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?
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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\$– Tony Stewart EE75Commented Jan 6, 2019 at 1:21
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\$\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 CreaseyCommented Jan 6, 2019 at 1:31
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\$\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\$– analogsystemsrfCommented Jan 6, 2019 at 1:34
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\$\begingroup\$ @JackCreasey exactly. Just two wires and it switches on when a high enough voltage is achieved. \$\endgroup\$– user180969Commented Jan 6, 2019 at 2:47
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1\$\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 Слава УкраїніCommented Jan 6, 2019 at 3:36
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2 Answers
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The below schematic offers a high current sink and MOSFET gate protection and about 250mV of hysteresis.
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.
answered Jan 6, 2019 at 3:33
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\$\begingroup\$ But this needs 3 wires, so does not meet the requirements. \$\endgroup\$ Commented Jan 6, 2019 at 3:57
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\$\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\$– user105652Commented Jan 6, 2019 at 5:07
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\$\begingroup\$ The source voltage would go as high as 16V, but no higher. \$\endgroup\$ Commented Jan 6, 2019 at 18:09
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\$\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\$ Commented Jan 6, 2019 at 18:36
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1\$\begingroup\$ yes, current will flow from
+to-- about 2mA but no current will flow through the load. \$\endgroup\$ Commented Jan 6, 2019 at 19:15