Build your own dumb USB power strip

diy-usb-power-strip

Here’s a USB charging center which [Kenneth Finnegan] built using parts from his junk bin. We’d like to reiterate our claim that he must have the most magical of junk bins (the last thing we saw him pull out of it was a 24-port managed Ethernet switch).

The jack on the side accepts the barrel connector from a 12V wall wart. [Kenneth] mentions that the 2.1mm jack is a standard he uses in all of his projects. Inside there’s a switch mode power supply that provides the regulated 5V to each USB port. We really like the fact that he added some protection; diy is no fun if you end up frying your beloved multi-hundred dollar devices. The yellow components are polyfuses which will cut the power if 600 mA of current is exceeded. This works great for almost all of his devices, but his iPod 4G doesn’t like the system. It sees the voltage dip just a bit and stops charging entirely.

Getting around the Raspi’s USB Host current limit

For being such a revolutionary device, there are still a few problems with the Raspberry Pi. For one, the USB host ports are only able to source 140 mA per port, while the USB ports on your desktop, laptop, and even tablet are able to send a full 500 mA per port.

The official ‘fix’ for this problem is to use a powered USB port for any device that requires more than 140 mA,  something that didn’t sit well with [Manis]. He came up with an easy fix , though, that only requires a few bits of wire and a soldering iron.

The USB ports on the Raspi are current limited to 140 mA by a pair of polyfuses. [Manis] bridged these fuses, effectively taking them out of the circuit with a short length of wire. This allowed him to use a USB hard drive (powered by USB, of course) with the Raspi.

There’s one small problem with sending that much current through the Raspi’s USB port. Sometimes, when the high-power USB device is powered on, the voltage will sag, resetting the SoC and rebooting the system. [Manis] did his homework and discovered USB 2.0-spec ports should use a 120 μF, low-ESR capacitor to prevent this. The Raspi comes stock with a 47 μF cap used for this purpose. Replacing this cap (C32) might be a good idea if you’re planning on using high-power devices with your Raspi.

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