LaxSlash
Posts: 1
Joined: Sat Jun 03, 2017 7:48 pm

DIY Power supply units

Sat Jun 03, 2017 8:11 pm

Hi all,

I'm taking on a project of making a Raspberry Pi cluster using the Pi 3 Model B.

Doing some research online, I've come to conclusions so far:
- Each pi should have 2.5A supplied to it to allow for the usage of peripherals (Ie, a LCD display through USB and extra indicator LEDs) and other connections (the individual pi cases I'm planning on getting/making/using will make use of either fans or heatsinks, but would like the potential to use fans).

- The best way to do this is with the 5V Rail on a computer power supply

Now, past this... information I'm finding online seems to be largely out of date, and for the Pi 2 or earlier.

Any specific PSU that's recommended for doing this? Or rather, a specific wattage? Keep in mind that I'm also planning on attaching two large fans to it for the overall cluster case. The idea is I'm starting out small (currently using just 4 Pis), but plan on building it to be much larger. I want to be able to split everything between multiple PSUs in order to distribute my power supply/eliminate single points of failure once I have enough pis in the cluster to seriously consider/plan that... but for now, one PSU should do for as small as this project is right now.

As to whether I want the ability to power 16 per PSU or 8 per PSU, I guess that's where I'm looking for some input/suggestions. What do people typically do/what's possible to do with the full 2.5 amp current? I'd like to have each USB port be switchable on/off if possible, as well. Can someone recommend where I can get parts for this/parts that they have used for projects like this and where to get them?

(This is where still having RadioShack would be nice..... lol)

Thanks.

IanS
Posts: 229
Joined: Wed Jun 20, 2012 2:51 pm
Location: Southampton, England

Re: DIY Power supply units

Thu Jun 08, 2017 1:23 pm

There are a number of articles around on how to convert an unused PC power supply into a bench power supply, such as http://www.electronics-tutorials.ws/blo ... upply.html. One of the main problems, especially with newer PSUs is that they need a certain minimal load in order to keep going.
You also need to allow for differing voltages having different current limits. Do not think: 500W PSU = 5V * 100A = 40 RPis. I happen to have a 700W spare PSU to hand, and the limits are 3.3V@35A, 5V@45A. 12V@28A, -12V@0.8A, 5V standby @2A.

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stefanv
Posts: 36
Joined: Wed Oct 19, 2016 12:08 pm
Location: Ontario, Canada
Contact: Website

Re: DIY Power supply units

Thu Jun 08, 2017 4:21 pm

If you have enough Pi 3s connected to the power supply, it will meet the minimum load requirements, so you won't need to do anything special. As for how many you can run from one PSU, you need to look at the current limit of the PSU's 5V output, and divide that by the per-Pi current.

In my experience, I've never had a Pi 3, _with_ attached Official Touchscreen, go much over 1A, so 1.5A per Pi should be more than sufficient (and probably 1A is sufficient without a screen attached), especially since you're not likely to attach a lot of peripherals to all of them. IMHO, the 2.5A PSU suggestion is to allow for peripherals, and to ensure that the supply is still providing >=5V under real-world maximum loads (e.g. a 5V 1A wall wart might drop to 4.9V at 1A, which a Pi would not like). A good computer PSU should stay at 5V all the way up to its stated limit.

So, if you have a PSU like the one IanS wrote about above, you could probably run 30 to 40 Pi 3s from it.
Stefan Vorkoetter: Programmer, hobbyist, amateur watchmaker, pilot, and collector of fountain pens, slide rules, calculators, and watches.

IanS
Posts: 229
Joined: Wed Jun 20, 2012 2:51 pm
Location: Southampton, England

Re: DIY Power supply units

Fri Jun 09, 2017 9:32 am

Multiple RPis running would provide a base load, although with a modern PSU, 'multiple' might need to be 'lots'.
Obviously, different manufacturers will have different requirements, but assuming the article I referenced before is typical:
The newer type ATX12V PSU’s are a little more tricky to convert as they use a ‘soft’ power switch function and require a much larger external load resistance. To get them to start-up, or switch-ON, the supply must be loaded to at least 20W or 10% of the rated power for the larger 600W+ PSU’s. Anything below this the power supply may run, but regulation will be very poor less than 50%.
20W of raspberry power, if they do not have screens or power hungry USB connected peripherals, is probably four boards. If you use a large PSU and need a 10% load, then obviously you will need a considerable size cluster just to keep the PSU awake.
Start-up and shut-down are also issues to consider. If there is a ramp up/down in the number of active boards, then you will have a lower load at each end.
When you say you want to switch the USB ports, do you mean the connected peripherals or the power to the RPi? Something like https://www.amazon.co.uk/d/Cables/charg ... 00F9Q6TNW/ will do the latter, and you can get similar leads that have male/female standard USB connectors at the ends that can then go in-line with a peripheral connection and just cut the power to the peripheral. (I would guess from your reference to RadioShack that you are more likely US than UK, since RadioShack were never common in the UK, but I am sure amazon.com has something similar.)

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