I am attempting to design a circuit that monitors the Line voltage and switch a 12V SLA to the load in the event of a power outage. I am using a LM311, 12V single supply with hysteresis. to turn on a relay that is sourced by a 2N3904. Needless to say I am having a difficult time getting it to work. I am using a 12V reference on the inverting input which I expect would keep the output in a low state. However, that is not the case. I am getting near Vcc voltage at the output. What is wrong with my design? I have tried several different approaches with other comparators including LM339 all with the same result. I have used formulas culled from component manufacturers' websites and various other sources to determine the resistor value for the hysteresis network.

 

Look at R1, R4 and R5 in your circuit. R4 and R5 may or may not be needed or it could be that their values need to be different than what you are using. You did not include values. For your LM311 to go to zero on pin 7 (the output), it would need to sink enough current through R5 to make that happen. If R5 is too small, it will keep the output from going low enough. You have to understand that your low is provided through your ground at pin 4. The current to give you that ground at pin 7 goes through your chip.

 

Normally in simple battery backups you just use two diodes and these set which power source is being used.
That also means that the mains supply needs to have a higher voltage than the battery supply to be preferred.