Power Switch

NOTE: This is for a non-FRC application.

I have a robot that I would like to power from a Battery (3S LiPo with a Traxxas Female Connector) or a Power Supply (12V with a Traxxas Female Connector). Among the electronics on the robot is a linux board that would not do very well with removing power without a proper shutdown process. While in development the robot would be powered off the power supply, and when testing would run off the battery. So switching between the two would be common. I don’t want to run the power supply and battery in parallel as that would not have very good consequences, so I’m looking for some type of power switch that can switch between either the power supply or the battery but acts as a “make before break”, so power is never interrupted to the rest of the robot.

Does anyone have any recommendations for a switch like this? The cost ideally would be around $20-$30, the robot sees at most 30A @ 12V and can’t be too big, maybe an inch or two. Here are some reference pictures: https://github.com/dgitz/icarus_rover/wiki

A couple of diodes and resistors can make a battery backup circuit. You could use a couple of smaller batteries to power your board once the main power cuts off. Once you sense a main power disconnect, issue a command to shut down your Linux board.

If you place a capacitor in parallel with your inputs, you can use a switch that breaks and then joins on the other side, and the capacitor can carry you through the transition without resetting the linux board. The more current that the circuit draws, the longer the transition time, and the more sensitive the circuit (in terms of the maximum acceptable voltage drop), the larger the capacitor you will need. Assuming that the allowable voltage drop is small compared to the nominal voltage, The necessary capacitance is simply
C = t * I / ΔV, where t is the switching time, I is the current draw, and ΔV is the acceptable voltage drop.