Drive mosfet with arduino

[bonuscafe]

Hello,

I would like to drive a mosfet with Arduino Nano RP2040 Connect to control a fan.
I can see proper square signal at one of the digital output on arduino. I connected this output to the mosfet gate (no resistor). Source is connected to GND on arduino.

I was curious and measured mosfet's drain and source pin with oscilloscope and I surprised I can see same square signal which goes to gate.
I though there should be nothing to measure if there is not connected anything to mosfet D and S recently. Somehow gate leaks the whole signal? I tested with STD16N65M5 and STU10NM60N mosfets.
Can you help me out why is this situation?

[ataradov]

Can you show your circuit and measurement points. You say that you have connected source to the ground, yet you somehow measure the same voltage as on the gate?

[bonuscafe]

Hello Alex,

I attached a drawing.
Yes, I can see the same signal as on the gate.

[ataradov]

Ok, so you are measuring S-D voltage? You are likely observing parasitic coupling. Add even slightest load (500 kOhm) to the drain and your signal will go away.

[Manul]

Yes, I can see the same signal as on the gate.

Question is, how much same same signal is? Does it match exactly by amplitude and shape? If that is so, it's not because of parasitic capacitance, it must be something else. Maybe source is not connected to Arduino GND.

[MarkF]

With those two N-channel MOSFETs mentioned and a 3.3V Arduino Nano, you will barely
be able to turn ON the MOSFET.  If at all.

It would be better to use a P-channel MOSFET instead.  (Leave out R4.)

[aliarifat794]

Can you specify the model of the MOSFET that you are using?

[bonuscafe]

Hello,

Finally it seems root cause of the issue my four mosfets were broken, all of them.
I don't understand why this happened because I only tested with some milliamps.
Now I used brand new and I cannot see previous strange square signals.
Thank you very much everybody's help!

Anyway, I attached measurement with the good mostfet (STD16N65M5).
I marked with green two points on the blue measurement. Maybe this is indicating parasitic coupling?

[MarkF]

If your measurements are with the circuit you posted earlier, they are totally meaningless.

You have:
  - No voltage source
  - No load
  - No current limiting
  - A gate voltage that barely meets the min V_GS of 3V in the datasheet (4V typical)
  - No valid circuit to speak of

You have a MOSFET hanging out in the breeze.  I'm not surprised you blew up all of your old ones.
Until you've connected the MOSFET in some sensible manner, there is no point in discussing your readings.

[mianos]

Yes, in case it's not clear, you can't use a MOSFET like that with no load.