'buffer' some energy is a good tip too. it is basically a micro UPS. Will test it out.