Fig.1: the USB Power Injector is essentially a switch and a 5V regulator. The Vbus supply from USB socket CON1 turns on transistor Q1 which then turns on Mosfet Q2. This then feeds unregulated DC to REG1 which in turn supplies 5V to the downstream USB socket CON2.

EACH USB SOCKET of a PC or self-powered USB (Universal Serial Bus) hub can supply up to 500mA at 5V DC, which can be used to power many USB peripherals directly. That’s one of the advantages of USB and many of the newer peripherals are designed to be powered in this way.

Many low-cost USB hubs are also designed to take their own power from the PC, via their "upstream" USB cable. That’s fine in most cases, as the hub’s internal circuitry only needs a few tens of milliamps to operate.

However, things start to get a little more complicated if you try to connect a number of bus-powered USB peripherals to your PC via such a hub, because the hub’s "downstream" output sockets can each only supply a maximum of 100mA. That’s because all of their power must ultimately come from the PC itself, of course.

What happens if you have one of these hubs already powering say, three USB powered peripherals and then you buy a USB-powered scanner or label printer that needs to draw more than 100mA? Ah, that is a problem. Luckily it’s easily solved; all you need is the USB Power Injector described here. It’s designed to be connected in series with the USB cable to your new peripheral and also to a 9V AC or DC plugpack.

When it detects 5V DC coming from the PC and/or hub, it switches power from the plugpack through to a built-in 5V regulator, to provide your new peripheral with its own 5V power at up to 500mA.

All of the components used in the USB Power Injector are mounted directly on a very small PC board, which fits snugly inside a small jiffy box.