I have designed this circuit to turn off the mosfet as soon as I apply 1.2A of current for 2ms. Also I want to know the status through the LED whether the mosfet is turned off or not.When I simulated this circuit,LED starts glowing before the current reaches to zero.Could you please suggest me whether this circuit serves the purpose.enter image description here

Simulation profile:

I am using FDC5614P from Fairchild semiconductor.Here is my simulation snapshot of drain current Vs Voltage across the load enter image description here

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    \$\begingroup\$ Re: "as soon as I apply 1.2A of current for 2ms". Where do you expect to apply this current? \$\endgroup\$
    – Fizz
    Nov 12, 2015 at 13:47
  • \$\begingroup\$ It is possibly better you describe how you think the circuit works so we can point out the flaw, than to have us guess where the mistake in your thinking lies since no one can think of a way why the circuit would do what you expect it to do. \$\endgroup\$
    – PlasmaHH
    Nov 12, 2015 at 13:50
  • \$\begingroup\$ When +12Vdc is applied to the circuit,the mosfet turns on till it reaches the gate threshold to turn it off.I have set the resistor value 10 ohms to allow 1.2A of current.This works perfectly fine. But I need to see whether the mosfet turned off or not by means of LED \$\endgroup\$
    Nov 12, 2015 at 13:51
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    \$\begingroup\$ Different MOSFETs have different characteristics, in particular the Vgs threshold, triode region and Vgs required for saturation. Use an actual MOSFET with a proper model or post the model the simulator is using and we may be able to help. \$\endgroup\$ Nov 12, 2015 at 14:00
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    \$\begingroup\$ You do know the 1.2A continues to go through the base of TR3 if the FET is off? \$\endgroup\$
    – Icy
    Nov 12, 2015 at 14:14

1 Answer 1


All you are missing is a base resistor basically. Assuming you have a 2mA LED try:

enter image description here


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