Low voltage at N-Channel source pin

Question

I made a pcb which uses an N-Channel Mosfet for enabling/disabling the NTC thermistor as in the schematic below. The problem is I get 2V at the Drain pin instead of 3.3V. I measure 3.3V at NTC_TRIG and power pins with a voltmeter so there isn't any issure there.

Currently using this N-Channel mosfet whose gate threshold voltage(1.6V typical) seems suitable for my application. MCU is STM32F030C8 powered with 3.3V

Schematic:

Answer 1

You can either swap drain and source and use a P-channel MOSFET, or drive the gate with several volts higher than the 3.3V supply and stay with the 2N7002 (likely not convenient for you).

You have made a source follower so for output current 250uA it will drop Vgs(th) from the gate voltage (source voltage effectively reduces Vgs).

In the case of using the P-channel MOSFET, input low will turn the MOSFET “on” and high will turn it “off”.

Suitable P-channel MOSFETs might include the BSS84 and the AO3401A for lower Rds(on).

Answer 2

With NMOS connected at high-side (Drain to Vcc) you cannot supply NTC with more then 3v3-Vth.

Use NMOS at low-side (Source connected to GND) if you can.

Or PMOS according schematic below - remember the logic will be swapped, so Pin=High will disconnect the NTC.

^{simulate this circuit – Schematic created using CircuitLab}