When I take my amateur radio equipment out for portable operating (such as up a mountain), I usually take a large 12 volt battery as my power source. Recently there have been a couple of occasions where I’ve wanted to charge gadgets like my mobile phone but didn’t have a means (nor the will to carry a USB specific power bank). To address this I decided to get myself a small 12 volt to 5 volt USB power supply. Below I will share some photos and information on the solution I ultimately went with.
The solution needed to support Anderson PowerPole connectors on the 12 volt side, as I use these exclusively for all of my amateur radio equipment. After a bit of research I identified four possible options:
- Buy a PowerWerx USBbuddy
- Buy a car cigarette lighter to USB adapter cable from eBay
- Buy a readymade DC step-down PSU from eBay
- Make a DIY DC step-down PSU circuit
The first option was ideal but disregarded as there don’t seem to be any European resellers of this product, making it rather expensive to buy from the USA. I didn’t like the second option as I am not a fan of the ‘cigarette lighter’ power connector. The last option was also out as I don’t have the time right now to research, design and build a custom PCB. So, this left the option of a cheap ready-made DC to DC step down converter from Chinese suppliers on eBay.
There is a wide variety of DC to DC step down converter circuits available on eBay. I went with this 9-36V to 5V 5A buck converter unit, which was a meagre £4.59 including postage. Unlike most of the converters I saw on eBay this one had a metal case and had a high output current rating. The wide input voltage is also handy should I want to operate my portable radio station with LiPo or LiFe batteries.
The power supply unit arrived reasonably quickly from China. The photo below shows how the unit arrived (minus the packaging):
Removing the lid of the metal case revealed a pretty clean looking circuit board, although I did consider the amount of solder used to be a bit frugal.
The buck converter IC is marked “XLSEMI XL4015E1”. XLSEMI is a Chinese IC manufacturer, one which I have never encountered before. The datasheet for this chip can be found here. A close up picture of the IC is shown below:
In order to provide the case with some basic insulation and protection from the elements, I applied glue-backed heatshrink tubing to the unit as shown below. I also soldered PowerPole connectors and some extra wire flex to the included barrel jack pigtail wire. Incidentally, there is some good information on the Anderson PowerPole connector on G0HWC’s website here. Purely as a preventative step, I also wound the input power lead around a clip on ferrite RF choke.
The USB PSU was connected up to a 12 volt SLA battery and SOTABEAMS PowerPole power distribution box for the initial test:
Using a handy inline USB power meter I was able to easily take some measurements from the device. With no (significant) load on the device, the output was read as 5.35 volts. This is notably higher than the 5.00 volts required by USB devices. However the USB specification allows for 5V ±10%, so anything between 4.5V and 5.5V is (in theory at least) fine.
With a 1 amp dummy load attached to the unit the voltage sags slightly to 5.32 volts:
Doubling up to 2 amps of load, the voltage drops to 5.30 volts:
I left the unit running for half an hour without incident. Even with the heatshrink covering much of the case, it did not get warm while supplying the 2 amps to the dummy load. This bodes well for the maximum rating of 5 amps, although I must admit I have not tested the unit to it’s full rated power yet.
From an RF point of view I haven’t had any noticeable interference issues when using the power supply around my radios, something that is always a concern with switch-mode power supplies.
Overall, for less than £5 I’ve been pleased with this little power supply and I’ve already made use of it a few times to charge my phone and GPS when out all day operating portable. If you have ever thought about adding a USB power source to your portable or mobile amateur radio installation, this cheap and cheerful step-down converter is well worth checking out.
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