emergency power supply problem

[sparkball]

I'm working on a project using microcontroller 89s51. The system needs to run continuously 24 hours a day and is powered by the rectified D.C. +5V from the A.C. mains supply.
The problem is: how do I safely connect the system to an emergency battery supply in case of a power cut. I suppose a relay would be too slow.

Does anyone have any idea. Please help!

 

[ionp]

A common technique is to diode-or the DC at a point before the regulator.
That is, insert in series with the input of your 5V regulator a power rectifier. Add another rectifier, also feeding DC into your regulator except from the battery. Whichever voltage source is higher will run your circuit.

To be clear, the junction of the 2 rectifiers has the cathodes connected to the 5V regulator input.

 

[clairvoyant13]

Battery backup with charging.
When power is supplied the battery is charged by 'trickle charging' less than 20mA current. ( red line )
For example if R=220ohm:
( 9V-7.2V )/220ohm =8.2mA
Bye!!!