less than 4 milliseconds. I need this AC voltage failure signal to switch change over relay. This relay oscillate when no filter capacitor. I use this signal to turn off MOSFET that drive this relay.You need to define your requirements. How fast a method do you require? Less than 1 cycle (16.67 milliseconds) or less than some number of cycles? Tell us.
That is called an online UPS but most UPS have relays to switch to the inverter.The UPS I depend on always sends the output of it's battery run inverter to my load.
Most UPS's don't need to switch over in < 1/2 cycle either. But it's the OP's place to determine how fast it really needs to go.That is called an online UPS but most UPS have relays to switch to the inverter.