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So I'm finding it a little difficult to figure out how I should be powering my setup. I'll have Waveshares IO board here: https://www.amazon.com/gp/product/B08VJBGD1Q/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1 along with the Compute Module 4 Lite (8GB RAM) and I'd like to have the capability of fully powering one of those PoE ports and at least 1 USB port.... plus the Pi of course. So assuming both the PoE and USB device are drawing power at the max, what does that result in? I started reading about voltage, amperage etc and realized I don't want to blow anything up (Pi 4s are rare and very expensive these days) because of my misunderstanding so I tried looking up some power supply calculators but all seem to be for PC desktops. I'm pretty confident that this: https://www.waveshare.com/ord-psu-12v2a-5.5-2.1.htm is enough, but I wanted to check with you Pi folks first. That would provide enough power, right? Please feel free to link any good calculator sites out there as well, already got a nice collection for my new Pi 4 rolling :)

2 Answers 2

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Warning The Raspberry Pi 4 DOES NOT requires a 3.0A minimum - that is a recommendation.

That will not work. The Raspberry Pi 4 requires a 3.0A minimum. Start by adding the worse case current for each or the other hardware devices then add 20% now add the 3A minimum the Pi requires, this gives you a minimum requirement for your power supply. Take into account the loads you add to the board. Your link does not give any information for current or wattage for the Waveshares IO board. Be careful that you use the correct voltages.

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  • It says "POWER INPUT--7V~36V" on the waveshare amazon web page. So I add all the amps together... the Pi is 3, the USB is 2.5, and the PoE is also 2.5. I need an 8A 7V power supply with a dc jack, right? I read that wattage is volts times amps, so I should be on the look out for a 56w power supply. I think 65 is pretty common, so I'll grab one of those universal ones and just make sure it meets the minimum amperage + volt needs, right?
    – Riptide
    Commented Jan 18, 2022 at 10:16
  • Ok I can't find anything over 3.5a on amazon, I must be missing something. I'm just going to get some sleep and worry about it tomorrow.
    – Riptide
    Commented Jan 18, 2022 at 10:25
  • Sorry to inform you Amazon does not have everything! ebay, Newark, and others have them. Going over on the amps is ok.
    – Gil
    Commented Jan 19, 2022 at 0:40
  • I'm A-OK going somewhere else, but most of my search results bring me to a shady 3rd party off-shore website with poor wording and limited random stock. I guess it would help if I knew exactly what I was looking for. You said I should be careful about what voltage I use, but the Waveshare link I provided in the OP says a range of acceptable voltage values... this leads me to believe that I'm misunderstanding you or them. And the only way to find out what works is start buying things I think would work and probably end up with an exploding Pi haha. Can you give me an example of what would work?
    – Riptide
    Commented Jan 19, 2022 at 10:35
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There are different voltage rails (supply circuits) in this system. On a board like this, switch-mode regulators are typically used, which has almost the same power in as power out (often around 90% efficient). In other words, the current into the regulator and current out to the circuit it powers are not the same, but the power in Watts is close.

To work out the total DC supply current, first we need to work out the power (Watts) of each section. This is Voltage times Current.

The Pi Compute Module 4 has a typical power draw of 5V x 1.4A = 7W. Let's round that up to 10W to cover high processing loads.

The PoE section of the IO board mentions IEEE 802.3af, which can supply up to 15.4W.

There are 4 USB 3.0 ports on the IO board. Typical peak current I've seen for USB 3.0 (e.g. phone charging) is 3A. I'll say two ports at full draw just for some extra power instead of just one. So 2 x 5V x 3A = 30W.

The IO board is powered through a switch-mode regulator which converts 7-36V to the 5V for the Compute Module and USB (and the PoE supply voltage). Assuming 90% efficiency for the switch-mode after adding the powers above: (10+15.4+30)/0.9 = 61.6W PSU power.

At a minimum I'd say you want a power supply that can give 65W, but more would be better. As the IO board specs says, the voltage can be anywhere between 7V and 36V; 12V is fine. For a 12V supply, it should handle a current draw of 65W/12V = 5.4A (or more).

You might also be able to use an old laptop charger. Some chargers run at 19V which the IO board can take, and come in 65W, 90W, etc. You'll have to check the DC plug size, since there are lots of different sizes out there.

Note: the official Compute IO board says the PCIe socket and the fan are directly powered from the DC barrel jack. If the Waveshare board is the same and you use a 19V laptop charger then you can't use PCIe or a 12V fan.

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