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I would like to use N-channel as a touch switch which will be an input to the microcontroller. Currenty I am concerning about power draw. The whole device will be in metal case and I am planing to connect this case to battery GND. Now, if I use the Nmos connected like this (R3 is a touch plate):

enter image description here

i will have in off state power consumption on R1. Only way how to avoid this comsumption is put a touch plate instead of R2 and switch to VCC. I am asking, does make sense to make a metal case connected to VCC instead of GND? Or is better to use a P-channel mosfet wich will be grounded during on state? Is there a way to use high impedance of uC imput to make it without mosfets?

EDIT:

I forgott to clarify, the touch plate will be made like mosfet gate to metal case touch switch.

EDIT:

Until now I see the most suitable solution to use a P-channel like this (no waste, no need to low finger resistance):

enter image description here

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  • \$\begingroup\$ Basically, don't do it. Touching the gate of a MOSFET sounds like a good idea. Then you do it with a static charge of 10,000 volts and destroy the FET. \$\endgroup\$ Jan 26, 2021 at 1:57
  • \$\begingroup\$ According to static charge, it can be solved with simple diode protection. Would you better to use a high hfe BJT or Darlington? \$\endgroup\$
    – user208862
    Jan 26, 2021 at 2:01
  • \$\begingroup\$ There speaks somebody who has never done it. Safely handling direct application of static discharge is not simple. \$\endgroup\$ Jan 26, 2021 at 2:04
  • \$\begingroup\$ ESD control is done by fast diodes to each rail in 2 stages with 10k series current limiters in all CMOS not 2M \$\endgroup\$ Jan 26, 2021 at 3:47
  • \$\begingroup\$ I think if you move D1 so it is connected directly to the gate it will not be any ESD problem for the MOSFET. \$\endgroup\$
    – mkeith
    Jan 26, 2021 at 4:45

1 Answer 1

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Why using a Mosfet while a bunch of MCU has integrated capacitive touch functionality that requires no external components?

enter image description here

Using a MOSFET isn't a good idea, this design will give you reliability issue and is more expensive.

You can protect the gate against ESD to some extent, but that will reduce sensitivity, and your circuit will work differently depending on temperature and humidity, it might work in the summer and stop working altogether in the winter.

MCU with touch capabilities offers a stack that takes care of all this.

Based on your question, you will need an MCU anyway, get one with touch capability.

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  • \$\begingroup\$ Thank you, but it is a very small device with almost all front panel covered with display, so there is no place to put pads like this. All I can place there is a small 5mm pads, isolate them from metal case and a do a switching by finger crossing between pads and case. \$\endgroup\$
    – user208862
    Jan 26, 2021 at 5:16
  • \$\begingroup\$ Nothing stops you from having 5mm pads. If for some reason you don't want to use capacitive touch, go with physical switches. This MOSFET solution will most likely never be reliable. \$\endgroup\$
    – Damien
    Jan 26, 2021 at 6:47

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