Temporary energy holding circuit (2 Seconds,

[anroop]

Hi,

This is aimed at reducing the unnecessary battery drain during presence of external supply.

i am trying to build a circuit that powers a device that operate at 3.3v to 5v max of 2A. It contains a battery. I would like to isolate the battery from supplying current to the load in presence of External supply and to switch over from supply to battery power during power cut. The problem I could face is that once the External power is cut it takes a delay to switch from Supply to Battery which could make the device switch off for an instinct causing loss of data. I prefer to maintain the Current for a duration of at least 2 seconds after power cut so that the switching from Supply to Battery could occur without any fluctuation. I had an idea of using a Capacitor but its size may be huge(my assumption). I request you to suggest me an solution to my problem.

Note:
Energy Holding time - Minimum 2 Seconds.
Voltage - 3.3V to 5V
Curretn - 2A

 

[(*steve*)]

It's called a "capacitor" (or alternately "a battery")

If you use a battery then you need approximately 1.5 farad (1.5F)

The other side of the coin is that charging this capacitor will impose an enormous load on your power supply.

As an alternative, if you already have a backup battery, you can probably arrange for the power to switch from external power to battery instantly with the use of a couple of diodes.

 

[anroop]

Actually, I am trying to add a backup battery as you said at-last, but to power the device only when external Supply is absent. I have planned to give an input of 12 instead of 5v use a 0.87F capacitor in parallel, regulate the output to 5v followed by battery controlled by switch with feedbak from supply .then the load. Refer Schematic. I'll use a transistor circuit in place of NOT gate IC.

 

[anroop]

Image attached to this thread.

 

[(*steve*)]

What voltage is the battery?

if it's between 6 and 11 volts, then you could just use this:



GIven that you only need 3.3V, a 6V battery a Schottky diode for D2, and a 5V LDO regulator should keep your output voltage well above 4V