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I have two Questions about MOSFET as sensor device.

1. I had been using resistance to discharge MOSFET. but I can not figure out minimum requirement of the resistance which could work on 12V power supply? enter image description here

2. Can I use diode to discharge MOSFET? as given in following circuit diagram..

Diode for discharging MOSFET

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    \$\begingroup\$ How do you expect that diode to conduct? Resistor value you need depends on how fast you want it to turn off vs. how much current you can afford to waste from the battery to keep the MOSFET gate charged. \$\endgroup\$
    – Trevor_G
    Commented Oct 26, 2017 at 18:49
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    \$\begingroup\$ Could you please draw your circuit with the ground below and power above? Your way of drawing gives me a headache. And what is R3 doing at that location, and why 220k? \$\endgroup\$
    – Wouter van Ooijen
    Commented Oct 26, 2017 at 18:57
  • \$\begingroup\$ @WoutervanOoijen haha! sorry for the Circuit diagram. R3 (Resistance - 220 ohm or 0.220k not 220K) is to protect LED. \$\endgroup\$
    – VBAbyMBA
    Commented Oct 26, 2017 at 19:02
  • \$\begingroup\$ @WoutervanOoijen if you look close its actually 0.220K... \$\endgroup\$
    – Trevor_G
    Commented Oct 26, 2017 at 19:02
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    \$\begingroup\$ HUH? Me thinks you are totally confused. \$\endgroup\$
    – Trevor_G
    Commented Oct 26, 2017 at 19:18

1 Answer 1

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Ok lets draw your circuit properly...

schematic

simulate this circuit – Schematic created using CircuitLab

Now lets say when the switch is closed you only want 0.1mA of current to be wasted.

\$R2 = 12V/0.1mA = 120K\$

Now, from the data-sheet an IRF530 has a max gate capacitance of 800pF. So the gate will discharge exponentially with an RC time constant of 9.6uS when the switch opens.

That seems fast enough to me...

Maybe you want 1uA... R would be 12Meg and that would put the time constant close to 1mS.

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