The below image shows the circuit schematic:


simulate this circuit – Schematic created using CircuitLab

I wanted to check the turn off time for the mosfet switch varying the pulldown resistance.

  1. resistance=2ohm
  2. Resistance=100kohm

But in both cases there wasn't any noticeable difference in the turn of time . theoretically increasing the resistance should increase the time constant (RC) therefore increase the turn off time .

what do you guys think is happening here ?

Mosfet :IRF 1405PbF(sorry I couldn't upload the image showing the electrical characteristics of the mosfet) enter image description here

  • 1
    \$\begingroup\$ The RC time constant for 5.5nF and 100K ohms is 1ms, are you measuring things accurately enough to measure the difference between 1ms and 11ns for the 2 ohm resistor? You won't notice this difference but you can easily measure it. \$\endgroup\$ Commented Feb 2, 2018 at 7:15
  • \$\begingroup\$ How is the time constant measured? What instrument, and what probes do you use? \$\endgroup\$
    – MrGerber
    Commented Feb 2, 2018 at 7:33
  • 2
    \$\begingroup\$ Very important to add a diode in parallel with the motor. Otherwise you are going to blow up your mosfet the first time you try to turn it off. \$\endgroup\$
    – user57037
    Commented Feb 2, 2018 at 7:38
  • 1
    \$\begingroup\$ Add flyback diode and add pulldown resistor between gate-source. \$\endgroup\$
    – winny
    Commented Feb 2, 2018 at 9:35
  • 1
    \$\begingroup\$ The energy stored in an inductor is 1/2 * LI^2. So what is the winding inductance, and what is the peak current at which you might turn off the FET? Substitute those into the equation to calculate energy, in Joules. Then look at the datasheet. The datasheet has a few different tables showing tradeoffs between time and voltage or current. But the single pulse rating is 560 mJ. If you are far below that, then don't worry. If you are not far below that, look at the tables, or ask a separate question if you can't figure them out. \$\endgroup\$
    – user57037
    Commented Feb 3, 2018 at 6:18

1 Answer 1


This is a pretty simple answer. Judging from the question posed by Dean Franks:

Are you measuring things accurately enough to measure the difference between 1ms and 11ns for the 2 ohm resistor?

And the answer by OP:

No measuring instruments just the naked eye lol

I think the answer is to get an oscilloscope or something similar that you can take measurements with. Of course there is no noticeable difference with the naked eye between 1 ms and 11 ns.


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