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I am working on a small Raspberry Pi cluster that I am going to use for web hosting, home automation, cloud storage, VPN, and so on. I plan on using the 3D printer and laser cutter/engraver at my high school to make a small "server rack" for the cluster using rather thin plexiglass, and PBA plastics. Instead of using up many wall outlets or using a power splitter, I would like to develop a small PCB that has one wall plug, and splits in to 4 USB female ports, and one or two 3 Pin computer fan ports. What I'm not so sure about, is can I just split the 5V input to 5V and GND wires on the USB ports or is there other components that need to be integrated such as resistors, ICs, capacitors an such? I have seen many boards like what I'm looking for, but they all seem to be complicated which leads me to believe it cant be as simple as I think this is.

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  • \$\begingroup\$ It is not as simple as that if your question title is accurate and you're using 5V AC instead of 5V DC. \$\endgroup\$
    – Samuel
    Apr 3, 2014 at 0:28
  • \$\begingroup\$ Well, what I meant is plugging a cord in to a wall and plugging it in to the PCB i wish to create, I believe the cord converts to DC correct? \$\endgroup\$
    – SuperAdmin
    Apr 3, 2014 at 0:31
  • \$\begingroup\$ A cord does not convert AC to DC. If you're using the word cord to reference an entire power adapter, then perhaps it converts it. \$\endgroup\$
    – Samuel
    Apr 3, 2014 at 0:36
  • \$\begingroup\$ you should change the title of the question to '5V "wallwart" supply to 4 USBs and 3Pin fans' or something similar. The wallwart converts 110 - 240V AC into 5V DC. also you will find it hard to find a 5V DC wall pack that gives you the current output required to run 4+ Raspberry Pis at once, including fans. Also, your fans will probably be 6-12V DC fans, meaning they wont turn very fast at 5V. Each Raspberry Pi uses 2-3 Watts, as far as i remember. you will need to look for a ~3 Amp output 5V DC wall pack. \$\endgroup\$
    – KyranF
    Apr 3, 2014 at 2:08
  • \$\begingroup\$ while designing your PCB, look around online for DC fan control circuits - unless you are speed controlling the fans, you wont need 3-pin ones either (that us usually a tachometer for a closed loop speed -> voltage control). Basically you should put protection diodes in parallel with your fan power and ground pins, to protect your other circuitry on the 5V rail from negative spikes etc. You could use the third pin on your fans to connect to each R-pi, and write a little program to count RPM and tell you fan speed as a side project. \$\endgroup\$
    – KyranF
    Apr 3, 2014 at 2:21

2 Answers 2

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It can be as simple as that, but there are various enhancements you might consider, such as:

  • decoupling each load with ferrite beads and ceramic capacitors
  • providing overcurrent protection for each load with either polyfuses or active devices such as the LTC4210
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  • \$\begingroup\$ As I am only a high school student with an introductory knowledge of digital electronics through RIT, I don't quite understand the benefits decoupling with ferrite beads and ceramic capacitors. \$\endgroup\$
    – SuperAdmin
    Apr 2, 2014 at 23:27
  • \$\begingroup\$ The LTC4210 from Digikey is like $5 each.. would that be used to control the load of all Raspberry Pis or one hotswap controller for each? Pretty expensive option. \$\endgroup\$
    – KyranF
    Apr 3, 2014 at 2:07
  • \$\begingroup\$ @iOSNoob: You really need to fill in your profile so that answers can be tailored to your level of understanding. Otherwise, the rest of us start to feel as though we're wasting our time. \$\endgroup\$
    – Dave Tweed
    Apr 3, 2014 at 2:10
  • \$\begingroup\$ @KyranF: I just threw that in there because I used it on a power-distribution board in a previous project, and found it very useful. It provides a logic-level control input, soft start and precision overcurrent protection with low overhead (50 mV). It's probably the far end of the range of things the OP might want to do. \$\endgroup\$
    – Dave Tweed
    Apr 3, 2014 at 2:13
  • \$\begingroup\$ @DaveTweed yeah it seems like a capable and robust IC. out of interest, do you favour Texas Instruments power management ICs or Linear Technology ones? \$\endgroup\$
    – KyranF
    Apr 3, 2014 at 2:15
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First, have you noted that multi-USB power supplies exist as a product? Search for "USB power center" on Amazon for example.

That won't serve your fans though. Common fans run off 12V. The third wire is for speed regulation, which you could omit, (run them at full speed), or add a speed controller, or control them from one of the Pis.

If you want a more condensed solution, get a power supply unit from one of the electronics suppliers which provides the voltages at the currents you need. Such supplies are commonly available with both +5V and +12V in one unit.

Apparently the power requirement for Pi is 0.7 to 1.0A, which means your power supply needs to be built to provide at least 4A, better 5A or more. Wallwarts are generally not capable of that.

In addition, we want the voltage regulator in the supply (wallwart or power supply) to regulate based on an accurate measure of the voltage the load is actually receiving. For that reason, we don't want to interpose more wire length and connectors than necessary, as those introduce more resistance, and thus difference between the voltage the power supply is providing, and the voltage the load receives.

For that reason, having a power supply unit built in to your rack, with short cables to your Pis, would be more reliable than a wall wart --> USB sockets --> USB cables.

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  • \$\begingroup\$ @gwidman Thank you for you help. It would be my hopes that I could use the PWM pin to control the fan with the pi. As it seems that a wall wart is out of the question, I was hoping you would know of some extremely small power supply options I could consider. As I plan to make a case out of plexiglass, and about the size of 4 pis, I would hate to have to use a full sized ATX power supply \$\endgroup\$
    – SuperAdmin
    Apr 5, 2014 at 0:14

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