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I am building a circuit on a breadboard to see if I can get a 2N7000 NMOS to switch using a 3.3V V_gs signal. As far as I know, I've set up the circuit properly:

schematic

simulate this circuit – Schematic created using CircuitLab

However,

  • This circuit causes the LED to light up regardless if V_gs is 3.3V or 0V. The transistor conducts no matter what.
  • It works properly when I connect a 1MΩ resistor between Gate and Source. Now, when V_gs is 0V, the LED turns off and when V_gs is 3.3V, the LED turns on. I discovered this when I was probing Gate and Source with a multimeter, and I wondered if an 1MΩ resistor would achieve the same effect.

Can someone explain this phenomenon for me? Thank you very much.

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  • \$\begingroup\$ Is your transistor mounted upside down? \$\endgroup\$
    – winny
    Feb 1, 2019 at 10:20

1 Answer 1

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With your multimeter or the resistor, you are providing a path to ground at the gate. The gate is essentially a small capacitor, and is very high impedance. This means that charge may accumulate, and raise \$V_\mathrm{gs}\$ sufficiently to allow the MOSFET to conduct. Hence, your LED was lighting up. It is always good practice to include this "pull down" resistor.

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    \$\begingroup\$ As a side question, I'm intending to do something similar, can I use the pull down resistor of my microcontroller (STM32) and does it affect any timing? (I want to switch the transistor very fast, like 100 kHz). \$\endgroup\$ Feb 1, 2019 at 10:19
  • \$\begingroup\$ Thanks for your answer. Are there equations or guidelines that can help me choose an appropriate pull down resistor? \$\endgroup\$
    – Ivan Hu
    Feb 1, 2019 at 10:27
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    \$\begingroup\$ @MichelKeijzers Yes, putting the pull down at the driving end is equivalent (just draw the diagram, it's the same node). Yes, it will affect your timing as you are forming an RC filter at the gate, so you may need to put a "real" value there if you have problems, rather than relying on the generally less well defined uC pull downs. 100 kHz isn't too fast though if you're only switching small signals, large MOSFETs are a different story. \$\endgroup\$
    – awjlogan
    Feb 1, 2019 at 10:27
  • \$\begingroup\$ Thanks for the answers, I haven't started yet on my project, intending to use 2N7000 with a 16x8 or 8x8 or so LED matrix using multiplexing. \$\endgroup\$ Feb 1, 2019 at 10:34
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    \$\begingroup\$ @IvanHu If you are just switching slowly (few kHz) then the 100k-1 M resistor is fine. You are trying to balance two things: how much you load the driving node (it would prefer a high resistance), and how fast you need the gate to turn off again (faster -> lower resistance). To a first approximation, it's just a simple RC network - that would get you started if you are concerned about switching rate, but otherwise it's ok to start with just a high value resistor like you've done already (as a beginner at least). \$\endgroup\$
    – awjlogan
    Feb 1, 2019 at 10:41

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