Electronic – Power from battery when primary source fails


I'm attempting to design a circuit that will be powered from a 12V primary source but if the power fails then a relay switch to battery (which is maintained charged by an UC3906).


The idea is that if primary source is present the MOSFET is on, activating the relay. When primary source fails the MOSFET is off and the relay go to the position showed in the schematic, the application is now powered by the battery.
Do yo see this reliable?

I wish to ensure that no current is drawn from the battery if primary power is present and I prefer to not put the primary voltage above the battery voltage (which is already slightly above 12V), with the two diodes solution I added a pMOSFET to get it (may be D1 is not necessary now?).


Best Answer

This circuit idea is OK if the "application" can live with a momentary dropout of its supply voltage during the changeover. There are two things that cause this. First the 12V primary power must drop to less than about 2V or so before the 2N7000 N-FET will turn off. Secondly the relay contact switch over time will take a millisecond or two.

If you want uninterrupted power to the "application" during the changeover then you could consider several alternatives.

1) Add a comparator circuit that detects when the primary 12V has only dropped a small amount (such as to 11.4V) and switch the relay before the primary rail fails. Some charge storage capacitors on the "application side" can minimize the voltage droop there while the relay contacts are switching over.

2) Use a pair of power MOSFETs to switch the voltage instead of the relay. These would switch much faster than the relay and the "application side" hold up capacitors can be much smaller.

3) Use just two power Schottky diodes to OR the two power sources to the application. No switch over control logic required but you do lose some voltage from the sources to the application power rail.

NOTE: In your schematic you need to add a back biased diode across the relay coil so that at the time the the 2N7000 FET turns off the inductive kick of the coil will not take out the FET.

Tino proposed to change his circuit to use the two diode approach as follows:

enter image description here

He questioned as to whether the Diode D1 would still be needed. Unfortunately the diode would still be needed because of you look closely at the data sheet for the IRFU9024 you will note the body diode of the MOSFET....

enter image description here

You can quickly see the problem!