About six months ago a Chinese company on eBay had a special offer selling the item pictured below for US$2 each. We bought five and put them somewhere safe and then forgot what they were for.
With a bit of searching around we finally found the details for this product. It is an high current voltage regulator which takes an input voltage of from 12-23 VDC and outputs 9 VDC. No heat sinking is required up to a constant 2.5 Amps of output current, and they are rated to supply brief peaks of up to 4 Amps.
The pin connections (the three legs) have 2.54 mm (0.1 inch) pitch exactly the same as for the L7809 regulator, and the device is also of similar overall dimensions. Therefore, this little PCB can be used to substitute an L7809 (pictured below) where more current than the 1-1.5 Amps (with heat sink) maximum of the L7809 is required.
It could for example also be used instead of the LM317T we use in our standard 9V from 12V regulator (though we have designed that with R1 = 330, R2 = 2K8 so that we get an output of around 8.8V compared to the 9.3V this regulator module outputs – a lower voltage to reduce power consumption for renewable energy powered applications). With thicker cables for the connections this would give a 2.5 Amp constant current 9V from 12V regulator with no need for heat sinking.
This module has a small integrated circuit labelled MP2307DN which a quick search on Google shows up as a 3A constant load current DC-DC step-down power supply module and a monolithic synchronous buck regulator, which has been used in this case to provide a 9V output.
At the time of writing we can only find this device here: L7809 Alternative Module priced at US$4.99 plus US$1.99 for air mail delivery. (The same vendor also has similar L7812 and L7833 alternative modules for 12V and 3.3V outputs respectively.)