Circuit to retain some power after power off

Thread Starter

ep.hobbyiest

Joined Aug 26, 2014
201
Hi,
I am looking for the circuit which will retain power for 500ms to 1 sec. after power off the main power supply.
I want to do some operation before going shutdown like storing and all.
 

ErnieM

Joined Apr 24, 2011
8,377
To provide 200mA for 1 second with no more than a 0.5V droop (5V to 4.5v) would require 400mF (0.4F) of capacitance.
Which of course is a LOT of capacitance. If you go to a supercap then you still have a problem with the voltage rating.

Still, you can find one with these ratings for a few dollars. DO make sure you allow for the highest voltage it can see, and give it some margin anyway.
 

Thread Starter

ep.hobbyiest

Joined Aug 26, 2014
201
then i think i should minimize the consumption and i can use the 3.3V controller.
I can go down till 70mAmp and 3.3V of controller for 0.3 Sec.

If i want to retain for 0.3 Second and keep the voltage 3.3V then i will need regulator as well, right?
That means i will have connect the capacitor before regulator.
 

crutschow

Joined Mar 14, 2008
34,285
So you still have a 5V supply but the controller will operate at 3.3V from a regulator?
If so then yes, you put the capacitor before the regulator.
You will also need a low dropout regulator.

Maintaining 70mA for 1 second with a 1.7V drop would require 41mF of capacitance so a 47mF capacitor should be adequate.
 
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