OK folks, it looks like either the cheap/generic PoE system or the $1 DC-DC converter I’m using is starting to break down. It’s lasted all summer and has powered my Pi3B+ but over the last couple of weeks I’ve started to get the throttling errors again.
Bearing in mind that the equipment up the mast is relatively expensive, I don’t want to risk anything going horribly wrong and letting the magic smoke out.
It’s also time to get the Pi4 up there instead of the 3B+ so I need to do this properly. I have CAT6 cable from the shack to the top of the mast so it makes sense to carry on using PoE.
I’d like to replace my existing PoE injector with a proper 802.3af jobbie and something a bit better to get the voltage into the Pi.
Is the official RPi beefy enough to power the Pi4 and the Airspy Mini? I’ll do the same as I am now and take a direct feed from the GPIO to the bias-tee injector for the LNA so it needs to power that as well. If not, are there any suitable alternatives?
This is the PoE injector I have in mind. Will this do the job at the bottom end?
I have the RPI3B+ POE unit on one RPI. It powers a 3B+ with airspy OK. I haven’t used it on an RPI4 with airspy yet.
I just bought a couple of POE adapters from Amazon but haven’t tried them yet. They are only 2.5A so I am not 100% sure they will work with one or two airspies (One for 1090Mhz and one for 978Mhz).
Hoping to migrate the airspys to RPI4s in the next week or so.
I did see that - I wasn’t planning on using a splitter like that, there seems little point if using a PoE HAT.
See, this is what concerns me just a little. The official PSU for the Pi4 has been upped to 3A rather than the 2.5A of the previous version. I’d have expected a new HAT for the Pi4.
I think the reasoning for that was that USB 2 ports should be able to supply a nominal 500mA, whereas for USB 3 that has increased to 900mA. The consumption of the pi itself is a bit more than a 3b+, but not hugely so.
I haven’t actually made any power consumption measurements, but I can do so if it’s useful. I’m using a pi 4 with airspy mini and RTL-SDR LNA powered by a bias-t.
Had to do a little grinding to shoehorn it inside of the Flirc case, so it’s all nice and compact with passive cooling (silent). I’ll give it a go to see if it will handle the Airspy Mini at full capacity (w/bias-t) as well as an OC’d Pi4 and report back. In the middle of a few other tests ATM, so it will be a couple days most likely. Hope it works since it’s a nicely made compact unit that will fit in most cases as-is.
I do have a pi4. At the moment the v2 Poe hat is on my pi3b+. If I have enough time, I’ll try swapping it to the pi4 tomorrow and check the current draw with the airspy mini.
I’ve made some measurements on my system using an inline USB power meter:
The airspy dongle draws 80mA at idle, and a fairly consistent 300mA when the ads-b decoder is running. The internal bias-t is disabled.
The complete system, consisting of pi 4, airspy mini, bias-t (powering rtl-sdr.com lna via the GPIO pins) and a fan shim draws about 1.2A, with occasional peaks up to about 1.6A.
I would imagine that the PoE hat would be sufficient to power it.
OK, Using the official PoE HAT on a pi4 with an airspy mini, I am seeing peak power draw of about 8.5W from my POE switch.
This corresponds to 1.7A @ 5v
Seems pretty much the same as @caius measured
The added benefit of the hat is that the fan keeps the CPU cool. You can even set the speed at which it activates. I add a low profile heatsink with heatsink adhesive(not tape) just to be sure. I could only put one on the CPU as the Ethernet and memory don’t have enough clearance. The fan should be enough for most people. Most of mine are in my attic so get fairly hot in Summer.
Thanks Jon, I think it’s time mine was built into a new box so I’ll probably do something similar. I had considered a heatsink as well but wasn’t sure if it will fit below the PoE HAT. I wonder if you have any pictures of how yours looks with the heatsink on as well? Also, what heatsink and adhesive did you use?
I have a load of small heatsinks that will probably do the job but will need to grab some thermal adhesive as the tape is rubbish!
