First off, I am aware that premade devices like this exist however I have been unable to find any that fit my exact needs which are as follows.
- 0 power draw when in the off state
- Minimal voltage drop when in the on state
- Being able to set it to trigger at 10.5V
- Simple design
- Manually reset
The idea is that when I connect a battery with a voltage above 10.5V and S1 is pushed to jump start the circuit the normally open relay will then start keeping it's self closed. I would then have R2 tuned so that it had just enough resistance so that when the voltage on the main circuit dropped to 10.5V the voltage to the relay coil would become too low to keep the contacts closed. The contacts would then come open and cut power to the whole circuit.
In theory this is a nice simple elegant solution to the problem for a layman like me. The issue that I'm running into is that there isn't an "exact" voltage where a relay shuts off so it would be nearly impossible to get it to shut off when the main circuit is at exactly 10.5V. I can't use a transistor or similar device because even a small voltage drop on a 12V nominal circuit like this makes a huge difference. I need 10.5V reaching the end device when the battery is at 10.5V.
I am hoping to keep this circuit as simple as possible so that I can have it packed up nicely as a single rugged unit. Currently the only application I have for it draws a constant 5 amps but I would like to be able to modify the circuit to accommodate larger loads if needed in the future so an electromechanical component seems ideal for switching the main circuit.
My question is, is there a component or simple circuit that I could put in the place of R2 that would only allow voltages over 10.5V through and completely cut off below that? If I can just get that relatively low amperage (under 200mA) circuit to completely cut off at a designated voltage then I can switch the main circuit with whatever relay or contactor I want with minimal modification.