So this is for a fourth year engineering project, I'm on the power subsystem of a lunar penetrator. I'm not in electrical engineering, I'm in aero (mech), so forgive me for my lack of specialized knowledge. I want a system in place where if the main power source fails (RTG), the loads are automatically switched to a backup battery.
From researching switches I have come to the (perhaps incorrect) conclusion that a relay switch would be too big/bulky for the penetrator (~7cm internal diameter "artillery shell"), so perhaps transistor switches are a better option. I have found that a BJT transistor can be used as a switch (programmed to "expect" a certain input voltage, and if it does not receive said voltage it can open/close a switch).
So if these are used the transistor can be "programmed" to expect 4.5 V, and if the RTG fails, and the voltage drops to zero, it opens the switch. My question is, how do I get the switch on the backup battery to close? If I used the same theoretical transistor setup, there would be no change. 0 Volts, RTG fails, still 0 volts. So basically I don't know how to go about approaching this problem. It seems to me that I'm missing something very easy here, I mean it's pretty much the same idea as an alarm clock having a backup battery. I'm just having trouble finding info on this...
edit: For instance, I find patents like this, but in the context of this project I think a more simple solution might be more practical considering it's a preliminary design. http://www.freepatentsonline.com/4528459.pdf