I'm looking to build a circuit where current only flows when there is a potential difference across the entire circuit of more than 12 volts (+- 0.1 V). I also need there to be only a small amount of resistance across the part of the circuit that controls whether current will flow, in order to prevent overheating. I'm not sure what I should use to control this; I was thinking of using a zener diode, but cannot find any with a forward voltage above 7V. I'm thinking that there might be some other type of semiconductor I could use, but I'm not sure what it would be. Perhaps I could use a zener diode but put it in reverse orientation rather than forward? I'm not sure how much resistance that would add however. What would you guys recommend using?