Finalized and purchased components for supporting time for orderly shutdown of DE10-Nano

SWITCHED OVER TO MORE FLEXIBLE BATTERY BASED APPROACH

I decided that rather than supercapacitors that would get expensive for increasing numbers of seconds feeding 1.5+ amps to the DE10-Nano board, I would use a rechargeable battery system. This will allow me to support longer times if that proves necessary. 

SELECTING BATTERY, CHARGER AND INPUT POWER FOR THIS APPROACH

The board I am using will be drawing nearly 2A from the 5V supply at peak times, which will add a noticeable extra load on the IBM 1130 power rail I attach it to. The 12VDC rail I was using only supplies 4A in total and there are demands on that throughout the computer itself. Even with the ratio of 5/12 for current draw, ignoring conversion losses, that would put a bit under 7/8A on the 12V rail, over 20% of the capacity of the supply that I would be adding. 

I therefore switched over to the 48VDC rail, which has more capacity and benefits from the reduction of the draw to a 5/48 ratio or around 1/5th of an Ampere. There will be a DC-DC Buck Converter to drop the voltage to the levels I need for the battery charging and to power the DE10-Nano board itself. 

For convenience in sourcing batteries with the ability to deliver a couple of Amperes for perhaps 10 seconds without damage to the cells, I chose to look for 12V rechargeable batteries as these are widely used in many applications. I only need about 100 mAH (milli Amp Hour) of capacity but to also meet the ability to deliver 1 Ampere without strain I instead will be buying something with single digit AH rating. 

That implies that I have a second DC-DC Buck Converter to take the battery voltage level down to the 5V that the board requires. This is 12V when only the battery is feeding power and rises to 13-14V when the charging circuit from the first converter is active because the 1130 is feeding 48V to us. 

Add in two diodes so that this converter is fed from either the 48V to 13+V converter or the 12V battery pack without back feed to the computer.  The first converter's power is fed to the battery and then diode 1 takes that voltage to the board. The first converter's power is also passed through diode 2 to the board. The second converter does have some voltage drop from the diodes, so we are seeing from perhaps 11.4V to 13.4V depending on whether it is battery only during a power failure or not. 

THE POWER FAILURE ALERT SIGNAL DESIGN

I chose to use an NPN transistor to pull the user button on the board to ground when the power from the computer is available. The pullup on the button circuit will make that register as digital 1 when the power has failed but 0 while we are driving the transistor from the first converter's 13-14V. 

A subprocess running under my application on the Linux system on the board will be polling that button's state. When it sees a 1, it will inject a command line to an attached shell that shuts down the Linux system gracefully. 

The gate of the transistor is fed from my converter output, before the diode that connects to the DE10-Nano board, so that it is driven while the 48V supply to the converter is active. The collector goes to the user key on the side that connects to the processor and has a pullup resistor. The emitter is hooked to ground.