Electrical – Capacitor size across power source selector relay

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:

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....

You can quickly see the problem!

One issue is that you do not have a fixed grounding resistor on the transistor. This may not always be needed. However the Uno ports are not programmed at reset or when there is no program running. In those cases the port may be set to a high impedance input with or with out a light pull up resistance. In this case you might be unknowingly turning the transistor on intermittently with noise or pull up leakage current.

To insure stable operation be sure to set the controlling port pin quickly to a digital output and set the initial logic value. Adding a resistor to ground, (before or after the existing 10k) will help prevent an open port or a light pull up from activating the transistor. With a 5v Uno a matching 10k should be OK. Lower resistor values reduce the chance of unknowingly activating the transistor, but at some point the high signal will not be able to turn it on.

If immediate switching time is not critical adding a small cap on the transistor base to ground can prevent fast multiple ON-OFF cycles that could reduce the life of the bulb. (Maybe 1uf).