How to detect AC line Voltage failure quickly? I am trying to built UPS. The filter capacitor in diode bridge cause delay, when AC voltage failure. Please help me.

 

A normally closed relay. When power is lost, the relay will close and sound an alert.

 

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.

 

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.

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.

 

Less than 1/4 of a cycle. So the only way I can think of quickly, is to design a circuit that duplicates and then tracks the actual power signal. So any deviation of the power signal from the one you have synthesized will be rapidly detected. So you can start with a 60 Hz. quadrature oscillator made from a pair of op amps. Then use a transformer to create a low voltage AC signal from the mains. If you phase lock your oscillator to the transformer output you will know immediately when there is a deviation.

 

Do you really need to switch outputs with a damn slow relay?

Do you really need to switch outputs at all?

The UPS I depend on always sends the output of it's battery run inverter to my load. While AC power is up the battery is being charged as much as it is supplying, meaning the battery isn't being loaded. As soon as the AC input goes away the battery picks up the current with a zero switchover time.

 

The UPS I depend on always sends the output of it's battery run inverter to my load.

That is called an online UPS but most UPS have relays to switch to the inverter.

 

That is called an online UPS but most UPS have relays to switch to the inverter.

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.

 

So true. In fact most relays have an operate time of around 10ms plus there is some bounce so maybe OP needs to discard it altogether.

 

Relay OFF time is about 4 milliseconds. But, my detected signal is about 3 ms. So, the total time to turn off relay is about 8 ms.