# Designing a circuit that opens when less than 12 volts are applied and closes when more than 12 are applied

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?

• Any comparator can act as low side switch up to xx mA using any Vref with input scaled down to Vref so if V+>V- the output goes from open to closed =0V then load to V+ and output – Sunnyskyguy EE75 Jan 6 at 1:21
• How many wires? For example do you want this to be like a switch with just two wires that comes on when the supply is more than 12V? And how much current? – Jack Creasey Jan 6 at 1:31
• How much current must be switched? How sloppy can the tripvoltage be (12 +- 2v?)? How fast must the switching occur, once the tripvoltage is detected? – analogsystemsrf Jan 6 at 1:34
• @JackCreasey exactly. Just two wires and it switches on when a high enough voltage is achieved. – user180969 Jan 6 at 2:47
• for switch mosfet, bipolar transistor or relay, for voltage sensor TL431 is good choice especially since you want better than 1% accuracy. – Jasen Jan 6 at 3:36

• yes, current will flow from + to - - about 2mA but no current will flow through the load. – Jasen Jan 6 at 19:15