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enter image description hereenter image description hereActually I was developing the overcurrent protection using op amp LM-358 as described in the second picture . I made the circuit as described on this link https://circuitdigest.com/electronic-circuits/overcurrent-protection-circuit-using-op-amp, But I am facing the problem that using comparator I am able to achieve about 0 Volt at gate of mosfet but still I can not switch off the load

I only performed the simulation , I have not done it practically

enter image description here

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  • \$\begingroup\$ Simulation only or real-world? Add the info to your post rather than bury it in the comments. \$\endgroup\$
    – Transistor
    Commented Feb 7, 2020 at 20:38
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    \$\begingroup\$ Look closely at the IRF540N symbol in your simulation schematic. Do you see the diode inside the symbol? It is not there for no reason. Now look at your first schematic. Why are the two schematics not the same? \$\endgroup\$
    – DKNguyen
    Commented Feb 7, 2020 at 20:38
  • \$\begingroup\$ @DKNguyen what do u mean I have checked the datasheet the terminals are according to datasheet, kindly give a clear answer. \$\endgroup\$ Commented Feb 7, 2020 at 20:43
  • \$\begingroup\$ Impossible. Because your schematics don't even match each other. They're not even the same component. One of them is NOT the same as the datasheet. \$\endgroup\$
    – DKNguyen
    Commented Feb 7, 2020 at 20:44
  • \$\begingroup\$ @DKNguyen the second mosfet in second one is wrong , but the first one is correct. The first one was there on the provided link \$\endgroup\$ Commented Feb 7, 2020 at 20:47

4 Answers 4

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When you push 900mA through 1K you get a 900V drop. 36% of that is 324V. The zener is probably breaking down at 100V and passing enough current to drag the 324V down to 114 and change. That pot will burn up pretty quickly in real life.

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An IRF540N is N-channel but Circuit Digest shows it as P-channel.

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    \$\begingroup\$ Apparently that is just his build schematic. Yes, it's wrong, but his simulation circuit does match up correctly and it's his simulation that is messing up. \$\endgroup\$
    – DKNguyen
    Commented Feb 7, 2020 at 21:08
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enter image description here

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  1. Look at the diode in the symbol of your simulation schematic. It is there for a reason.
  2. Are those even the same component symbol?

Your ideal current source is your problem in your simulation. You have a thousand volts across RV2 and Q1 is probably in some kind of breakdown.

It also doesn't make sense because your IC1A and and Q1 is also a current source when combined with R1 so you are making two current sources fight each other.

You're not planning to build this are you? Because it is not realistic as it is.

Other issues:

  • I'm not sure why you grounded one end of RV2. It seems you want to use it as a variable resistor, not a divider.
  • 1K for RV2 is way too high to push enough current through to turn on a lightbulb. You would need unreasonable voltages.
  • 5V is not enough to turn on the NMOS. Gate threshold voltage is just when the MOSFET barely starts to turn on. If you are using the NMOS as a switch, you want to turn it all the way on which means you want to use the Vgs used to obtain the rated RDson which is 10V according to the datasheet.

enter image description here But you are using it as a linear device. But that still means the 5V maximum output from your op-amp will still not be enough. In fact, you may need even more than 10V since Vs will rise as more current flows through R1, thereby reducing Vgs if you are applying a gate voltage that is referenced to ground.

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The Mosfet is operating at its absolute maximum Vds so maybe its Zener diode is at its knee and is conducting a little.

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