PSU MANUAL

V1.0

The PSU board has the same mechanical outline and fixing centres as the Raspberry Pi 3B. The GPIO port has been omitted.It is intended that this board should be used by stacking the Serial Node board on top of it using 20mm spacers. It may also be used with the Raspberry Pi models to provide battery backup.

The charging connector is type USB-C, the battery connector is JST PH 2.0 and the output connector is a 2-pole 5mm miniature terminal block. Alternatively it is also possible to use a 3.5mm barrel jack socket for the output.

Power is provided by a dual 18650 3.7V lithium cell battery pack. This is connected to a charger module which also provides additional cell protection to that in the battery pack. The output from the charger modules feeds a DC-DC buck-boost converter to provide 5V at up to 600mA. The Enable pin is switched to turn it on and off. There is provision for an output indicator LED. By degfault this is powered from the raw battery voltage into the DC/DC converter, but a solder blob link allows it to be powered from the 5V output.

The charger can supply up to 1A output, so a typical 2400mAh battery pack should fully charge within approximately 2.5hrs from "flat".

The available running time from a fully charged battery at the full 600mA output should also be approximately 2.5hrs, allowing for some inefficiency in the conversion. At a more usable 200mA output the running time should be approximately 7hrs.

For safety reasons it is recommended that the battery is unplugged if the PSU is being transported for any distance or if it is going to be unused for a long period of time.. The umit must not be left unattended while the battery is being charged.


Bill Of Materials

Name	Value	Comment

	PCB	Custom
C1	100n		Ceramic capacitor
K1	JST PH 2.0	2mm battery connector
K2	5mm 2-pole	output terminal block
LD1			3mm high efficiency LED	
R1	1K2		resistor
S1	MSK-12C02	horizontal SMD slide switch from AE / ebay etc.
U1	charger module	from AE / ebay etc.
U2	S09 5V0	DC/DC buck-boost converter from AE / ebay etc.

Note:
The S09 buck-boost converter is available in several voltages (3.3V, 4.2V, 5V, 9V, 12V). This unit could use one of those if you wanted a different voltage supply. Also, additional component positions have been provided on the PCB for S2 and R2. These allow the board to work at two alternative voltages, V2 being higher than the default V1. To utilise this you need to temporarily parallel a resistor across the first resistor after the row of unused pairs of pads on the S09. This changes the feedback loop and hence the output voltage. Having found your desired voltage use that value for R2, fit a wire link between the top end of the SMD resistor on the S09 and the pad provided close to that corner and fit S2. S2 has been positioned so that it cannot be operated accidentally. I discovered that 33K, when used with a 3V3 module, gave almost exactly 5V. Officially the highest available voltage from the chip is 24V, but attempting that may damage other components and may mean changing the other feedback resistor too.

I've not included the battery pack in the Bill of Materials as these are available in various capacities at varying prices. As the battery is intended to be fixed using cable ties I've included additional slots in the centre so that a single 18650 can be used.

Input voltage to the S09 is 2V to 15V it would be feasible to omit the charger and fit a carrier for 3x AA alkaline cells.
