1
\$\begingroup\$

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

Improve this question
\$\endgroup\$
6
  • 1
    \$\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 '15 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 '15 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\$ – ANONYMOUS Nov 12 '15 at 13:51
  • 1
    \$\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\$ – Peter Smith Nov 12 '15 at 14:00
  • 3
    \$\begingroup\$ You do know the 1.2A continues to go through the base of TR3 if the FET is off? \$\endgroup\$ – Icy Nov 12 '15 at 14:14
2
\$\begingroup\$

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

enter image description here

Improve this answer
\$\endgroup\$
0

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.