Available from various suppliers on the internet at a givaway price is a real gem of a 5 volt regulator. At first glance this little PCB doesn't look anything special, but it's actually quite impressive. Conveniently for the voltage input (9V to 40V claimed) there's a choice of a two way terminal block or a standard 2.1mm DC socket. For the 5 volts output there's another two way terminal block or a type 'A' USB socket. My only negative comment would be that it could do with some mounting holes. I don't know what the official name is for this board, but it always appears as in the image shown below.

This solar charger project came about because I had in my garden shed an old 40 watt solar panel just sitting there doing nothing. Also in the shed was a nice little electronics enclosure begging to be put to good use. I was already using one of these regulator PCBs indoors as part of my normal 12 volt solar  power system to charge my phone and other USB gadgets, and as they came as a pack of two I had a spare. So I thought why not throw all these parts together to make a self contained (sort of) solar powered gadget charger.

This regulator is of the 'buck' variety which makes it very efficient compared to linear types. There are other buck regulators out there which are much smaller and feature a simpler circuit design but they do get rather hot. This one can get a bit warm too, but reassuringly not as much. The main regulating IC is a CX8571 that drives two output MOSFETS. The board also has another IC, the RZC7512 which is a dedicated charging port controller. This automatically enables connected devices to know how much power they are able to draw, and will stop your phone telling you off for using an incompatible or faulty charging cable! Although the RZC5712 is a two channel device, only one channel is used here (pins 3 & 4) as there is only one USB socket. Some boards may use a CX2901A instead of an RZC7512 but their datasheets suggest they are effectively the same IC.

An important factor for me was that this buck regulator behaved itself if used with a solar panel. I didn't want it to produce any voltages below 5V if, for example, there was a cloudy sky. I wanted it to be either completely 'ON' supplying 5 volts or completeley 'OFF'. I did not want a possible in-between state that might confuse any connected devices. A mobile phone will tolerate the occasional interupted charge, especially so if it's not actually going to be used, but I don't think it would appreciate something like 4 volts or less! This though behaves perfectly, producing 5 volts at the output when the voltage on the input rises above 7.5V and going back to 0V when the input drops below 7.1V, with no undetermined state. A bit of welcomed hysteresis.