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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? enter image description here

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  • \$\begingroup\$ 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 \$\endgroup\$ – Sunnyskyguy EE75 Jan 6 at 1:21
  • \$\begingroup\$ 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? \$\endgroup\$ – Jack Creasey Jan 6 at 1:31
  • \$\begingroup\$ 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? \$\endgroup\$ – analogsystemsrf Jan 6 at 1:34
  • \$\begingroup\$ @JackCreasey exactly. Just two wires and it switches on when a high enough voltage is achieved. \$\endgroup\$ – user180969 Jan 6 at 2:47
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    \$\begingroup\$ for switch mosfet, bipolar transistor or relay, for voltage sensor TL431 is good choice especially since you want better than 1% accuracy. \$\endgroup\$ – Jasen Jan 6 at 3:36
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The below schematic offers a high current sink and MOSFET gate protection and about 250mV of hysteresis.

schematic

simulate this circuit – Schematic created using CircuitLab

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you could do this perhaps, I've drawn it rotated 180 degrees from your diagram.

schematic

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  • \$\begingroup\$ But this needs 3 wires, so does not meet the requirements. \$\endgroup\$ – Jack Creasey Jan 6 at 3:57
  • \$\begingroup\$ How do you protect the MOSFET gate if Vcc goes way above 12 volts? OP did imply the source voltage was not regulated. \$\endgroup\$ – Sparky256 Jan 6 at 5:07
  • \$\begingroup\$ The source voltage would go as high as 16V, but no higher. \$\endgroup\$ – user180969 Jan 6 at 18:09
  • \$\begingroup\$ I might be missing something here, but couldn't the current take the outside path going through R1 and R2 even when the voltage is far below 12V? \$\endgroup\$ – user180969 Jan 6 at 18:36
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    \$\begingroup\$ yes, current will flow from + to - - about 2mA but no current will flow through the load. \$\endgroup\$ – Jasen Jan 6 at 19:15

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