USB Power For Your 12V Circuits

August 22, 2015 at 11:36 pm | Posted in Uncategorized | Leave a comment
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If your desk is anything like mine you have a pile of USB cables and chargers squirrelled away in a drawer. These are great for powering 5V or 3.3V circuits, especially when paired with one of the USB power sticks that have become so popular. But what about circuits that require a little more voltage?

I run into this problem occasionally and the solution I had in mind was a small step-up regulator that could be paired with the appropriate connector to create a voltage converter that could be plugged directly into a USB port. After looking around for a while I found just the thing I needed being offered by Pololu; a series of step-up regulators with a wide variety of input voltages, output voltages, and current capabilities. I bought a few of the 12V variety to try them out.

The specific one I used was the UV312F12 and the circuit couldn’t be simpler. A type A USB connector at one end supplying 5V from the USB port and a dual pin connector at the other for the 12V output. Two prototypes I put together are shown below. For the first I used an old USB cable from which I removed the plug and attached the 5V and GND lines to the regulator input. As an alternative you could replace the cable with a mini/micro-USB connector and use a USB wall charger.

Voltage Converter Prototype 1

For the second I mounted a type A connector directly to a piece of stripboard with the regulator.

In both cases when I plugged them into a USB port the measured output voltage was exactly 12V. I’ve been using the first to power my WSPR receiver for the last few days with no problems.

One limit of which you need to be aware. The Polulo site lists the regulator’s maximum input current. The available output current will depend on the voltage conversion ratio and conversion losses. So for example, with a USB port providing 100mA at 5V I wouldn’t expect more than about 40mA after converting it to 12V. If you use a USB power stick you’ll probably have more power available. The one I use can supply 5V at up to 2.1A, which is actually more current than the input specification for the regulator I used. With the appropriate regulator you could probably get enough power out for a small QRP transmitter.

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