I would like to build essentially a UPS for my computerized pellet stove. I live in the southwest where there are frequent powr blips and outages which could last from a few minutes to days.
I would like to build an Arduino based backup UPS with one or two 12 volt car batteries (could be sealed type), a charging circuit, an inverter from 12 to 120V (pure sine is best, but perhaps semi might work) and of course the necessary instantaneous transfer switch. Much of this can use off the shelf hardware. However, instantaneous transfer switches are very expensive. Typically they seem to have somewhere in the range of a 30 millisecond transfer from mains to battery time.
I am not sure if this is within the possibility of an Uno R3 driven project or not. I imagine that solid state relays would be the way to go. I have yet to begin investigating them. I believe the basic processing/sensing/reacting time of the Uno may be the critical first factor.
Thoughts on the feasibility of this and perhaps suggestions for critical parts of this system?
First question that comes to mind is what is the Arduino's role? To detect when the grid power goes off? How will you get the AC power to 0 to 5V that the Arduino can handle?
Second question is how will you power the Arduino? A 7.5V wall wart is probably not a good idea.
Third question is more basic. Why does the pellet stove need an uninterrupted power supply? Why doesn't it use EEPROM and some smarts to store temperature set points, etc., so that it can tolerate a power interruption.
If the purpose of the UPS is to keep the Arduino running continuously the solution is very simple. Run the Arduino from a battery. At the same time use a suitable charger to keep the battery fully charged. If the power supply fails the Arduino won't notice until the battery runs out. No switches needed.
If you want to detect the power failure put a diode at the battery end of the charger wire (so the battery can't back-feed to the charger) and using a suitable voltage divider connect the charger wire (upstream of the diode) to an Arduino I/O pin. When the power is on the pin will read HIGH and when it isn't it won't.